Relationship between knockdown resistance, metabolic detoxification and organismal resistance to pyrethroids in Anopheles sinensis

Strong resistance to β-cyfluthrin in a strain of the beetle Alphitobius diaperinus: a de novo transcriptome analysis

The lesser mealworm, Alphitobius diaperinus, is an invasive tenebrionid beetle and a vector of pathogens. Due to the emergence of insecticide resistance and consequent outbreaks that generate significant phytosanitary and energy costs for poultry farmers, it has become a major insect pest worldwide. To better understand the molecular mechanisms behind this resistance, we studied a strain of A. diaperinus from a poultry house in Brittany that was found to be highly resistant to the β-cyfluthrin. The strain survived β-cyfluthrin exposures corresponding to more than 100 times the recommended dose. We used a comparative de novo RNA-Seq approach to explore genes expression in resistant versus sensitive strains. Our de novo transcriptomic analyses showed that responses to β-cyfluthrin likely involved a whole set of resistance mechanisms. Genes related to detoxification, metabolic resistance, cuticular hydrocarbon biosynthesis and proteolysis were found to be constitutively overexpressed in the resistant compared to the sensitive strain. Follow-up enzymatic assays confirmed that the resistant strain exhibited high basal activities for detoxification enzymes such as cytochrome P450 monooxygenase and glutathione-S-transferase. The in-depth analysis of differentially expressed genes suggests the involvement of complex regulation of signaling pathways. Detailed knowledge of these resistance mechanisms is essential for the establishment of effective pest control.

Human activities and densities shape insecticide resistance distribution and dynamics in the virus-vector Culex pipiens mosquitoes from Morocco

BACKGROUND

Mosquitoes of the Culex pipiens complex are widely distributed vectors for several arboviruses affecting humans. Consequently, their populations have long been controlled using insecticides, in response to which different resistance mechanisms have been selected. Moreover, their ecological preferences and broad adaptability allow C. pipiens mosquitoes to breed in highly polluted water bodies where they are exposed to many residuals from anthropogenic activities. It has been observed for several mosquito species that anthropization (in particular urbanization and agricultural lands) can lead to increased exposure to insecticides and thus to increased resistance. The main objective of the present study was to investigate whether and how urbanization and/or agricultural lands had a similar impact on C. pipiens resistance to insecticides in Morocco.

METHODS

Breeding sites were sampled along several transects in four regions around major Moroccan cities, following gradients of decreasing anthropization. The imprint of anthropogenic activities was evaluated around each site as the percentage of areas classified in three categories: urban, agricultural and natural. We then assessed the frequencies of four known resistance alleles in these samples and followed their dynamics in five urban breeding sites over 4 years.

RESULTS

The distribution of resistance alleles revealed a strong impact of anthropization, in both agricultural and urbanized lands, although different between resistance mutations and between Moroccan regions; we did not find any clear trend in the dynamics of these resistance alleles during the survey.

CONCLUSIONS

Our study provides further evidence for the role of anthropic activities in the selection and maintenance of mutations selected for resistance to insecticides in mosquitoes. The consequences are worrying as this could decrease vector control capacities and thus result in epizootic and epidemic outbreaks. Consequently, concerted and integrated disease control strategies must be designed that include better management regarding the consequences of our activities.

New insights into cypermethrin insecticide resistance mechanisms of Culex pipiens pallens by proteome analysis

BACKGROUND

Due to the development of insecticide resistance in mosquitoes, with worldwide mosquito-borne diseases resurgence in recent years, recent advances in proteome technology have facilitated a proteome-wide analysis of insecticide resistance-associated proteins in mosquitoes. Understanding the complexity of the molecular basis of insecticide resistance mechanisms employed by mosquitoes will help in designing the most effective and sustainable mosquito control methods.

RESULTS

After 30 generations, insecticide-selected strains showed elevated resistance levels to the cypermethrin used for selection. Proteome data allowed the detection of 2892 proteins, of which 2885 differentially expressed proteins (DEPs) achieved quantitative significances in four stages (egg, larvae, pupae, adult) of Culex pipiens pallens cypermethrin-resistant strain as compared to the susceptible strain. Among them, a significant enrichment of proteins, including cuticular proteins, enzymes involved in the detoxification (cytochrome P450, glutathione S-transferases, esterase, ATP-binding cassette) and some biological pathways (oxidative phosphorylation, hippo signalling) that are potentially involved in cypermethrin resistance, was observed. Thirty-one representative DEPs (cytochrome P450, glutathione S-transferase, cuticle protein) during Cx. pipiens pallens developmental stages were confirmed by a parallel reaction monitoring strategy.

CONCLUSIONS

The present study confirmed the power of isobaric tags for relative and absolute quantification for identifying concomitantly quantitative proteome changes associated with cypermethrin in Cx. pipiens pallens. Proteome analysis suggests that proteome modifications can be selected rapidly by cypermethrin, and multiple resistance mechanisms operate simultaneously in cypermethrin-resistance of Cx. pipiens pallens, Our results interpret that an up-regulated expression of proteins and enzymes like cytochrome P450, glutathione S-transferases, esterase etc. has an impact in insecticide resistance. Previously neglected penetration resistance (cuticular proteins) may play an important role in the adaptive response of Cx. pipiens pallens to insecticides. This information may serve as a basis for future work concerning the possible role of these proteins in cypermethrin resistance in mosquito Cx. pipiens pallens. © 2022 Society of Chemical Industry.

Multidisciplinary Investigations of Sustained Malaria Transmission in the Greater Mekong Subregion

In the course of malaria elimination in the Greater Mekong Subregion (GMS), malaria epidemiology has experienced drastic spatiotemporal changes with residual transmission concentrated along international borders and the rising predominance of Plasmodium vivax. The emergence of Plasmodium falciparum parasites resistant to artemisinin and partner drugs renders artemisinin-based combination therapies less effective while the potential spread of multidrug-resistant parasites elicits concern. Vector behavioral changes and insecticide resistance have reduced the effectiveness of core vector control measures. In recognition of these problems, the Southeast Asian International Center of Excellence for Malaria Research (ICEMR) has been conducting multidisciplinary research to determine how human migration, antimalarial drug resistance, vector behavior, and insecticide resistance sustain malaria transmission at international borders. These efforts allow us to comprehensively understand the ecology of border malaria transmission and develop population genomics tools to identify and track parasite introduction. In addition to employing in vivo, in vitro, and molecular approaches to monitor the emergence and spread of drug-resistant parasites, we also use genomic and genetic methods to reveal novel mechanisms of antimalarial drug resistance of parasites. We also use omics and population genetics approaches to study insecticide resistance in malaria vectors and identify changes in mosquito community structure, vectorial potential, and seasonal dynamics. Collectively, the scientific findings from the ICEMR research activities offer a systematic view of the factors sustaining residual malaria transmission and identify potential solutions to these problems to accelerate malaria elimination in the GMS.

Population genetic structure and evolutionary genetics of Anopheles sinensis based on knockdown resistance (kdr) mutations and mtDNA-COII gene in China-Laos, Thailand-Laos, and Cambodia-Laos borders

BACKGROUND

Vector control is still a pivotal method for preventing malaria, and its potency is weakened by the increasing resistance of vectors to chemical insecticides. As the most abundant and vital malaria vector in Southeast Asia, the chemical insecticide resistance status in Anopheles sinensis remains elusive in Laos, which makes it imperative to evaluate the true nature of chemical insecticide resistance-associated genetic mutations in An. sinensis in Laos.

METHODS

Adult An. sinensis were collected from three border regions in Laos. DNA was extracted from individual mosquitoes. PCR amplification and DNA sequencing of a fragment containing codon 1014 of the voltage-gated sodium channel (vgsc) gene were completed to study the kdr allele frequency distribution, kdr intron polymorphism, population genetic diversity, and the evolutionary status of the kdr codon. The mitochondrial cytochrome c oxidase subunit II gene (COII) was amplified and sequenced to examine population variations, genetic differentiation, spatial population structure, population expansion, and gene flow patterns.

RESULTS

Nine wild kdr haplotypes of the vgsc gene were detected in this study, and eight of them, namely 1014L1, 1014L2, 1014L4, 1014L7, 1014L9, 1014L10, 1014L11, and 1014L21, were discovered in the China-Laos border (northern Laos), while 1014L3 was only detected in the Thailand-Laos border (northwestern Laos) and Cambodia-Laos border (southern Laos). The newly identified haplotype, 1014L21, was uniquely distributed in the China-Laos border and was not identified in other countries. Based on sequence analysis of the mitochondrial COII genes, significant genetic differentiation and limited gene flow were detected between the China-Laos and Cambodia-Laos An. sinensis populations, which suggested that those two regions were genetically isolated. The distinct distribution of the kdr haplotype frequencies is probably the result of geographical isolation in mosquito populations.

CONCLUSIONS

Lack of kdr mutations in the vgsc gene was probably due to genetic isolation and the absence of intense selection pressure in the three border regions of Laos. This study reveals that pyrethroid-based chemical insecticides are still appropriate for battling An. sinensis in parts of Laos, and routine monitoring of chemical insecticide resistance should be continuously implemented and focused on more restricted areas as part of chemical insecticide resistance management.

Community structure and insecticide resistance of malaria vectors in northern-central Myanmar

BACKGROUND

Myanmar is one of the six countries in the Greater Mekong Subregion (GMS) of Southeast Asia. Malaria vectors comprise many Anopheles species, which vary in abundance and importance in malaria transmission among different geographical locations in the GMS. Information about the species composition, abundance, and insecticide resistance status of vectorial systems in Myanmar is scarce, hindering our efforts to effectively control malaria vectors in this region.

METHODS

During October and November 2019, larvae and adult females of Anopheles mosquitoes were collected in three sentinel villages of Banmauk township in northern Myanmar. Adult female mosquitoes collected by cow-baited tent collection (CBTC) and adults reared from field-collected larvae (RFCL) were used to determine mortality rates and knockdown resistance (kdr) against deltamethrin using the standard WHO susceptibility test. Molecular species identification was performed by multiplex PCR and ITS2 PCR, followed by DNA sequencing. The kdr mutation at position 1014 of the voltage-gated sodium channel gene was genotyped by DNA sequencing for all Anopheles species tested.

RESULTS

A total of 1596 Anopheles mosquitoes from seven morphologically identified species groups were bioassayed. Confirmed resistance to deltamethrin was detected in the populations of An. barbirostris (s.l.), An. hyrcanus (s.l.), and An. vagus, while possible resistance was detected in An. annularis (s.l.), An. minimus, and An. tessellatus. Anopheles kochi was found susceptible to deltamethrin. Compared to adults collected by CBTC, female adults from RFCL had significantly lower mortality rates in the four species complexes. A total of 1638 individuals from 22 Anopheles species were molecularly identified, with the four most common species being An. dissidens (20.5%) of the Barbirostris group, An. peditaeniatus (19.4%) of the Hyrcanus group, An. aconitus (13.4%) of the Funestus group, and An. nivipes (11.5%) of the Annularis group. The kdr mutation L1014F was only detected in the homozygous state in two An. subpictus (s.l.) specimens and in a heterozygous state in one An. culicifacies (s.l.) specimen.

CONCLUSIONS

This study provides updated information about malaria vector species composition and insecticide resistance status in northern Myanmar. The confirmed deltamethrin resistance in multiple species groups constitutes a significant threat to malaria vector control. The lack or low frequency of target-site resistance mutations suggests that other mechanisms are involved in resistance. Continual monitoring of the insecticide resistance of malaria vectors is required for effective vector control and insecticide resistance management.

Identification and phylogenetic analysis of voltage-gated sodium channel haplotypes in the malaria vector Anopheles sinensis using a high-throughput amplicon sequencing approach

Background

Anopheles sinensis is a dominant vector for malaria transmission in Asian countries. Voltage-gated sodium channel (VGSC) mutation-mediated knock-down resistance (kdr) has developed in many A. sinensis populations because of intensive and long-term use of pyrethroids. Our previous study showed that multiple mutations at position 1014 of the VGSC were heterogeneously distributed in A. sinensis populations across Sichuan, China.

Methods

To understand resistance genotypes at the haplotype level and reconstruct the phylogenetic relationship of VGSC haplotypes, a cost-effective next-generation sequencing (NGS)-based amplicon sequencing approach was established to clarify haplotypes containing codon 1014 of the VGSC gene from a total of 446 adults collected in 12 locations of Sichuan, China.

Results

Nineteen (19) haplotypes were identified, including 11 wild 1014L, 6 resistance 1014F, and 2 resistance 1014C haplotypes. We found that resistance haplotypes of A. sinensis VGSC were widely distributed at frequencies ranging from 3.67 to 92.61%. The frequencies of the 1014C haplotype in the southeast of Sichuan (Luzhou, Guangan, and Suining) were relatively higher than those in other sampling locations. Phylogenetic analyses support that kdr-type mutation at position 1014 is not singly originated and resistance 1014C haplotypes evolve from TTT-encoding 1014F.

Conclusions

A cost-effective next-generation sequencing (NGS)-based amplicon sequencing approach has been established in this study. The data revealed the patchy distribution of VGSC resistance haplotypes with overall high frequencies in Sichuan, China. Phylogenetic analyses support multiple origins and sequential evolution (1014L → 1014F → 1014C) for kdr-type mutations in A. sinensis.

Graphical abstract

Widespread multiple insecticide resistance in the major dengue vector Aedes albopictus in Hainan Province, China

BACKGROUND

Aedes albopictus is a highly invasive mosquito and has become a potential vector of dengue, chikungunya and Zika viruses. Insecticide-based mosquito interventions are the main tools for vector-borne disease control. However, mosquito resistance to insecticides is a major threat to effective prevention and control. Five Ae. albopictus populations across Hainan Province, China were investigated for susceptibility to multiple insecticide and resistance mechanisms.

RESULTS

Larval bioassays indicated that resistance to pyrethroids was common in all larval populations. Adult bioassays revealed all populations were either resistant or highly resistant to at least four of the six synthetic insecticides (deltamethrin, permethrin, cyfluthrin, propoxur, malathion, and DDT) tested. Pre-exposure of mosquitoes to the synergistic agent piperonyl butoxide (PBO) increased mosquito mortality by 2.4-43.3% in bioassays to DDT, malathion, and permethrin and rendered mosquito sensitive to deltamethrin, cyfluthrin, and propoxur. The frequency of knockdown resistance (kdr) mutations (F1534S and F1534C) ranged from 69.8% to 89.3% and from 38.1% to 87.0% in field-resistant and sensitive populations, respectively. F1534S mutation was significantly associated with pyrethroid resistance. No mutation was detected in the acetylcholinesterase (ace-1) gene in the two examined populations.

CONCLUSION

This study provides evidence of widespread resistance to multiple insecticides in Ae. albopictus in Hainan Province, China. Both kdr mutations and metabolic detoxification were potential causes of insecticide resistance for Ae. albopictus. Our findings highlight the need for insecticide resistance management and mosquito control measures that do not entirely depend on synthetic insecticides. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

From Global to Local-New Insights into Features of Pyrethroid Detoxification in Vector Mosquitoes

The threat of mosquito-borne diseases continues to be a problem for public health in subtropical and tropical regions of the world; in response, there has been increased use of adulticidal insecticides, such as pyrethroids, in human habitation areas over the last thirty years. As a result, the prevalence of pyrethroid-resistant genetic markers in natural mosquito populations has increased at an alarming rate. This review details recent advances in the understanding of specific mechanisms associated with pyrethroid resistance, with emphasis on features of insecticide detoxification and the interdependence of multiple cellular pathways. Together, these advances add important context to the understanding of the processes that are selected in resistant mosquitoes. Specifically, before pyrethroids bind to their targets on motoneurons, they must first permeate the outer cuticle and diffuse to inner tissues. Resistant mosquitoes have evolved detoxification mechanisms that rely on cytochrome P450s (CYP), esterases, carboxyesterases, and other oxidation/reduction (redox) components to effectively detoxify pyrethroids to nontoxic breakdown products that are then excreted. Enhanced resistance mechanisms have evolved to include alteration of gene copy number, transcriptional and post-transcriptional regulation of gene expression, as well as changes to cellular signaling mechanisms. Here, we outline the variety of ways in which detoxification has been selected in various mosquito populations, as well as key gene categories involved. Pathways associated with potential new genes of interest are proposed. Consideration of multiple cellular pathways could provide opportunities for development of new insecticides.

A survey of insecticide resistance-conferring mutations in multiple targets in Anopheles sinensis populations across Sichuan, China

Background

Sichuan province is located in the southwest of China, and was previously a malaria-endemic region. Although no indigenous malaria case has been reported since 2011, the number of imported cases is on the rise. Insecticide-based vector control has played a central role in the prevention of malaria epidemics. However, the efficacy of this strategy is gravely challenged by the development of insecticide resistance. Regular monitoring of insecticide resistance is essential to inform evidence-based vector control. Unfortunately, almost no information is currently available on the status of insecticide resistance and associated mechanisms in Anopheles sinensis, the dominant malaria vector in Sichuan. In this study, efforts were invested in detecting the presence and frequency of insecticide resistance-associated mutations in three genes that encode target proteins of several classes of commonly used insecticides.

Methods

A total of 446 adults of An. sinensis, collected from 12 locations across Sichuan province of China, were inspected for resistance-conferring mutations in three genes that respectively encode acetylcholinesterase (AChE), voltage-gated sodium channel (VGSC), and GABA receptor (RDL) by DNA Sanger sequencing.

Results

The G119S mutation in AChE was detected at high frequencies (0.40–0.73). The predominant ace-1 genotype was GGC/AGC (119GS) heterozygotes. Diverse variations at codon 1014 were found in VGSC, leading to three different amino acid substitutions (L1014F/C/S). The 1014F was the predominant resistance allele and was distributed in all 12 populations at varying frequencies from 0.03 to 0.86. The A296S mutation in RDL was frequently present in Sichuan, with 296SS accounting for more than 80% of individuals in six of the 12 populations. Notably, in samples collected from Chengdu (DJY) and Deyang (DYMZ), almost 30% of individuals were found to be resistant homozygotes for all three targets.

Conclusions

Resistance-related mutations in three target proteins of the four main classes of insecticides were prevalent in most populations. This survey reveals a worrisome situation of multiple resistance genotypes in Sichuan malaria vector. The data strengthen the need for regular monitoring of insecticide resistance and establishing a region-customized vector intervention strategy.

Graphical abstract

Biochemical Mechanism of Insecticide Resistance in Malaria Vector, Anopheles gambiae s.l in Nigeria

Background

Malaria is a parasitic vector-borne disease endemic in the tropical and subtropical countries of the world. The aim of this study was to investigate the current activities of the detoxification enzymes in resistant and susceptible Anopheles gambiae s.l. in northern Nigeria.

Methods

Anopheles larvae were collected from northeast and northwestern Nigeria between Aug and Nov 2018. Biochemical analyses was carried out on the mosquitoes exposed to various insecticides (deltamethrin, DDT, bendiocarb, malathion) to measure and compare the enzymatic activities of the major detoxification enzymes (P₄₅₀, GSTs, Esterase).

Results

High levels of resistance was observed; DDT 37%-53% (95%, CI: 29-61), bendiocarb 44%-55% (CI: 39-60) and deltamethrin 74%-82% (CI: 70-86). However, these mosquitoes were found to be susceptible to malathion 99%-100% (CI: 98-100). The P450 and GSTs enzymes were found to be elevated in the resistant mosquitoes exposed to deltamethrin (1.0240±0.1902); (1.3088±1.2478), DDT (1.7703±1.4528); (1.7462±0.9418) and bendiocarb (1.1814±0.0918); (1.4479±1.0083) compared to the Kisumu strain (0.764±0.4226); (0.6508±0.6542), (0.3875±0.3482); (0.4072±0.4916) and (0.6672±0.3949); (0.7126±0.7259) at P<0.05. Similarly, the resistant mosquitoes expressed increased activity to esterase (0.7606±1.1477), (0.3269±1.1957) and (2.8203±0.6488) compared to their susceptible counterpart (0.6841±0.7597), (0.7032±0.5380) and (0.6398±0.4159) at P<0.05. The enzyme ratio was found to be: P450 (1.341, 4.568 and 1.77); GSTs (2.011, 4.288 and 2.031); Esterases (1.111, 0.469 and 4.408). One way Anova and single sample t-test were also conducted to determine the effect of the enzymes on the resistant and susceptible strains.

Conclusion

High level of insecticide resistance was observed with significant elevation of detoxification enzymes activities in the resistant mosquitoes.

Phenotypic, genotypic and biochemical changes during pyrethroid resistance selection in Anopheles gambiae mosquitoes

The directional selection for insecticide resistance due to indiscriminate use of insecticides in public health and agricultural system favors an increase in the frequency of insecticide-resistant alleles in the natural populations. Similarly, removal of selection pressure generally leads to decay in resistance. Past investigations on the emergence of insecticide resistance in mosquitoes mostly relied on field survey of resistance in vector populations that typically had a complex history of exposure to various public health and agricultural pest control insecticides in nature, and thus the effect of specific insecticides on rate of resistance emergency or resistance decay rate is not known. This study examined the phenotypic, genotypic, and biochemical changes that had occurred during the process of selection for pyrethroid resistance in Anopheles gambiae, the most important malaria vector in Africa. In parallel, we also examined these changes in resistant populations when there is no selection pressure applied. Through repeated deltamethrin selection in adult mosquitoes from a field population collected in western Kenya for 12 generations, we obtained three independent and highly pyrethroid-resistant An. gambiae populations. Three susceptible populations from the same parental population were generated by removing selection pressure. These two lines of mosquito populations differed significantly in monooxygenase and beta-esterase activities, but not in Vgsc gene mutation frequency, suggesting metabolic detoxification mechanism plays a major role in generating moderate-intensity resistance or high-intensity resistance. Pre-exposure to the synergist piperonyl butoxide restored the susceptibility to insecticide among the highly resistant mosquitoes, confirming the role of monooxygenases in pyrethroid resistance. The rate of resistance decay to become fully susceptible from moderate-intensity resistance took 15 generations, supporting at least 2-years interval is needed when the rotational use of insecticides with different modes of action is considered for resistance management.

Anopheles metabolic proteins in malaria transmission, prevention and control: a review

The increasing resistance to currently available insecticides in the malaria vector, Anopheles mosquitoes, hampers their use as an effective vector control strategy for the prevention of malaria transmission. Therefore, there is need for new insecticides and/or alternative vector control strategies, the development of which relies on the identification of possible targets in Anopheles. Some known and promising targets for the prevention or control of malaria transmission exist among Anopheles metabolic proteins. This review aims to elucidate the current and potential contribution of Anopheles metabolic proteins to malaria transmission and control. Highlighted are the roles of metabolic proteins as insecticide targets, in blood digestion and immune response as well as their contribution to insecticide resistance and Plasmodium parasite development. Furthermore, strategies by which these metabolic proteins can be utilized for vector control are described. Inhibitors of Anopheles metabolic proteins that are designed based on target specificity can yield insecticides with no significant toxicity to non-target species. These metabolic modulators combined with each other or with synergists, sterilants, and transmission-blocking agents in a single product, can yield potent malaria intervention strategies. These combinations can provide multiple means of controlling the vector. Also, they can help to slow down the development of insecticide resistance. Moreover, some metabolic proteins can be modulated for mosquito population replacement or suppression strategies, which will significantly help to curb malaria transmission.

Genomic Variant Analyses in Pyrethroid Resistant and Susceptible Malaria Vector, Anopheles sinensis

Anopheles sinensis is a major malaria vector in Southeast Asia. Resistance to pyrethroid insecticides in this species has impeded malaria control in the region. Previous studies found that An. sinensis populations from Yunnan Province, China were highly resistant to deltamethrin and did not carry mutations in the voltage-gated sodium channel gene that cause knockdown resistance. In this study, we tested the hypothesis that other genomic variants are associated with the resistance phenotype. Using paired-end whole genome sequencing (DNA-seq), we generated 108 Gb of DNA sequence from deltamethrin -resistant and -susceptible mosquito pools with an average coverage of 83.3× depth. Using a stringent filtering method, we identified a total of 916,926 single nucleotide variants (SNVs), including 32,240 non-synonymous mutations. A total of 958 SNVs differed significantly in allele frequency between deltamethrin -resistant and -susceptible mosquitoes. Of these, 43 SNVs were present within 37 genes that code for immunity, detoxification, cuticular, and odorant proteins. A subset of 12 SNVs were randomly selected for genotyping of individual mosquitoes by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and showed consistent allele frequencies with the pooled DNA-seq derived allele frequencies. In addition, copy number variations (CNVs) were detected in 56 genes, including 33 that contained amplification alleles and 23 that contained deletion alleles in resistant mosquitoes compared to susceptible mosquitoes. The genomic variants described here provide a useful resource for future studies on the genetic mechanism of insecticide resistance in this important malaria vector species.

Preliminary Selection and Analysis of Deltamethrin-Resistant Strains of Aedes albopictus in the Laboratory

Background: Aedes albopictus is a major vector for transmission of many viral pathogens. Deltamethrin resistance was analyzed by catching A. albopictus in the field; the analysis effect is affected by many insecticides that often interact with A. albopictus in the field environment. Materials and Methods: This study examined the development of deltamethrin resistance in A. albopictus mosquitoes under controlled laboratory conditions, focusing on morphological changes, reproductive fitness, and mutation of the knockdown resistance (kdr) gene. Deltamethrin-resistant strains were selected up to the 20th generation. To determine the level of resistance, the lethal concentration 50 (LC₅₀) of deltamethrin in the larvae was obtained, followed by the resistance ratio (RR), and in adult mosquitoes, mortality rates were calculated using the contact tube method. Results: An increase in the LC₅₀ from 0.0070 to 0.0563 mg/L was observed in resistant versus sensitive strains, with an increase of 11.26 in the RR. Overall, the results of the larval resistance bioassay showed that resistant larvae had medium resistance; however, by the 20th generation, adult mosquitoes showed strong resistance. PCR amplification, cloning, and sequencing of sodium channel domain III gene fragments were subsequently carried out using selected resistant and sensitive female mosquitoes. As a result, a number of base mutations were observed in the kdr gene in the resistant strain; however, no amino acid sequence mutations were observed, suggesting that base sequence changes did not affect protein expression. Results of morphological changes between resistant and sensitive strains showed that significant differences in the body, foreleg, mid leg, and hind leg length, as well as wing length and width, antenna length, and proboscis length were observed between 18^th-generation resistant and sensitive strains of A. albopictus. On analysis of reproductive fitness associated with deltamethrin resistance in selection of mosquitoes, observation results showed differences between resistant and sensitive strains; the female/male ratio of mosquitoes decreased after pupa hatching, with more females and fewer males. Conclusions: The model of deltamethrin-resistant selection of A. albopictus was successfully established in the laboratory. The morphological phenotypes of the deltamethrin-resistant population of A. albopictus mosquitoes had changed. The kdr gene of the 19th and 20th generations of deltamethrin-resistant A. albopictus mosquitoes had silent mutations at several sites. After deltamethrin resistance selection, the female/male ratio of mosquitoes increased after pupa hatching, with more females and fewer males, hinting at increased chances of more female mosquitoes transmitting diseases.

Mitochondrial genes associated with pyrethroid resistance revealed by mitochondrial genome and transcriptome analyses in the malaria vector Anopheles sinensis (Diptera: Culicidae)

BACKGROUND

Insecticides are still the main method of mosquito control, but mosquito resistance presents a large obstacle. The function of mitochondrial genes in the evolution of insecticide resistance is still poorly understood. Pyrethroid is the most commonly used insecticide, and Anopheles sinensis is an important malaria vector in China and Southeast Asia. In this study, we investigated the mitochondrial genes associated with pyrethroid resistance through their genetic and expression variation based on analyses of transcriptomes and 36 individuals with resequencing in three geographical populations in China.

RESULTS

The nucleotide diversity (Pi) in 18 resistant individuals was much lower than that in 18 susceptible individuals, which suggests that some sites experienced purifying selection subject to pyrethroid stress. Ka/Ks and amino acid analyses showed that ND4 experienced positive selection and had 23 amino acid mutations due to pyrethroid stress. These mutations might change the ND4 structure and function and thus alter the efficiency of the respiratory chain. ND5 was significantly upregulated, and ATP8 was significantly downregulated in these three pyrethroid resistant populations, which suggests that these two genes function in the production and maintenance of pyrethroid resistance. There are differences in mitochondrial genes involved in pyrethroid resistance among these three populations.

CONCLUSION

This is the first study to reveal the association of mitochondrial genes in the evolution of insecticide resistance through amino acid mutation and expression patterns and can help us further understand insecticide resistance mechanisms. © 2019 Society of Chemical Industry.

Resistance to pyrethroid and organophosphate insecticides, and the geographical distribution and polymorphisms of target-site mutations in voltage-gated sodium channel and acetylcholinesterase 1 genes in Anopheles sinensis populations in Shanghai, China

Background

In the final phase of China’s national programme to eliminate malaria by 2020, it is vitally important to monitor the resistance of malaria vectors for developing effective vector control strategies. In 2017 Shanghai declared that it had eliminated malaria; however, the insecticide resistance status of the primary malaria vector Anopheles sinensis remains unknown.

Methods

We examined the pyrethroid and organophosphate resistance of An. sinensis via a bioassay of two populations from the Chongming District of Shanghai. The voltage-gated sodium channel (VGSC) and acetylcholinesterase 1 (ace-1) genes were partially sequenced to examine the association between resistance phenotype and target site genotype. In addition, the geographical distribution, polymorphism and genotype frequencies of insecticide resistance genes were examined using samples collected during routine mosquito surveillance in 2016 and 2017 from Chongming, Songjiang, Jiading and Qingpu Districts.

Results

In Chongming District, the An. sinensis population near Dongtan National Nature Reserve showed resistance to pyrethroids, sensitivity to organophosphate, no knockdown resistance (kdr) mutations in the VGSC gene, and a low frequency (1.71%) of the ace-1 119S allele. An An. sinensis population near the Chongming central area (CM-Xinhe population) showed high resistance to pyrethroids and organophosphates and high frequencies of kdr 1014F and 1014C (80.73%) and ace-1 119S (85.98%) alleles. A significant association was detected between the homozygous kdr mutation 1014F/1014F and pyrethroid resistance in the CM-Xinhe population, indicating that the kdr mutation is probably recessive. Eight kdr genotypes with 1014F and 1014C substitutions were detected in the four surveyed districts of Shanghai. TTT and GGC/AGC were the dominant kdr allele and ace-1 genotype, respectively, and were prevalent in most Shanghai An. sinensis populations.

Conclusions

On the basis of our assessment of insecticide resistance gene mutations in Shanghai, we identified a kdr mutation-free population in Chongming Dongtan. However, high frequencies of target-site mutations of insecticide resistance genes were observed in most areas of Shanghai. Bioassays of An. sinensis populations in the central Chongming District indicated the high insecticide resistance status of An. sinensis populations in Shanghai. We accordingly recommend a restriction on insecticide usage and development of effective integrated pest/vector management interventions to support disease control efforts.

Development and application of a tri-allelic PCR assay for screening Vgsc-L1014F kdr mutations associated with pyrethroid and organochlorine resistance in the mosquito Culex quinquefasciatus

BACKGROUND

Culex quinquefasciatus has a widespread distribution across tropical and sub-tropical regions, and plays an important role in the transmission of vector-borne diseases of public health importance, including lymphatic filariasis (LF) and multiple arboviruses. Increased resistance to insecticides threatens the efficacy and sustainability of insecticide-based anti-vector interventions which mitigate the burden of mosquito transmitted diseases in endemic regions. In C. quinquefasciatus two non-synonymous voltage gated sodium channel (Vgsc) variants, both resulting in a leucine to phenylalanine change at codon 1014, are associated with resistance to pyrethroids and DDT. This tri-allelic variation has compromised the ability to perform high-throughput single-assay screening. To facilitate the detection and monitoring of the Vgsc-1014 locus in field-caught mosquitoes, an Engineered-Tail Allele-Specific-PCR (ETAS-PCR) diagnostic assay was developed and applied to wild mosquitoes from Brazil, Tanzania and Uganda.

RESULTS

This new cost-effective, single-tube assay was compared to two, well-established, genotyping approaches, pyrosequencing and TaqMan. The ETAS-PCR assay showed high specificity for discriminating the three alleles at Vgsc-L1014F, with genotyping results strongly correlated with pyrosequencing and TaqMan results (98.64% and 100% respectively).

CONCLUSIONS

Our results support the utility of the ETAS-PCR/Vgsc-1014 diagnostic assay, which stands as an effective alternative for genotyping tri-allelic variants.

The polymorphism and geographical distribution of knockdown resistance of adult Anopheles sinensis populations in eastern China

Background

Anopheles sinensis is one of the major malaria vectors in China and other southeast Asian countries, including Vietnam, Cambodia, Thailand. Vector control is considered to be the critical measure for malaria control, while the increasing prevalence of insecticide resistance caused by long-term use of insecticides, especially pyrethroids, is threatening the successful control of An. sinensis. In order to understand the underlying resistance mechanisms involved and molecular basis, the principal malaria vector, An. sinensis from Jiangsu and Anhui provinces, Southeast China, was investigated.

Methods

The adult Anopheles mosquitoes were sampled from multiple sites across Jiangsu and Anhui provinces, and sufficient mosquitoes collected from eleven sites for insecticide susceptibility bioassays. The DIIS4–DIIS6 region of the para-type sodium channel gene was amplified and sequenced, then multiple PCR and Taqman assays were used to assess the frequencies of kdr mutations at the target gene.

Results

In the present study, most of the adult An. sinensis populations were pyrethroids resistant, which indicated the presence of kdr resistance mutations in the para-type sodium channel gene. Sequence analyses demonstrated the kdr mutation existed at codon 1014 in Jiangsu and Anhui provinces. In adult An. sinensis, three mutant types (TTT L1014F, TTC L1014F, and TGT L1014C) of kdr alleles were detected, while no wild type (TTG L1014) was observed. The TTC L1014F mutation was first reported in Anhui province.

Conclusions

The highly polymorphic kdr alleles were observed in all the adult An. sinensis populations, which suggested that in-depth studies are required for carrying on insecticide resistance monitoring and specific resistance mechanisms studying into establish effective long-term malaria vector control program in eastern China.

Insecticide Resistance Status and Mechanisms of Anopheles sinensis (Diptera: Culicidae) in Wenzhou, an Important Coastal Port City in China

Although scaled-up interventions and effective control efforts have drastically reduced malaria morbidity and mortality, malaria remains a serious threat to public health worldwide. Anopheles sinensis Wiedemann 1828 is a historically important vector of Plasmodium vivax (Haemosporida: Plasmodiidae) malaria in China. Insecticide resistance has become a major obstacle to vector-borne disease control. However, little is known about the insecticide resistance of An. sinensis in Wenzhou, an important coastal port city in Zhejiang province, China. The aim of this study was to examine insecticide resistance and mechanisms in An. sinensis field mosquito populations. Evidence of multiple insecticide resistance was found in An. sinensis adult female populations. Medium to high frequencies of target site kdr together with fixed ace-1 mutations was detected in both the Ruian and Yongjia populations. Both populations showed an association between kdr L1014 mutation and resistance phenotype when tested against deltamethrin and DDT. Significantly different metabolic enzyme activities were found between the susceptible laboratory strain and field-collected mosquitoes from both Ruian and Yongjia. Both field collected An. sinensis populations exhibited significantly higher P450 enzyme activity compared with the laboratory strain, while the field-collected resistant mosquitoes exhibited various GST and COE enzyme activities. These results indicate multiple resistance mechanisms in An. sinensis field populations. Effective implementation of insecticide resistance management strategies is urgently needed. The data collected in this study will be valuable for modeling insecticide resistance spread and vector-control interventions.

Modification of contact avoidance behaviour associated with pyrethroid resistance in Anopheles sinensis (Diptera: Culicidae)

Background

Anopheles sinensis is the primary vector of vivax malaria in China and its control is under great threat as the development of insecticide resistance. In contrast to physiological resistance, there is no report of behavioural modifications of resistant An. sinensis after long-term insecticide use, despite their huge potential impact on malaria transmission.

Methods

Larvae or pupae of An. sinensis were collected from Yuanyang, Bishan, and Wuhe counties from southwestern to eastern China. Resistance to deltamethrin was assayed using the standard World Health Organization (WHO) susceptibility test. The frequency distribution of the kdr allele of the para-type sodium channel gene was determined by polymerase chain reaction (PCR) amplification and DNA sequencing. Contact repellency to deltamethrin-impregnated bed nets was evaluated using a modified WHO cone bioassay.

Results

All contemporary field populations for all three geographic locations were resistant to deltamethrin, with mortality ranging from 6.00 to 26.79%. Three kdr genotypes with either an L1014F or L1014C substitution with frequencies of 76.10–100% were identified in the Bishan and Wuhe populations, but no kdr mutations were detected in the Yuanyang samples despite high phenotypic resistance. The susceptible mosquitoes exhibited significantly longer flying time and more takeoffs on deltamethrin-treated bed nets (DTN) than on untreated bed nets (UTN), suggestive of robust avoidance behaviour. However, no significant increases in the frequency of takeoffs or flying time were observed in deltamethrin-resistant An. sinensis populations when exposed on DTNs, regardless of the presence of a kdr mutation. Moreover, the first takeoff from DTNs by resistant mosquitoes significantly lagged behind compared to susceptible mosquitoes.

Conclusion

The An. sinensis populations were highly resistant to deltamethrin and exhibited decreased avoidance behaviour. Behavioural modification significantly associated with deltamethrin resistance, but not directly related to the presence of kdr mutations, indicating that there are additional factors contributing to the changes.

Trends in insecticide resistance in Culex pipiens pallens over 20 years in Shandong, China

Background

Culex pipiens pallens is the most abundant Culex mosquito species in northern China and is an important vector of bancroftian filariasis and, potentially, West Nile virus. Insecticides, particularly pyrethroids, are widely used for adult mosquito control. Insecticide resistance has become common in several mosquito species, and vector control is the main method currently available to prevent disease transmission. The voltage-gated sodium channel (Vgsc) gene is the target site of pyrethroids, and mutations in this gene cause knockdown resistance (kdr).

Methods

Culex pipiens pallens larvae were collected from May to November over two decades, from 1992 to 2018, in four cities in Shandong Province, China. The World Health Organization (WHO) standard resistance bioassay was applied to test the resistance levels of Cx. p. pallens larvae to five different insecticides and to test deltamethrin resistance in adults, using the F1 generation. Mutations at Vgsc codon 1014 were also screened in 471 adult samples collected in 2014 to determine the association between kdr mutations and phenotypic resistance.

Results

Larval resistance against deltamethrin showed an increasing trend from the 1990s until 2018, which was statistically significant in all populations; resistance to cypermethrin increased significantly in mosquitoes from the Zaozhuang population. However, larval resistance to other insecticides remained relatively stable. Larval resistance against deltamethrin was consistent with adult bioassays in 2014, in which all tested populations were highly resistant, with mortality rates ranging from 39.4 to 55.23%. The L1014S and L1014F mutations were both observed in five Cx. p. pallens populations, with L1014F significantly associated with deltamethrin resistance.

Conclusions

The long-term dataset from Shandong demonstrates major increases in pyrethroid resistance over a 20-year period. The L1014F kdr mutation may be considered a viable molecular marker for monitoring pyrethroid resistance in Cx. p. pallens.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3416-9) contains supplementary material, which is available to authorized users.

Target-site mutations (AChE-G119S and kdr) in Guangxi Anopheles sinensis populations along the China-Vietnam border

Background

In South Asia, the epidemiology of malaria is complex, and transmission mainly occurs in remote areas near international borders. Vector control has been implemented as a key strategy in malaria prevention for decades. A rising threat to the efficacy of vector control efforts is the development of insecticide resistance, thus it is important to monitor the type and frequency of insecticide resistant alleles in the disease vectors such as An. sinensis along the China-Vietnam border. Such information is needed to synthesize effective malaria vector control strategies.

Methods

A total of 208 adults of An. sinensis, collected from seven sites in southwest Guangxi along the China-Vietnam border, were inspected for the resistance-conferring G119S mutation in acetylcholinesterase (AChE) by PCR-RFLP (polymerase chain reaction restriction fragment length polymorphism) and kdr mutations in the voltage-gated sodium channel (VGSC) by sequencing. In addition, the evolutionary origin of An. sinensis vgsc gene haplotypes was analyzed using Network 5.0.

Results

The frequencies of mutant 119S of AChE were between 0.61–0.85 in the seven An. sinensis populations. No susceptible homozygote (119GG) was detected in three of the seven sites (DXEC, LZSK and FCGDX). Very low frequencies of kdr (0.00–0.01) were detected in the seven populations, with most individuals being susceptible homozygote (1014LL). The 1014F mutation was detected only in the southeast part (FCGDX) at a low frequency of 0.03. The 1014S mutation was distributed in six of the seven populations with frequencies ranging from 0.04 to 0.08, but absent in JXXW. Diverse haplotypes of 1014L and 1014S were found in An. sinensis along the China-Vietnam border, while only one 1014F haplotype was detected in this study. Consistent with a previous report, resistant 1014S haplotypes did not have a single origin.

Conclusions

The G119S mutation of AChE was present at high frequencies (0.61–0.85) in the An. sinensis populations along the China-Vietnam border, suggesting that the vector control authorities should be cautious when considering carbamates and organophosphates as chemicals for vector control. The low frequencies (0.00–0.11) of kdr in these populations suggest that pyrethroids remain suitable for use against An. sinensis in these regions.

Evaluation of Pyrethroid Susceptibility in Culex pipiens of Northern Izmir Province, Turkey

BACKGROUND

Mosquitoes, being a nuisance species, are considered as one of the most important species in public health control programs due to their role as a vector in mosquito-borne diseases observed in humans and animals. We evaluated the susceptibility status of Culex pipiens collected from northern Izmir, Turkey in 2011-16.

METHODS

Mosquito larvae, collected from three different locations in northern İzmir, were reared in the laboratory. Adult susceptibility bioassays were performed using the WHO insecticide-impregnated papers including deltamethrin 0.05%, permethrin 0.75%, α-cypermethrin 0.05% and cyfluthrin 0.15%. In addition, adult bioassays were performed after the pre-exposure to piperonyl butoxide (PBO) to determine the contribution of P450 detoxification enzymes to the phenotypic resistance.

RESULTS

In all of the three populations, high levels of resistance were observed (mortalities<63%) to all of the four pyrethroids. Different pyrethroids but with the same mode of action can exhibit significantly different phenotypic resistance in a single population. PBO bioassays also showed that P450 detoxification enzymes can have diverse effects on different pyrethroids.

CONCLUSION

Using just one chemical in a class of insecticide can be misleading for resistance studies.

Comparative transcriptome analysis and RNA interference reveal CYP6A8 and SNPs related to pyrethroid resistance in Aedes albopictus

Wide and improper application of pyrethroid insecticides for mosquito control has resulted in widespread resistance in Aedes albopictus mosquitoes, an important dengue vector. Therefore, understanding the molecular regulation of insecticide resistance is urgently needed to provide a basis for developing novel resistance diagnostic methods and vector control approaches. We investigated the transcriptional profiles of deltamethrin-resistant and -susceptible Ae. albopictus by performing paired-end sequencing for RNA expression analysis. The analysis used 24 independent libraries constructed from 12 wild-caught resistant and 12 susceptible Ae. albopictus female adults. A total of 674,503,592 and 612,512,034 reads were obtained, mapped to the Ae. albopictus genome and assembled into 20,091 Ae. albopictus transcripts. A total of 1,130 significantly differentially expressed genes included 874 up-regulated genes and 256 down-regulated genes in the deltamethrin-resistant individuals. These differentially expressed genes code for cytochrome P450s, cuticle proteins, glutathione S-transferase, serine proteases, heat shock proteins, esterase, and others. We selected three highly differentially expressed candidate genes, CYP6A8 and two genes of unknown function (CCG013931 and CCG000656), to test the association between these 3 genes and deltamethrin resistance using RNAi through microinjection in adult mosquitoes and oral feeding in larval mosquitoes. We found that expression knockdown of these three genes caused significant changes in resistance. Further, we detected 1,162 single nucleotide polymorphisms (SNPs) with a frequency difference of more than 50%. Among them, 5 SNPs in 4 cytochrome P450 gene families were found to be significantly associated with resistance in a genotype-phenotype association study using independent field-collected mosquitoes of known resistance phenotypes. Altogether, a combination of novel individually based transcriptome profiling, RNAi, and genetic association study identified both differentially expressed genes and SNPs associated with pyrethroid resistance in Ae. albopictus mosquitoes, and laid a useful foundation for further studies on insecticide resistance mechanisms.

Correlation between adult pyrethroid resistance and knockdown resistance (kdr) mutations in Aedes albopictus (Diptera: Culicidae) field populations in China

BACKGROUND

Arboviral disease transmitted by Aedes albopictus such as dengue fever is an important threat to human health. Pyrethroid resistance raises a great challenge for mosquito control. A systematic assessment of Ae. albopictus resistance status in China is urgently needed, and the study of correlation between pyrethroid resistance and knockdown resistance (kdr) mutations would provide information to guide the control of the Ae. albopictus vector.

METHODS

Five field populations of Ae. albopictus were collected from Jinan (JN), Hangzhou (HZ), Baoshan (BS), Yangpu (YP) and Haikou (HK), China in 2017. Insecticide-impregnated papers were prepared with four pyrethroid chemicals, deltamethrin, permethrin, beta-cypermethrin and lambda-cyhalothrin. The susceptibility of Ae. albopictus to pyrethroids was tested by the WHO tube assay. Kdr mutations were identified by PCR and sequencing. Moreover, the correlation analysis between kdr alleles and pyrethroid resistance was performed.

RESULTS

All five populations of Ae. albopictus showed resistance to four pyrethroid insecticides. One kdr mutant allele at codon 1532 and three at 1534 were detected with frequency of 5.33% (I1532T), 44.20% (F1534S), 1.83% (F1534 L) and 0.87% (F1534C), respectively. Both 1532 and 1534 mutation mosquitoes were found in the BS and YP populations. Allele I1532T was negatively correlated with deltamethrin resistance phenotype (OR < 1), while F1534S mutation was positively correlated with deltamethrin and permethrin resistance (OR > 1).

CONCLUSIONS

The five field populations of Ae. albopictus adults were all resistant to deltamethrin, permethrin, beta-cypermethrin and lambda-cyhalothrin. Mutant F1534S was clearly associated with pyrethroid resistance phenotype in Ae. albopictus and this could be developed as a molecular marker to monitor the pyrethroid resistance problem in China.

The Effect of Permethrin Resistance on Aedes aegypti Transcriptome Following Ingestion of Zika Virus Infected Blood

Aedes aegypti (L.) is the primary vector of many emerging arboviruses. Insecticide resistance among mosquito populations is a consequence of the application of insecticides for mosquito control. We used RNA-sequencing to compare transcriptomes between permethrin resistant and susceptible strains of Florida Ae. aegypti in response to Zika virus infection. A total of 2459 transcripts were expressed at significantly different levels between resistant and susceptible Ae. aegypti. Gene ontology analysis placed these genes into seven categories of biological processes. The 863 transcripts were expressed at significantly different levels between the two mosquito strains (up/down regulated) more than 2-fold. Quantitative real-time PCR analysis was used to validate the Zika-infection response. Our results suggested a highly overexpressed P450, with AAEL014617 and AAEL006798 as potential candidates for the molecular mechanism of permethrin resistance in Ae. aegypti. Our findings indicated that most detoxification enzymes and immune system enzymes altered their gene expression between the two strains of Ae. aegypti in response to Zika virus infection. Understanding the interactions of arboviruses with resistant mosquito vectors at the molecular level allows for the possible development of new approaches in mitigating arbovirus transmission. This information sheds light on Zika-induced changes in insecticide resistant Ae. aegypti with implications for mosquito control strategies.

Evidence for multiple-insecticide resistance in urban Aedes albopictus populations in southern China

BACKGROUND

Aedes albopictus (Skuse) is an invasive mosquito that has become an important vector of chikungunya, dengue and Zika viruses. In the absence of specific antiviral therapy or a vaccine, vector management is the sole method available for reducing Aedes-induced disease morbidity. Determining the resistance status of Ae. albopictus to insecticides and exploring the resistance mechanisms is essential for future vector control planning.

METHODS

Aedes albopictus larvae and pupae were sampled from six sites (two sites each from urban, suburban and rural) in Guangzhou. The resistance bioassays were conducted against Bacillus thuringiensis israelensis (Bti): deltamethrin, propoxur and malathion for larvae; and deltamethrin, DDT, propoxur and malathion for adults. P450 monooxygenase (P450s), glutathione S-transferase (GSTs) and carboxylesterase (COEs) activities of adult mosquitoes were measured. Mutations at the knockdown resistance (kdr) gene were analyzed, and the association between kdr mutations and phenotypic resistance was tested.

RESULTS

Adult bioassays revealed varied susceptibility against DDT, deltamethrin and propoxur in the six Ae. albopictus populations. Significantly lower mortality rates were found in urban populations than suburban and rural populations. Urban mosquito populations showed resistance against DDT, deltamethrin and propoxur, while one rural population was resistant to DDT. All populations tested were susceptible to malathion. Larval bioassays results indicated that all populations of Ae. albopictus were sensitive to the larvicide Bti and malathion. Resistance to deltamethrin and propoxur was common in larval populations. The F1534S and F1534 L mutations were found to be significantly associated with deltamethrin resistance. Biochemical assays indicated elevated detoxification enzyme activities in the field mosquito populations.

CONCLUSIONS

Aedes albopictus populations in Guangzhou, especially in urban areas, have developed resistance to the commonly used insecticides, primarily DDT and deltamethrin. This finding calls for resistance management and developing counter measures to mitigate the spread of resistance.

Toxicity and Metabolism of Zeta-Cypermethrin in Field-Collected and Laboratory Strains of the Neotropical Predator Chrysoperla externa Hagen (Neuroptera: Chrysopidae)

Resistance to pesticides has been studied in several insect pests, but information on the natural enemies of pests-including the Neotropical predator Chrysoperla externa Hagen (Neuroptera: Chrysopidae), a major biological control agent in South America-is lacking. We report here a comparative study between a field-collected strain of C. externa subjected to monthly sprayings of pyrethroids and neonicotinoids and a laboratory strain without exposure to pesticides. The tolerance of both strains against zeta-cypermethrin was similar, and addition of the synergist piperonyl butoxide increased the toxicity by 30% in both strains. Gas-chromatography analyses and mixed-function-oxidase measurements indicated similar values in both strains and also confirmed the key role of oxidative metabolism in this species. Because C. externa has maintained a tolerance to zeta-cypermethrin without previous pesticide exposure, this species could potentially be mass-reared and released in fields in the presence of pesticide pressure.

Genetic diversity and population structure of the primary malaria vector Anopheles sinensis (Diptera: Culicidae) in China inferred by cox1 gene

Background

Anopheles sinensis is a primary vector for Plasmodium vivax malaria in most regions of China. A comprehensive understanding of genetic variation and structure of the mosquito would be of benefit to the vector control and in a further attempt to contribute to malaria elimination in China. However, there is only inadequate population genetic data pertaining to An. sinensis currently.

Methods

Genetic variations and structure among populations of An. sinensis was examined and analyzed based on the nucleotide sequences of a 662 nt variable region of the mitochondrial cox1 gene among 15 populations from 20 collection sites in China.

Results

A total of 453 individuals in 15 populations were analyzed. The cox1 gene sequences were aligned, and 247 haplotypes were detected, 41 of these shared between populations. The range of haplotype diversity was from 0.709 (Yunnan) to 0.998 (Anhui). The genealogic network showed that the haplotypes were divided into two clusters, cluster I was at a high level of homoplasy, while cluster II included almost all individuals from the Yunnan population. The Yunnan population displayed a significantly high level of genetic differentiation (0.452−0.622) and a restricted gene flow with other populations. The pairwise F_ST values among other populations were lower. The AMOVA result showed that the percentage of variation within populations (83.83%) was higher than that among populations (16.17%). Mantel test suggested that geographical distance did not significantly contribute to the genetic differentiation (R² = 0.0125, P = 0.59). Neutral test and mismatch analysis results showed that the An. sinensis population has undergone demographic expansions.

Conclusions

Anopheles sinensis populations showed high genetic polymorphism by cox1 gene. The weak genetic structure may be a consequence of low genetic differentiation and high gene flow among populations, except the Yunnan samples. The Yunnan population was isolated from the other populations, gene flow limited by geographical distance and barriers. These findings will provide a theoretical basis for vector surveillance and vector control in China.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2013-z) contains supplementary material, which is available to authorized users.

A neural network prediction of environmental determinants of Anopheles sinensis knockdown resistance mutation to pyrethroids in China

Selection pressure caused by long-term intensive use of insecticides is the key driving force in resistance development. Additional parameters such as environmental conditions may affect both the mosquito response to insecticides and the selection of resistance mechanisms. In this context, we analyzed the environmental determinants of kdr prevalence in Anopheles sinensis across China. We collected kdr frequency from 48 sites across central and southern China, together with key environmental factors including long-term climatic data, topographic features, main crops, and land cover types. Trend surface analysis found that the distribution of kdr frequency can be partitioned into three regions, namely central China (kdr frequency >80%), western China (kdr frequency varies from 0% to 60%), and southern China (kdr frequency <10%). Seven predictor variables were selected based on a radial basis function neural network model. A multilayer perceptron (MLP) network model revealed that the number of crops in a year was the most important predictor for the kdr mutation rate. Topography, long-term mean climate and land cover all contributed to the kdr mutation rate. The observed mean kdr frequency was 53.0% and the MLP network model-predicted mean was 52.6%, a 0.1% relative error. Predicted kdr frequencies closely matched the observed values. The model explained 92% of the total variance in kdr frequency. The results indicated that kdr was associated with the intensity of pesticide usage. Crop cultivation information, together with environmental factors, may well predict the spatial heterogeneity of kdr mutations in An. sinensis in China.

Diversity and frequency of kdr mutations within Anopheles sinensis populations from Guangxi, China

BACKGROUND

Anopheles sinensis is a major vector of malaria in China and its control is under great threat as the development of insecticide resistance. Voltage-gated sodium channel (VGSC) is the target of several classes of insecticides. Genetic mutations of VGSC have been documented to confer knockdown resistance (kdr) to dichlorodiphenyltrichloroethane (DDT) and pyrethroids in mosquitoes. To control this vector efficiently, it is important to know the resistance-associated genetic mutations, their distribution frequencies and genealogical relations.

METHODS

Three hundreds and thirteen (313) adults of An. sinensis collected from nine locations across Guangxi Zhuang Autonomous Region were used. The partial sequence of the An. sinensis voltage gated sodium channel gene (AS-VGSC) containing codon 1014 was sequenced. PHASE2.1 was used to construct the haplotypes of each individual, and the accuracy of haplotypes was further confirmed by clone sequencing. The genealogical relations of kdr mutations in AS-VGSC was analysed using TCS 2.1 and Network 5.0.

RESULTS

Sixteen AS-VGSC haplotypes including seven haplotypes carrying non-synonymous mutations at codon 1014, and fifty-five AS-VGSC genotypes were identified from 313 mosquitoes collected from nine geographical locations across Guangxi. The number of haplotypes in each of the nine populations ranged from 5 to 13. The frequency of haplotypes carrying kdr mutations ranged from 2.7 to 80.0 % within the nine populations, of which 1014C was unexpectedly high in the northeast of Guangxi. Genealogical analysis suggested multiple origins of kdr mutations in An. sinensis.

CONCLUSION

Diverse haplotypes of AS-VGSC are distributed in Guangxi. The presence of haplotypes carrying mutations at codon 1014 indicates a risk of pyrethroid and DDT resistance. The kdr mutations show differential distribution geographically, with high frequencies occurred in the northeast of Guangxi. Genealogical analysis suggests multiple origins of kdr mutations in An. sinensis populations in Guangxi. These findings have important practical implications for the sustainability of An. sinensis control programmes.

Genome-wide identification and characterization of odorant-binding protein (OBP) genes in the malaria vector Anopheles sinensis (Diptera: Culicidae)

Anopheles sinensis is a major malaria vector. Insect odorant-binding proteins (OBPs) may function in the reception of odorants in the olfactory system. The classification and characterization of the An. sinensis OBP genes have not been systematically studied. In this study, 64 putative OBP genes were identified at the whole-genome level of An. sinensis based on the comparison between OBP conserved motifs, PBP_GOBP, and phylogenetic analysis with An. gambiae OBPs. The characterization of An. sinensis OBPs, including the motif's conservation, gene structure, genomic organization and classification, were investigated. A new gene, AsOBP73, belonging to the Plus-C subfamily, was identified with the support of transcript and conservative motifs. These An. sinensis OBP genes were classified into three subfamilies with 37, 15 and 12 genes in the subfamily Classic, Atypical and Plus-C, respectively. The genomic organization of An. sinensis OBPs suggests a clustered distribution across nine different scaffolds. Eight genes (OBP23-28, OBP63-64) might originate from a single gene through a series of historic duplication events at least before divergence of Anopheles, Culex and Aedes. The microsynteny analyses indicate a very high synteny between An. sinensis and An. gambiae OBPs. OBP70 and OBP71 earlier classified under Plus-C in An. gambiae are recognized as belonging to the group Obp59a of the Classic subfamily, and OBP69 earlier classified under Plus-C has been moved to the Atypical subfamily in this study. The study established a basic information frame for further study of the OBP genes in insects as well as in An. sinensis.

Multi-country Survey Revealed Prevalent and Novel F1534S Mutation in Voltage-Gated Sodium Channel (VGSC) Gene in Aedes albopictus

Background

Aedes albopictus is an important dengue vector because of its aggressive biting behavior and rapid spread out of its native home range in Southeast Asia. Pyrethroids are widely used for adult mosquito control, and resistance to pyrethroids should be carefully monitored because vector control is the only effective method currently available to prevent dengue transmission. The voltage-gated sodium channel gene is the target site of pyrethroids, and mutations in this gene cause knockdown resistance (kdr). Previous studies reported various mutations in the voltage-gated sodium channel (VGSC) gene, but the spatial distribution of kdr mutations in Ae. albopictus has not been systematically examined, and the association between kdr mutation and phenotypic resistance has not been established.

Methods

A total of 597 Ae. albopictus individuals from 12 populations across Asia, Africa, America and Europe were examined for mutations in the voltage-gated sodium channel gene. Three domains for a total of 1,107 bp were sequenced for every individual. Two populations from southern China were examined for pyrethroid resistance using the World Health Organization standard tube bioassay, and the association between kdr mutations and phenotypic resistance was tested.

Results

A total of 29 synonymous mutations were found across domain II, III and IV of the VGSC gene. Non-synonymous mutations in two codons of the VGSC gene were detected in 5 populations from 4 countries. A novel mutation at 1532 codon (I1532T) was found in Rome, Italy with a frequency of 19.7%. The second novel mutation at codon 1534 (F1534S) was detected in southern China and Florida, USA with a frequency ranging from 9.5–22.6%. The WHO insecticide susceptibility bioassay found 90.1% and 96.1% mortality in the two populations from southern China, suggesting resistance and probable resistance. Positive association between kdr mutations with deltamethrin resistance was established in these two populations.

Conclusions

Two novel kdr mutations, I1532T and F1534S were found in Ae. albopictus. This is the first report of I1532T mutations in Italy and F1534S mutation in China and US. Significant association between kdr mutation and protection from deltamethrin raised the possibility that kdr mutation may be a viable biomarker for pyrethroid resistance surveillance in Ae. albopictus. The patchy distribution of kdr mutations in Ae. albopictus mosquitoes calls for developing global surveillance plan for pyrethroid resistance and developing countermeasures to mitigate the spread of resistance.

Aedes albopictus is a major dengue and Chikungunya vector and highly invasive. In the absence of effective treatment for the arbovirus infections, vector control is the only viable option. Pyrethroids are the most widely used insecticide for vector control programs due to low mammalian toxicity and rapid knockdown action. Extensive and prolonged use of pyrethroids imposes selection pressure on mosquito populations and eventually increases the potential of resistance. Monitoring pyrethroid resistance is essential to effective management of resistance. The voltage-gated sodium channel gene is the target site of pyrethroids, and mutations in this gene result in knockdown resistance (kdr). Previous studies reported various mutations in the VGSC gene, but the spatial distribution of kdr mutations in Aedes albopictus has not been systematically examined, and the association between kdr mutation and resistance has not been established. In the present study, we examined kdr mutation distribution in 12 populations from Asia, Africa, America and Europe. We found two novel and abundant kdr mutations, and established significant positive association between kdr mutations with deltamethrin resistance. This finding raised the possibility that kdr mutation may be a viable biomarker for pyrethroid resistance surveillance in Ae. albopictus. The patchy distribution of kdr mutations in Ae. albopictus mosquitoes calls for developing global surveillance plan for pyrethroid resistance and developing countermeasures to mitigate the spread of resistance.

Seasonal dynamics of insecticide resistance, multiple resistance, and morphometric variation in field populations of Culex pipiens

Resistance to insecticides that impairs nervous transmission has been widely investigated in mosquito populations as insecticides are crucial to effective insect control. The development of insecticide resistance is also of special interest to evolutionary biologists since it represents the opportunity to observe the genetic consequences of a well-characterized alteration in the environment. Although the frequencies of resistance alleles in Culex pipiens populations against different groups of insecticides have been reported, no detailed information is available on the relative change in these allele frequencies over time. In this study, we collected mosquitoes of the Cx. pipiens complex from six locations in three seasons in the Aegean region of Turkey and examined the i) seasonal variations in resistance to four different chemical classes of insecticides, ii) seasonal fluctuations in frequencies of resistance-associated target-site mutations of the three genes (ace-1, kdr, and Rdl), and iii) potential seasonal variations in wing morphometric characters that may be modified in resistant mosquitoes. Our bioassay results indicated the presence of different levels of resistance to all tested insecticides for all three seasons in all locations. The results of the PCR-based molecular analysis revealed low frequencies of mutations in ace-1 and Rdl that are associated with resistance to malathion, bendiocarb, and dieldrin and no obvious seasonal changes. In contrast, we detected high frequencies and striking seasonal changes for two kdr mutations associated with resistance to DDT and pyrethroids. In addition, the evaluation of the field populations from all seasons in terms of the combinations of polymorphisms at four resistance-associated mutations did not reveal the presence of insects that are resistant to all pesticides. Results from the morphological analysis displayed a similar pattern for both wings and did not show a clear separation among the samples from the three different seasons. The results of this study have advanced our knowledge of the potential dynamics of insecticide resistance among populations of the Cx. pipiens complex. The implications of these results to the understanding of the evolution of insecticide resistance and the management of resistance in mosquitoes are discussed.

Landscape genetic structure and evolutionary genetics of insecticide resistance gene mutations in Anopheles sinensis

Background

Anopheles sinensis is one of the most abundant vectors of malaria and other diseases in Asia. Vector control through the use of insecticides is the front line control method of vector-borne diseases. Pyrethroids are the most commonly used insecticides due to their low toxicity to vertebrates and low repellency. However, the extensive use of insecticides has imposed strong selection pressure on mosquito populations for resistance. High levels of resistance to pyrethroid insecticides and various mutations and haplotypes in the para sodium channel gene that confers knockdown resistance (kdr) have been detected in An. sinensis. Despite the importance of kdr mutations in pyrethroid resistance, the evolutionary origin of the kdr mutations is unknown. This study aims to examine the evolutionary genetics of kdr mutations in relation to spatial population genetic structure of An. sinensis.

Methods

Adults or larvae of Anopheles sinensis were collected from various geographic locations in China. DNA was extracted from individual mosquitoes. PCR amplification and DNA sequencing of the para-type sodium channel gene were conducted to analyse kdr allele frequency distribution, kdr codon upstream and downstream intron polymorphism, population genetic diversity and kdr codon evolution. The mitochondrial cytochrome c oxidase COI and COII genes were amplified and sequenced to examine population variations, genetic differentiation, spatial population structure, population expansion and gene flow patterns.

Results

Three non-synonymous mutations (L1014F, L1014C, and L1014S) were detected at the kdr codon L1014 of para-type sodium channel gene. A patchy distribution of kdr mutation allele frequencies from southern to central China was found. Near fixation of kdr mutation was detected in populations from central China, but no kdr mutations were found in populations from southwestern China. More than eight independent mutation events were detected in the three kdr alleles, and at least one of them evolved multiple times subsequent to their first divergence. Based on sequence analysis of the mitochondrial COI and COII genes, significant and large genetic differentiation was detected between populations from southwestern China and central China. The patchy distribution of kdr mutation frequencies is likely a consequence of geographic isolation in the mosquito populations and the long-term insecticide selection.

Conclusion

Our results indicate multiple origins of the kdr insecticide-resistant alleles in An. sinensis from southern and central China. Local selection related to intense and prolonged use of insecticide for agricultural purposes, as well as frequent migrations among populations are likely the explanations for the patchy distribution of kdr mutations in China. On the contrary, the lack of kdr mutations in Yunnan and Sichuan is likely a consequence of genetic isolation and absence of strong selection pressure. The present study compares the genetic patterns revealed by a functional gene with a neutral marker and demonstrates the combined impact of demographic and selection factors on population structure.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1513-6) contains supplementary material, which is available to authorized users.

RNA-seq analyses of changes in the Anopheles gambiae transcriptome associated with resistance to pyrethroids in Kenya: identification of candidate-resistance genes and candidate-resistance SNPs

Background

The extensive use of pyrethroids for control of malaria vectors, driven by their cost, efficacy and safety, has led to widespread resistance. To favor their sustainable use, the World Health Organization (WHO) formulated an insecticide resistance management plan, which includes the identification of the mechanisms of resistance and resistance surveillance. Recognized physiological mechanisms of resistance include target site mutations in the para voltage-gated sodium channel, metabolic detoxification and penetration resistance. Such understanding of resistance mechanisms has allowed the development of resistance monitoring tools, including genotyping of the kdr mutation L1014F/S in the para gene.

Methods

The sequence-based technique RNA-seq was applied to study changes in the transcriptome of deltamethrin-resistant and -susceptible Anopheles gambiae mosquitoes from the Western Province of Kenya. The resulting gene expression profiles were compared to data in the most recent literature to derive a list of candidate resistance genes. RNA-seq data were analyzed also to identify sequence polymorphisms linked to resistance.

Results

A total of five candidate-resistance genes (AGAP04177, AGAP004572, AGAP008840, AGAP007530 and AGAP013036) were identified with altered expression between resistant and susceptible mosquitoes from West and East Africa. A change from G to C at position 36043997 of chromosome 3R resulting in A101G of the sulfotransferase gene AGAP009551 was significantly associated with the resistance phenotype (odds ratio: 5.10). The kdr L1014S mutation was detected at similar frequencies in both phenotypically resistant and susceptible mosquitoes, suggesting it is no longer fully predictive of the resistant phenotype.

Conclusions

Overall, these results support the conclusion that resistance to pyrethroids is a complex and evolving phenotype, dependent on multiple gene functions including, but not limited to, metabolic detoxification. Functional convergence among metabolic detoxification genes may exist, with the role of each gene being modulated by the life history and selection pressure on mosquito populations. As a consequence, biochemical assays that quantify overall enzyme activity may be a more suitable method for predicting metabolic resistance than gene-based assays.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1083-z) contains supplementary material, which is available to authorized users.

Development of Resistance to Pyrethroid in Culex pipiens pallens Population under Different Insecticide Selection Pressures

Current vector control programs are largely dependent on pyrethroids, which are the most commonly used and only insecticides recommended by the World Health Organization for insecticide-treated nets (ITNs). However, the rapid spread of pyrethroid resistance worldwide compromises the effectiveness of control programs and threatens public health. Since few new insecticide classes for vector control are anticipated, limiting the development of resistance is crucial for prolonging efficacy of pyrethroids. In this study, we exposed a field-collected population of Culex pipiens pallens to different insecticide selection intensities to dynamically monitor the development of resistance. Moreover, we detected kdr mutations and three detoxification enzyme activities in order to explore the evolutionary mechanism of pyrethroid resistance. Our results revealed that the level of pyrethroid resistance was proportional to the insecticide selection pressure. The kdr and metabolic resistance both contributed to pyrethroid resistance in the Cx. pipiens pallens populations, but they had different roles under different selection pressures. We have provided important evidence for better understanding of the development and mechanisms of pyrethroid resistance which may guide future insecticide use and vector management in order to avoid or delay resistance.

Knockdown resistance of Anopheles sinensis in Henan province, China

Background

Vivax malaria was historically epidemic in Henan Province of China and Anopheles sinensis was the main vectors and poor farming communities bare the greatest burden of disease. Knockdown resistance in An. sinensis is one of the mechanisms of resistance against pyrethroids. In the present study, the frequency of mutations from An. sinensis was examined in Henan province, China.

Methods

Anopheles was collected from Kaifeng, Tongbai, Tanghe, Pingqiao, Shihe, and Yongcheng counties of Henan province in 2013. Molecular identification of Anopheles species was conducted by polymerase chain reaction (PCR) amplifying the internal transcribed spacer 2 (ITS2). Part of the IIS6 domain of the para-type sodium channel protein gene was polymerase chain reaction-amplified and directly sequenced. Frequency and geographic difference of kdr gene mutant types were analysed.

Results

208 Anopheles were received molecular identification, of which 169 (81.25%) were An. sinensis, 25 (12.02%) were Anopheles yatsushiroensis, and 12 (5.77%) were Anopheles lesteri. A 325 bp fragment of the para-type sodium channel gene including position 1014 was successfully sequenced from 139 Anopheles, of which 125 (89.93%) were An. sinensis, 12 (8.63%) were An. yatsushiroensis, 2 (1.44%) were An. lesteri. The molecular analyses revealed that mutations existed at codon 1014 in An. sinensis but not in An. yatsushiroensis and An. lesteri. Frequency of kdr mutation was 73.60% (92/125) from population of An. sinensis in Henan province, of which L1014F (TTT + TTC) allele frequencies accounted for 46.40% (58/125), and was higher than that of L1014C(TGT) which accounted for 27.20% (34/125) ( χ2 = 55.423, P < 0.001). The frequency of kdr mutation in Kaifeng county was 100% (49/49), and was higher than that of 37.93% (11/29) in Tongbai, 54.55% (6/11) in Pingqiao, 50.00% (3/3) in Shihe, and 62.50% (10/16) in Yongcheng county, respectively (χ2 = 39.538, P < 0.001; χ2 = 24.298, P < 0.001; χ2 = 25.913, P < 0.001; χ2 = 20.244, P < 0.001). While 92.86% (13/14) frequency of kdr mutation was found in Tanghe county, which was higher than that in Tongbai county (χ2 = 11.550, P = 0.0018).

Conclusions

A high frequency of kdr gene mutations from population of An. sinensis in Henan province was found.

Historical survey of the kdr mutations in the populations of Anopheles sinensis in China in 1996-2014

Background

Anopheles sinensis has become an important malaria vector in China. The long-term extensive utilization of pyrethroids for ITNs and IRS for mosquito control in the last three decades has resulted in the occurrence of resistant An. sinensis populations in many regions. Knockdown resistance (kdr), caused by point mutations in the VGSC gene, is one of the mechanisms that confer resistance to DDT and pyrethroids. Recently, several investigations revealed the kdr occurrence in some An. sinensis populations, however, no kdr data were available earlier than 2009. A survey tracking the dynamics of the kdr mutations in past decades would provide invaluable information to understand how the kdr alleles spread in mosquito populations temporally and spatially.

Methods

A survey was conducted on the kdr alleles at condon 1014 of the VGSC gene and their distributions in 733 specimens of An. sinensis and 232 specimens of the other eight member species of the Anopheles hyrcanus group that were collected from 17 provinces in China in 1996–2014.

Results

A total of three kdr alleles, TTT (F), TTG (F) and TGT (C) were detected, and TGT (C) and TTT (F) were already present in the specimens from Jiangsu and Shandong as early as 1997. The TTT (F) was the most frequent mutant allele, and largely distributed in central China, namely Shandong, Jiangsu, Anhui, Henan, Shanghai, Jiangxi and Hubei. When data were analysed in three time intervals, 1996–2001, 2005–2009, 2010–2014, the prevalence of kdr alleles increased progressively over time in the populations in central China. In contrast, the kdr alleles were less frequent in the samples from other regions, especially in Yunnan and Hainan, despite the documented presence of pyrethroid resistant populations in those regions. Interestingly, no mutant alleles were detected in all 232 specimens of eight other species in the An. hyrcanus group.

Conclusion

The survey revealed that the kdr occurrence and accumulation in the An. sinensis populations were more frequent in central China than in the other regions, suggesting that the kdr mutations may contribute significantly to the pyrethroid resistance in the mosquitoes in central China.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0644-0) contains supplementary material, which is available to authorized users.

Development of insecticide resistance in malaria vector Anopheles sinensis populations from Shandong province in China

BACKGROUND

Anopheles sinensis is a major vector of malaria and among the dominant species in Shandong province of China. Insecticide resistance is an important threat to vector-borne disease control. However, there are only few reports about insecticide resistance of An. sinensis populations from Shandong province.

METHODS

From 2003 to 2012, six districts in Shandong province were selected as the study areas. Insecticide susceptibility bioassay were tested on F1 progeny of An. sinensis to 4% DDT, 0.05% deltamethrin, 0.15% cyfluthrin, and 5% malathion, using the standard WHO resistance tube assay.

RESULTS

The resistance status of An. sinensis showed a significant decrease in the mortality rates in DDT, deltamethrin and cyfluthrin during the past ten years. Whereas obvious increase of mortality to malathion was observed throughout the assay, ranging from 47.37% to 86.62%.

The colonization of pyrethroid resistant strain from wild Anopheles sinensis, the major Asian malaria vector

Background

Anopheles sinensis is one of the most important malaria vectors in Asian countries. The rapid spread of insecticide resistance has become a major obstacle for insecticide-based strategies for vector control. Therefore, it is necessary to prepare an insecticide-resistant strain of An. sinensis to further understand the insecticide resistance mechanisms in this species to facilitate genetic approaches to targeting the insecticide-resistant population of this important malaria vector.

Methods

An. sinensis mosquitoes were collected from regions where pyrethroid resistance had been reported. The mosquitoes were subjected to continuous pyrethroid selection after species confirmation, and the forced copulation method was used to increase the mating rate. In addition, the knockdown-resistance (kdr) mutation frequencies of each generation of An. sinensis were measured; and the metabolic enzyme activities of cytochrome P450 monoxygenases (P450s) and glutathione S-transferases (GSTs) were detected.

Results

The identification of field-captured An. sinensis was confirmed by both morphological and molecular methods. The population of An. sinensis exhibited stable resistance to pyrethroid after continuous generations of pyrethroid selection in the laboratory with high kdr mutation frequencies; and elevated levels of both P450s and GSTs were significantly found in field selected populations comparing with the laboratory susceptible strain. So far, the colonised strain has reached its eleventh generation and culturing well in the laboratory.

Conclusions

We colonised a pyrethroid-resistant population of An. sinensis in the laboratory, which provides a fundamental model for genetic studies of this important malaria vector.

Mutations in the voltage-gated sodium channel gene of anophelines and their association with resistance to pyrethroids - a review

Constant and extensive use of chemical insecticides has created a selection pressure and favored resistance development in many insect species worldwide. One of the most important pyrethroid resistance mechanisms is classified as target site insensitivity, due to conformational changes in the target site that impair a proper binding of the insecticide molecule. The voltage-gated sodium channel (NaV) is the target of pyrethroids and DDT insecticides, used to control insects of medical, agricultural and veterinary importance, such as anophelines. It has been reported that the presence of a few non-silent point mutations in the NaV gene are associated with pyrethroid resistance, termed as 'kdr' (knockdown resistance) for preventing the knockdown effect of these insecticides. The presence of these mutations, as well as their effects, has been thoroughly studied in Anopheles mosquitoes. So far, kdr mutations have already been detected in at least 13 species (Anopheles gambiae, Anopheles arabiensis, Anopheles sinensis, Anopheles stephensi, Anopheles subpictus, Anopheles sacharovi, Anopheles culicifacies, Anopheles sundaicus, Anopheles aconitus, Anopheles vagus, Anopheles paraliae, Anopheles peditaeniatus and Anopheles albimanus) from populations of African, Asian and, more recently, American continents. Seven mutational variants (L1014F, L1014S, L1014C, L1014W, N1013S, N1575Y and V1010L) were described, with the highest prevalence of L1014F, which occurs at the 1014 site in NaV IIS6 domain. The increase of frequency and distribution of kdr mutations clearly shows the importance of this mechanism in the process of pyrethroid resistance. In this sense, several species-specific and highly sensitive methods have been designed in order to genotype individual mosquitoes for kdr in large scale, which may serve as important tolls for monitoring the dynamics of pyrethroid resistance in natural populations. We also briefly discuss investigations concerning the course of Plasmodium infection in kdr individuals. Considering the limitation of insecticides available for employment in public health campaigns and the absence of a vaccine able to brake the life cycle of the malaria parasites, the use of pyrethroids is likely to remain as the main strategy against mosquitoes by either indoor residual spraying (IR) and insecticide treated nets (ITN). Therefore, monitoring insecticide resistance programs is a crucial need in malaria endemic countries.

Development and application of an AllGlo probe-based qPCR assay for detecting knockdown resistance (kdr) mutations in Anopheles sinensis

Background

Anopheles sinensis is one of the most important malaria vectors in China and other Southeast Asian countries. High levels of resistance have been reported in this species due to the long-term use of insecticides, especially pyrethroids, for public health and agricultural purposes. Knockdown resistance (kdr) caused by a single base pair mutation in the gene encoding the sodium channel is strongly associated with pyrethroid insecticide resistance in many Anopheles mosquitoes. There are few methods currently available for detecting kdr mutations in An. sinensis.

Methods

A novel AllGlo probe-based qPCR (AllGlo-qPCR) method was developed to screen for the predominant kdr mutations in An. sinensis mosquitoes from the Jiangsu Province. The results from AllGlo-qPCR, allele-specific PCR (AS-PCR), and TaqMan-MGB probe-based qPCR (TaqMan-qPCR) were compared. A comparative analysis of the equipment required, ease of use and cost of the available methods was also performed. Finally, the AllGlo-qPCR method was used to detect the frequencies of kdr mutations from the other four provinces in central China.

Results

Six kdr genotypes were detected in An. sinensis from the Jiangsu Province by DNA sequencing. The AllGlo-qPCR method detected all of the kdr genotypes with a high level of accuracy (97% sensitivity and 98% specificity). AllGlo-qPCR correctly determined the kdr genotypes of 98.73% of 158 An. sinensis samples, whereas TaqMan-qPCR and AS-PCR correctly identified 96.84% and 88.61% of mutations, respectively. Furthermore, the AllGlo-qPCR method is simpler to perform, requires less equipment, and exhibits a moderate expense cost comparing with the other tested methods of kdr mutation detection. Samples collected from four of the other provinces in central China showed a high frequency of kdr mutation in An. sinensis, as detected by the established AllGlo-qPCR method.

Conclusion

The novel AllGlo-qPCR method developed for kdr mutation detection in An. sinensis exhibits greater specificity and sensitivity than currently available methods and is more cost-effective; therefore, it represents a useful tool for entomological surveillance.

De novo transcriptome sequencing and sequence analysis of the malaria vector Anopheles sinensis (Diptera: Culicidae)

Background

Anopheles sinensis is the major malaria vector in China and Southeast Asia. Vector control is one of the most effective measures to prevent malaria transmission. However, there is little transcriptome information available for the malaria vector. To better understand the biological basis of malaria transmission and to develop novel and effective means of vector control, there is a need to build a transcriptome dataset for functional genomics analysis by large-scale RNA sequencing (RNA-seq).

Methods

To provide a more comprehensive and complete transcriptome of An. sinensis, eggs, larvae, pupae, male adults and female adults RNA were pooled together for cDNA preparation, sequenced using the Illumina paired-end sequencing technology and assembled into unigenes. These unigenes were then analyzed in their genome mapping, functional annotation, homology, codon usage bias and simple sequence repeats (SSRs).

Results

Approximately 51.6 million clean reads were obtained, trimmed, and assembled into 38,504 unigenes with an average length of 571 bp, an N50 of 711 bp, and an average GC content 51.26%. Among them, 98.4% of unigenes could be mapped onto the reference genome, and 69% of unigenes could be annotated with known biological functions. Homology analysis identified certain numbers of An. sinensis unigenes that showed homology or being putative 1:1 orthologues with genomes of other Dipteran species. Codon usage bias was analyzed and 1,904 SSRs were detected, which will provide effective molecular markers for the population genetics of this species.

Conclusions

Our data and analysis provide the most comprehensive transcriptomic resource and characteristics currently available for An. sinensis, and will facilitate genetic, genomic studies, and further vector control of An. sinensis.

Transcriptome profiling of pyrethroid resistant and susceptible mosquitoes in the malaria vector, Anopheles sinensis

BACKGROUND

Anopheles sinensis is a major malaria vector in China and other Southeast Asian countries, and it is becoming increasingly resistant to the insecticides used for agriculture, net impregnation, and indoor residual spray. Very limited genomic information on this species is available, which has hindered the development of new tools for resistance surveillance and vector control. We used the 454 GS FLX system and generated expressed sequence tag (EST) databases of various life stages of An. sinensis, and we determined the transcriptional differences between deltamethrin resistant and susceptible mosquitoes.

RESULTS

The 454 GS FLX transcriptome sequencing yielded a total of 624,559 reads (average length of 290 bp) with the pooled An. sinensis mosquitoes across various development stages. The de novo assembly generated 33,411 contigs with average length of 493 bp. A total of 8,057 ESTs were generated with Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation. A total of 2,131 ESTs were differentially expressed between deltamethrin resistant and susceptible mosquitoes collected from the same field site in Jiangsu, China. Among these differentially expressed ESTs, a total of 294 pathways were mapped to the KEGG database, with the predominant ESTs belonging to metabolic pathways. Furthermore, a total of 2,408 microsatellites and 15,496 single nucleotide polymorphisms (SNPs) were identified.

CONCLUSIONS

The annotated EST and transcriptome databases provide a valuable genomic resource for further genetic studies of this important malaria vector species. The differentially expressed ESTs associated with insecticide resistance identified in this study lay an important foundation for further functional analysis. The identified microsatellite and SNP markers will provide useful tools for future population genetic and comparative genomic analyses of malaria vectors.

Multiple resistances and complex mechanisms of Anopheles sinensis mosquito: a major obstacle to mosquito-borne diseases control and elimination in China

Malaria, dengue fever, and filariasis are three of the most common mosquito-borne diseases worldwide. Malaria and lymphatic filariasis can occur as concomitant human infections while also sharing common mosquito vectors. The overall prevalence and health significance of malaria and filariasis have made them top priorities for global elimination and control programmes. Pyrethroid resistance in anopheline mosquito vectors represents a highly significant problem to malaria control worldwide. Several methods have been proposed to mitigate insecticide resistance, including rotational use of insecticides with different modes of action. Anopheles sinensis, an important malaria and filariasis vector in Southeast Asia, represents an interesting mosquito species for examining the consequences of long-term insecticide rotation use on resistance. We examined insecticide resistance in two An. Sinensis populations from central and southern China against pyrethroids, organochlorines, organophosphates, and carbamates, which are the major classes of insecticides recommended for indoor residual spray. We found that the mosquito populations were highly resistant to the four classes of insecticides. High frequency of kdr mutation was revealed in the central population, whereas no kdr mutation was detected in the southern population. The frequency of G119S mutation in the ace-1 gene was moderate in both populations. The classification and regression trees (CART) statistical analysis found that metabolic detoxification was the most important resistance mechanism, whereas target site insensitivity of L1014 kdr mutation played a less important role. Our results indicate that metabolic detoxification was the dominant mechanism of resistance compared to target site insensitivity, and suggests that long-term rotational use of various insecticides has led An. sinensis to evolve a high insecticide resistance. This study highlights the complex network of mechanisms conferring multiple resistances to chemical insecticides in mosquito vectors and it has important implication for designing and implementing vector resistance management strategies.

Ribosomal protein S29 regulates metabolic insecticide resistance through binding and degradation of CYP6N3

Background

Many diseases are transmitted by mosquitoes, including malaria, dengue fever, yellow fever, filariasis, and West Nile fever. Chemical control plays a major role in managing mosquito-borne diseases. However, excessive and continuous application of insecticides has caused the development of insecticide resistance in many species including mosquito, and this has become the major obstacle to controlling mosquito-borne diseases. Insecticide resistance is the result of complex polygenic inheritance, and the mechanisms are not well understood. Ribosomal protein RPS29 was found to be associated with DM resistance in our previous study. In this study, we aim to further investigate the involvement of RPS29 in deltamethrin resistance.

Methodology and Principal Findings

In this study, tandem affinity purification was used to identify proteins that can interact with RPS29. Among the candidate proteins, CYP6N3, a member of the CYP450 superfamily, was identified, and binding to RPS29 was confirmed in vitro and in vivo by GST pull-down and immunofluorescence. CCK-8 assay was used to investigate the RPS29-CTP6N3 interaction in relation to DM resistance. CYP6N3 overexpression significantly enhanced DM resistance and insect cell viability, but this was reversed by RPS29 overexpression. Western blot was used to study the mechanism of interaction between RPS29 and CYP6N3. RPS29 increases CYP6N3 protein degradation through the proteasome.

Conclusions and Significance

These observations indicate that CYP6N3, a novel RPS29-interacting partner, could stimulate deltamethrin resistance in mosquito cells and RPS29 overexpression targeted CYP6N3 for proteosomal degradation, abrogating the CYP6N3-associated resistence to deltamethrin. Our findings provide a novel mechanism associated with CYP450s mediated DM resistance.

Insecticide resistance of Anopheles sinensis and An. vagus in Hainan Island, a malaria-endemic area of China

Background

Malaria is one of the most important public health problems in Southeast Asia, including Hainan Island, China. Vector control is the main malaria control measure, and insecticide resistance is a major concern for the effectiveness of chemical insecticide control programs. The objective of this study is to determine the resistance status of the main malaria vector species to pyrethroids and other insecticides recommended by the World Health Organization (WHO) for indoor residual sprays.

Methods

The larvae and pupae of Anopheles mosquitoes were sampled from multiple sites in Hainan Island, and five sites yielded sufficient mosquitoes for insecticide susceptibility bioassays. Bioassays of female adult mosquitoes three days after emergence were conducted in the two most abundant species, Anopheles sinensis and An. vagus, using three insecticides (0.05% deltamethrin, 4% DDT, and 5% malathion) and following the WHO standard tube assay procedure. P450 monooxygenase, glutathione S-transferase and carboxylesterase activities were measured. Mutations at the knockdown resistance (kdr) gene and the ace-1gene were detected by DNA sequencing and PCR-RFLP analysis, respectively.

Results

An. sinensis and An. vagus were the predominant Anopheles mosquito species. An. sinensis was found to be resistant to DDT and deltamethrin. An. vagus was susceptible to deltamethrin but resistant to DDT and malathion. Low kdr mutation (L1014F) frequency (<10%) was detected in An. sinensis, but no kdr mutation was detected in An. vagus populations. Modest to high (45%-75%) ace-1 mutation frequency was found in An. sinensis populations, but no ace-1 mutation was detected in An. vagus populations. Significantly higher P450 monooxygenase and carboxylesterase activities were detected in deltamethrin-resistant An. sinensis, and significantly higher P450 monooxygenase, glutathione S-transferase and carboxylesterase activities were found in malathion-resistant An. vagus mosquitoes.

Conclusions

Multiple insecticide resistance was found in An. sinensis and An. vagus in Hainan Island, a malaria-endemic area of China. Cost-effective integrated vector control programs that go beyond synthetic insecticides are urgently needed.

Anopheles sinensis mosquito insecticide resistance: comparison of three mosquito sample collection and preparation methods and mosquito age in resistance measurements

Background

Insecticide resistance monitoring in malaria mosquitoes is essential for guiding the rational use of insecticides in vector control programs. Resistance bioassay is the first step for insecticide monitoring and it lays an important foundation for molecular examination of resistance mechanisms. In the literature, various mosquito sample collection and preparation methods have been used, but how mosquito sample collection and preparation methods affect insecticide susceptibility bioassay results is largely unknown. The objectives of this study were to determine whether mosquito sample collection and preparation methods affected bioassay results, which may cause incorrect classification of mosquito resistance status.

Methods

The study was conducted in Anopheles sinensis mosquitoes in two study sites in central China. Three mosquito sample collection and preparation methods were compared for insecticide susceptibility, kdr frequencies and metabolic enzyme activities: 1) adult mosquitoes collected from the field; 2) F1 adults from field collected, blood-fed mosquitoes; and 3) adult mosquitoes reared from field collected larvae.

Results

Mosquito sample collection and preparation methods significantly affected mortality rates in the standard WHO tube resistance bioassay. Mortality rate of field-collected female adults was 10-15% higher than in mosquitoes reared from field-collected larvae and F1 adults from field collected blood-fed females. This pattern was consistent in mosquitoes from the two study sites. High kdr mutation frequency (85-95%) with L1014F allele as the predominant mutation was found in our study populations. Field-collected female adults consistently exhibited the highest monooxygenase and GST activities. The higher mortality rate observed in the field-collected female mosquitoes may have been caused by a mixture of mosquitoes of different ages, as older mosquitoes were more susceptible to deltamethrin than younger mosquitoes.

Conclusions

Female adults reared from field-collected larvae in resistance bioassays are recommended to minimize the effect of confounding factors such as mosquito age and blood feeding status so that more reliable and reproducible mortality may be obtained.