Serine proteinase over-expression in relation to deltamethrin resistance in Culex pipiens pallens

Abamectin exposure causes chronic toxicity and trypsin/chymotrypsin damages in Chironomus kiiensis Tokunaga (Diptera: Chironomidae)

Abamectin has been extensively used in paddy fields to control insect pests. However, little information is available regarding its effects on non-target insects. In this study, we performed acute (3rd instar larvae) and chronic toxicity (newly hatched larvae <24 h) to determine the toxicity effects of abamectin on Chironomus kiiensis. The median lethal concentration (LC50) values of 24 h and 10 d were 0.57 mg/L and 68.12 μg/L, respectively. The chronic exposure significantly prolonged the larvae growth duration and inhibited pupation and emergence. The transcriptome and biochemical parameters were measured using 3rd instar larvae exposed to acute LC10 and LC25 for 24 h. Transcriptome data indicated that five trypsin and four chymotrypsin genes were downregulated, and RT-qPCR verified a significant expression decrease in trypsin3 and chymotrypsin1 genes. Meanwhile, abamectin could significantly inhibit the activities of the serine proteases trypsin and chymotrypsin. RNA interference showed that silencing trypsin3 and chymotrypsin1 genes led to higher mortality of C. kiiensis to abamectin. In conclusion, these findings indicated that trypsin and chymotrypsin are involved in the abamectin toxicity against C. kiiensis, which provides new insights into the mechanism of abamectin-induced ecotoxicity to chironomids.

Global, asynchronous partial sweeps at multiple insecticide resistance genes in Aedes mosquitoes

Aedes aegypti (yellow fever mosquito) and Ae. albopictus (Asian tiger mosquito) are globally invasive pests that confer the world's dengue burden. Insecticide-based management has led to the evolution of insecticide resistance in both species, though the genetic architecture and geographical spread of resistance remains incompletely understood. This study investigates partial selective sweeps at resistance genes on two chromosomes and characterises their spread across populations. Sweeps at the voltage-sensitive sodium channel (VSSC) gene on chromosome 3 correspond to one resistance-associated nucleotide substitution in Ae. albopictus and three in Ae. aegypti, including two substitutions at the same nucleotide position (F1534C) that have evolved and spread independently. In Ae. aegypti, we also identify partial sweeps at a second locus on chromosome 2. This locus contains 15 glutathione S-transferase (GST) epsilon class genes with significant copy number variation among populations and where three distinct genetic backgrounds have spread across the Indo-Pacific region, the Americas, and Australia. Local geographical patterns and linkage networks indicate VSSC and GST backgrounds probably spread at different times and interact locally with different genes to produce resistance phenotypes. These findings highlight the rapid global spread of resistance and are evidence for the critical importance of GST genes in resistance evolution.

Characteristics of Trypsin genes and their roles in insecticide resistance based on omics and functional analyses in the malaria vector Anopheles sinensis

Trypsin is one of the most diverse and widely studied protease hydrolases. However, the diversity and characteristics of the Trypsin superfamily of genes have not been well understood, and their role in insecticide resistance is yet to be investigated. In this study, a total of 342 Trypsin genes were identified and classified into seven families based on homology, characteristic domains and phylogenetics in Anopheles sinensis, and the LY-Domain and CLECT-Domain families are specific to the species. Four Trypsin genes, (Astry2b, Astry43a, Astry90, Astry113c) were identified to be associated with pyrethroid resistance based on transcriptome analyses of three field resistant populations and qRT-PCR validation, and the knock-down of these genes significantly decrease the pyrethroid resistance of Anopheles sinensis based on RNAi. The activity of Astry43a can be reduced by five selected insecticides (indoxacarb, DDT, temephos, imidacloprid and deltamethrin); and however, the Astry43a could not directly metabolize these five insecticides, like the trypsin NYD-Tr did in earlier reports. This study provides the overall information frame of Trypsin genes, and proposes the role of Trypsin genes to insecticide resistance. Further researches are necessary to investigate the metabolism function of these trypsins to insecticides.

Comparative Transcriptomic Analysis of Insecticide-Resistant Aedes aegypti from Puerto Rico Reveals Insecticide-Specific Patterns of Gene Expression

Aedes aegypti transmits major arboviruses of public health importance, including dengue, chikungunya, Zika, and yellow fever. The use of insecticides represents the cornerstone of vector control; however, insecticide resistance in Ae. aegypti has become widespread. Understanding the molecular basis of insecticide resistance in this species is crucial to design effective resistance management strategies. Here, we applied Illumina RNA-Seq to study the gene expression patterns associated with resistance to three widely used insecticides (malathion, alphacypermethrin, and lambda-cyhalothrin) in Ae. aegypti populations from two sites (Manatí and Isabela) in Puerto Rico (PR). Cytochrome P450s were the most overexpressed detoxification genes across all resistant phenotypes. Some detoxification genes (CYP6Z7, CYP28A5, CYP9J2, CYP6Z6, CYP6BB2, CYP6M9, and two CYP9F2 orthologs) were commonly overexpressed in mosquitoes that survived exposure to all three insecticides (independent of geographical origin) while others including CYP6BY1 (malathion), GSTD1 (alpha-cypermethrin), CYP4H29 and GSTE6 (lambda-cyhalothrin) were uniquely overexpressed in mosquitoes that survived exposure to specific insecticides. The gene ontology (GO) terms associated with monooxygenase, iron binding, and passive transmembrane transporter activities were significantly enriched in four out of six resistant vs. susceptible comparisons while serine protease activity was elevated in all insecticide-resistant groups relative to the susceptible strain. Interestingly, cuticular-related protein genes (chinase and chitin) were predominantly downregulated, which was also confirmed in the functional enrichment analysis. This RNA-Seq analysis presents a detailed picture of the candidate detoxification genes and other pathways that are potentially associated with pyrethroid and organophosphate resistance in Ae. aegypti populations from PR. These results could inform development of novel molecular tools for detection of resistance-associated gene expression in this important arbovirus vector and guide the design and implementation of resistance management strategies.

Interactions between vector competence to chikungunya virus and resistance to deltamethrin in Aedes aegypti laboratory lines?

The urban mosquito species Aedes aegypti is the main vector of arboviruses worldwide. Mosquito control with insecticides is the most prevalent method for preventing transmission in the absence of effective vaccines and available treatments; however, the extensive use of insecticides has led to the development of resistance in mosquito populations throughout the world, and the number of epidemics caused by arboviruses has increased. Three mosquito lines with different resistance profiles to deltamethrin were isolated in French Guiana, including one with the I1016 knock-down resistant allele. Significant differences were observed in the cumulative proportion of mosquitoes with a disseminated chikungunya virus infection over time across these lines. In addition, some genes related to resistance (CYP6BB2, CYP6N12, GST2, trypsin) were variably overexpressed in the midgut at 7 days after an infectious bloodmeal in these three lines. Our work shows that vector competence for chikungunya virus varied between Ae. aegypti laboratory lines with different deltamethrin resistance profiles. More accurate verification of the functional association between insecticide resistance and vector competence remains to be demonstrated.

Inheritance, Fitness Cost, and Management of Lambda-Cyhalothrin Resistance in a Laboratory-Selected Strain of Ceratitis capitata (Wiedemann)

The management of the medfly, Ceratitis capitata, in Spanish citrus crops relies mainly on the use of insecticides and the release of sterile males. However, the development of resistance to different insecticides in field populations, including lambda-cyhalothrin, implies a threat for the sustainable control of this pest. The inheritance, fitness cost, and management of lambda-cyhalothrin resistance were examined in the laboratory-selected W-1Kλ strain. We have demonstrated that lambda-cyhalothrin resistance in W-1Kλ is autosomic, completely dominant, and polygenic. In addition, individuals from W-1Kλ showed a lower embryo to pupal viability, a slower developmental time from egg to pupae, and an increase in adults' weight and longevity. We did not find significant trade-offs in the activity of digestive hydrolytic enzymes, with the exception of higher α-amylase activity in W-1Kλ females. A comparative study with different insecticide treatment strategies showed that lambda-cyhalothrin resistance increased when several consecutive treatments with this insecticide were applied. However, the alternation of this insecticide with spinosad was enough to delay the development of resistance. Our results indicate that the rotation of lambda-cyhalothrin with spinosad-a practice already used in some fields-may contribute to prevent the development of resistance.

Tackling the Molecular Drug Sensitivity in the Sea Louse Caligus rogercresseyi Based on mRNA and lncRNA Interactions

Caligus rogercresseyi, commonly known as sea louse, is an ectoparasite copepod that impacts the salmon aquaculture in Chile, causing losses of hundreds of million dollars per year. This pathogen is mainly controlled by immersion baths with delousing drugs, which can lead to resistant traits selection in lice populations. Bioassays are commonly used to assess louse drug sensitivity, but the current procedures may mask relevant molecular responses. This study aimed to discover novel coding genes and non-coding RNAs that could evidence drug sensitivity at the genomic level. Sea lice samples from populations with contrasting sensitivity to delousing drugs were collected. Bioassays using azamethiphos, cypermethrin, and deltamethrin drugs were conducted to evaluate the sensitivity and to collect samples for RNA-sequencing. Transcriptome sequencing was conducted on samples exposed to each drug to evaluate the presence of coding and non-coding RNAs associated with the response of these compounds. The results revealed specific transcriptome patterns in lice exposed to azamethiphos, deltamethrin, and cypermethrin drugs. Enrichment analyses of Gene Ontology terms showed specific biological processes and molecular functions associated with each delousing drug analyzed. Furthermore, novel long non-coding RNAs (lncRNAs) were identified in C. rogercresseyi and tightly linked to differentially expressed coding genes. A significant correlation between gene transcription patterns and phenotypic effects was found in lice collected from different salmon farms with contrasting drug treatment efficacies. The significant correlation among gene transcription patterns with the historical background of drug sensitivity suggests novel molecular mechanisms of pharmacological resistance in lice populations.

Consequences of larval competition and exposure to permethrin for the development of the rodent malaria Plasmodium berghei in the mosquito Anopheles gambiae

Background

Mosquitoes and other vectors are often exposed to sublethal doses of insecticides. Larvae can be exposed to the run-off of agricultural use, and adults can be irritated by insecticides used against them and move away before they have picked up a lethal dose. This sublethal exposure may affect the success of control of insect-borne diseases, for it may affect the competence of insects to transmit parasites, in particular if the insects are undernourished.

Methods

We assessed how exposure of larvae and adults to a sublethal dose of permethrin (a pyrethroid) and how larval competition for food affect several aspects of the vector competence of the mosquito Anopheles gambiae for the malaria parasite Plasmodium berghei. We infected mosquitoes with P. berghei and measured the longevity and the prevalence and intensity of infection to test for an effect of our treatments.

Results

Our general result was that the exposure to the insecticide helped mosquitoes deal with infection by malaria. Exposure of either larvae or adults decreased the likelihood that mosquitoes were infected by about 20%, but did not effect the parasite load. Exposure also increased the lifespan of infected mosquitoes, but only if they had been reared in competition. Larval competition had no effect on the prevalence of infection, but increased parasite load. These effects may be a consequence of the machinery governing oxidative stress, which underlies the responses of mosquitoes to insecticides, to food stress and to parasites.

Conclusions

We conclude that insecticide residues are likely to affect the ability of mosquitoes to carry and transmit pathogens such as malaria, irrespective of the stage at which they are exposed to the insecticide. Our results stress the need for further studies to consider sublethal doses in the context of vector ecology and vector-borne disease epidemiology.

Developmental and comparative perspectives on mosquito immunity

Diseases spread by mosquitoes have killed more people than those spread by any other group of arthropod vectors and remain an important factor in determining global health and economic stability. The mosquito innate immune system can act to either modulate infection with human pathogens or fight off entomopathogens and increase the fitness and longevity of infected mosquitoes. While work remains towards understanding the larval immune system and the development of the mosquito immune system, it has recently become clearer that environmental factors heavily shape the developing mosquito immune system and continue to influence the adult immune system as well. The adult immune system has been well-studied and is known to involve multiple tissues and diverse molecular mechanisms. This review summarizes and synthesizes what is currently understood about the development of the mosquito immune system and includes comparisons of immune components unique to mosquitoes among the blood-feeding arthropods as well as important distinguishing factors between the anopheline and culicine mosquitoes. An explanation is included for how mosquito immunity factors into vector competence and vectorial capacity is presented along with a model for the interrelationships between nutrition, microbiome, pathogen interactions and behavior as they relate to mosquito development, immune status, adult female fitness and ultimately, vectorial capacity. Novel discoveries in the fields of mosquito ecoimmunology, neuroimmunology, and intracellular antiviral responses are highlighted.

A systemic study of indoxacarb resistance in Spodoptera litura revealed complex expression profiles and regulatory mechanism

The tobacco cutworm, Spodoptera litura, is an important pest of crop and vegetable plants worldwide, and its resistance to insecticides have quickly developed. However, the resistance mechanisms of this pest are still unclear. In this study, the change in mRNA and miRNA profiles in the susceptible, indoxacarb-resistant and field indoxacarb-resistant strains of S. litura were characterized. Nine hundred and ten co-up-regulated and 737 co-down-regulated genes were identified in the resistant strains. Further analysis showed that 126 co-differentially expressed genes (co-DEGs) (cytochrome P450, carboxy/cholinesterase, glutathione S-transferase, ATP-binding cassette transporter, UDP-glucuronosyl transferase, aminopeptidase N, sialin, serine protease and cuticle protein) may play important roles in indoxacarb resistance in S. litura. In addition, a total of 91 known and 52 novel miRNAs were identified, and 10 miRNAs were co-differentially expressed in the resistant strains of S. litura. Furthermore, 10 co-differentially expressed miRNAs (co-DEmiRNAs) had predicted co-DEGs according to the expected miRNA-mRNA negative regulation pattern and 37 indoxacarb resistance-related co-DEGs were predicted to be the target genes. These results not only broadened our understanding of molecular mechanisms of insecticide resistance by revealing complicated profiles, but also provide important clues for further study on the mechanisms of miRNAs involved in indoxacarb resistance in S. litura.

Maternal inheritance of deltamethrin resistance in the salmon louse Lepeophtheirus salmonis (Krøyer) is associated with unique mtDNA haplotypes

Parasitic infections by the salmon louse, Lepeophtheirus salmonis (Krøyer), cause huge economic damage in salmon farming in the northern hemisphere, with combined treatment costs and production losses in 2014 having been estimated at US$ 350 million for Norway (annual production 1.25 million tonnes). The control of L. salmonis relies significantly on medicinal treatments, supplemented by non-pharmacological approaches. However, efficacy losses have been reported for several delousing agents, including the pyrethroid deltamethrin. The aim of the present study was to analyse the genetic basis of deltamethrin resistance in L. salmonis. Deltamethrin median effective concentrations (EC50) were 0.28 μg L-1 in the drug susceptible L. salmonis strain IoA-00 and 40.1 μg L-1 in the pyrethroid resistant strain IoA-02. IoA-00 and IoA-02 were crossed to produce families spanning one parental and three filial generations (P0, F1-F3). In three families derived from P0 crosses between an IoA-00 sire and an IoA-02 dam, 98.8% of F2 parasites (n = 173) were resistant, i.e. remained unaffected after exposure to 2.0 μg L-1 deltamethrin. F3 parasites from these crosses showed a deltamethrin EC50 of 9.66 μg L-1. In two families of the inverse orientation at P0 (IoA-02 sire x IoA-00 dam), 16.7% of F2 parasites were resistant (n = 84), while the deltamethrin EC50 in F3 animals was 0.26 μg L-1. The results revealed a predominantly maternal inheritance of deltamethrin resistance. The 15,947-nt mitochondrial genome was sequenced and compared among six unrelated L. salmonis strains and parasites sampled from wild salmon in 2010. IoA-02 and three further deltamethrin resistant strains, established from isolates originating from different regions of Scotland, showed almost identical mitochondrial haplotypes. In contrast, the mitochondrial genome was variable among susceptible strains and L. salmonis from wild hosts. Deltamethrin caused toxicity and depletion of whole body ATP levels in IoA-00 but not IoA-02 parasites. The maternal inheritance of deltamethrin resistance and its association with mitochondrial haplotypes suggests that pyrethroid toxicity in L. salmonis may involve molecular targets encoded by mitochondrial genes.

Cypermethrin exposure induces metabolic and stress-related gene expression in copepodid salmon lice (Lepeophtheirus salmonis)

Cypermethrin has been administered for decades to control salmon lice (Lepeophtheirus salmonis) infestations in Atlantic salmon farming regions globally. However, resistance to cypermethrin and other available therapeutants has threatened the sustainability of this growing industry. To better understand the effects of cypermethrin on L. salmonis, a 38K oligonucleotide microarray and RT-qPCR analyses were applied to pools of copepodid larvae exposed to 1.0ppb cypermethrin or seawater controls for 24h. Phenotypic assessments and global gene expression profiles showed a significant disruption of homeostasis in copepodid L. salmonis exposed to cypermethrin. Multiple degradative enzymes were overexpressed in cypermethrin-treated lice including five trypsin-like serine proteases and three cytochrome p450s CYP3a24 (p=0.03, fold change (FC)=3.8; GenBank accession no. JP326960.1), CYP6w1 (p=0.008, FC=5.3; GenBank accession no. JP317875.1), and CYP6d4 (p=0.01; FC=7.9; GenBank accession no. JP334550.1). These enzymes represent preliminary markers for understanding the physiological response of L. salmonis to cypermethrin exposure. A general stress response was also observed in cypermethrin-treated lice which included differential expression of cell signaling genes involved in the induction of cell growth, solute transport, and metabolism. Lastly, a consensus-based analysis was completed with two previously published L. salmonis transcriptome studies revealing genes that respond to cypermethrin, emamectin benzoate (another delousing agent) and hyposalinity. This included concordant differential expression of heat shock beta-1, ammonium transporter Rh types B, and 72kDa type IV collagenase across different L. salmonis studies. This is currently the most comprehensive transcriptome assessment of chemical exposure on the first infectious stage of L. salmonis, providing novel markers for studying drug resistance and general stress in this important parasite.

Altered differential hemocyte count in 3rd instar larvae of Drosophila melanogaster as a response to chronic exposure of Acephate

Acephate, an organophosphate (OP) pesticide, was used to investigate the effects of its chronic exposure on hemocyte abundance in a non-target dipteran insect Drosophila melanogaster. For this purpose, six graded concentrations ranging from 1 to 6 μg/ml were selected, which are below the reported residual values (up to 14 μg/ml) of the chemical. 1^st instar larvae were fed with these concentrations up to the 3^rd instar stage and accordingly hemolymph smears from these larvae were prepared for differential hemocyte count. Three types of cells are found in Drosophila hemolymph, namely, plasmatocytes, lamellocytes and crystal cells. Plasmatocyte count was found to decrease with successive increase in treatment concentrations. Crystal cells showed an increasing trend in their number. Though the number of lamellocytes was very low, a bimodal response was noticed. Lamellocyte number was found to increase with the initial three concentrations, followed by a dose dependent reduction in their number. As hemocytes are directly linked to the immune system of fruit flies, fluctuations in normal titer of these cells may affect insect immunity. Hemocytes share homologies in their origin and mode of action with the immune cells of higher organisms including man. Thus the present findings suggest that immune cells of humans and other organisms may be affected adversely under chronic exposure to Acephate.

Analysis of differentially expressed genes between fluoride-sensitive and fluoride-endurable individuals in midgut of silkworm, Bombyx mori

Fluoride tolerance is an economically important trait of silkworm. Near-isogenic lines (NILs) of the dominant endurance to fluoride (Def) gene in Bombyx mori has been constructed before. Here, we analyzed the gene expression profiles of midgut of fluoride-sensitive and fluoride-endurable individuals of Def NILs by using high-throughput Illumina sequencing technology and bioinformatics tools, and identified differentially expressed genes between these individuals. A total of 3,612,399 and 3,567,631 clean tags for the libraries of fluoride-endurable and fluoride-sensitive individuals were obtained, which corresponded to 32,933 and 43,976 distinct clean tags, respectively. Analysis of differentially expressed genes indicates that 241 genes are differentially expressed between the two libraries. Among the 241 genes, 30 are up-regulated and 211 are down-regulated in fluoride-endurable individuals. Pathway enrichment analysis demonstrates that genes related to ribosomes, pancreatic secretion, steroid biosynthesis, glutathione metabolism, steroid biosynthesis, and glycerolipid metabolism are down-regulated in fluoride-endurable individuals. qRT-PCR was conducted to confirm the results of the DGE. The present study analyzed differential expression of related genes and tried to find out whether the crucial genes were related to fluoride detoxification which might elucidate fluoride effect and provide a new way in the fluorosis research.

Next-Generation Transcriptome Profiling of the Salmon Louse Caligus rogercresseyi Exposed to Deltamethrin (AlphaMax™): Discovery of Relevant Genes and Sex-Related Differences

Sea lice are one of the main parasites affecting the salmon aquaculture industry, causing significant economic losses worldwide. Increased resistance to traditional chemical treatments has created the need to find alternative control methods. Therefore, the objective of this study was to identify the transcriptome response of the salmon louse Caligus rogercresseyi to the delousing drug deltamethrin (AlphaMax™). Through bioassays with different concentrations of deltamethrin, adult salmon lice transcriptomes were sequenced from cDNA libraries in the MiSeq Illumina platform. A total of 78 million reads for females and males were assembled in 30,212 and 38,536 contigs, respectively. De novo assembly yielded 86,878 high-quality contigs and, based on published data, it was possible to annotate and identify relevant genes involved in several biological processes. RNA-seq analysis in conjunction with heatmap hierarchical clustering evidenced that pyrethroids modify the ectoparasitic transcriptome in adults, affecting molecular processes associated with the nervous system, cuticle formation, oxidative stress, reproduction, and metabolism, among others. Furthermore, sex-related transcriptome differences were evidenced. Specifically, 534 and 1033 exclusive transcripts were identified for males and females, respectively, and 154 were shared between sexes. For males, estradiol 17-beta-dehydrogenase, sphingolipid delta4-desaturase DES1, ketosamine-3-kinase, and arylsulfatase A, among others, were discovered, while for females, vitellogenin 1, glycoprotein G, transaldolase, and nitric oxide synthase were among those identified. The shared transcripts included annotations for tropomyosin, γ-crystallin A, glutamate receptor-metabotropic, glutathione S-transferase, and carboxipeptidase B. The present study reveals that deltamethrin generates a complex transcriptome response in C. rogercresseyi, thus providing valuable genomic information for developing new delousing drugs.

Deltamethrin Resistance Mechanisms in Aedes aegypti Populations from Three French Overseas Territories Worldwide

Background

Aedes aegypti is a cosmopolite mosquito, vector of arboviruses. The worldwide studies of its insecticide resistance have demonstrated a strong loss of susceptibility to pyrethroids, the major class of insecticide used for vector control. French overseas territories such as French Guiana (South America), Guadeloupe islands (Lesser Antilles) as well as New Caledonia (Pacific Ocean), have encountered such resistance.

Methodology/Principal Findings

We initiated a research program on the pyrethroid resistance in French Guiana, Guadeloupe and New Caledonia. Aedes aegypti populations were tested for their deltamethrin resistance level then screened by an improved microarray developed to specifically study metabolic resistance mechanisms. Cytochrome P450 genes were implicated in conferring resistance. CYP6BB2, CYP6M11, CYP6N12, CYP9J9, CYP9J10 and CCE3 genes were upregulated in the resistant populations and were common to other populations at a regional scale. The implication of these genes in resistance phenomenon is therefore strongly suggested. Other genes from detoxification pathways were also differentially regulated. Screening for target site mutations on the voltage-gated sodium channel gene demonstrated the presence of I1016 and C1534.

Conclusion /significance

This study highlighted the presence of a common set of differentially up-regulated detoxifying genes, mainly cytochrome P450 genes in all three populations. GUA and GUY populations shared a higher number of those genes compared to CAL. Two kdr mutations well known to be associated to pyrethroid resistance were also detected in those two populations but not in CAL. Different selective pressures and genetic backgrounds can explain such differences. These results are also compared with those obtained from other parts of the world and are discussed in the context of integrative research on vector competence.

Aedes aegypti is vector of Dengue, Chikungunya and Zika viruses, all causing emerging or re-emerging diseases worldwide. Fighting these diseases relies on the control of the vector. Therefore, insecticides have been extensively used worldwide, resulting in the development of insecticide resistance. In the French overseas territories, resistance to pyrethroids has been monitored for many years with high levels in the South American French territories. We then investigated the mechanisms underlying this resistance in populations from French Guiana, Guadeloupe and New Caledonia. Transcription levels of detoxification genes were measured and alongside screening for target site mutations. Upregulation of cytochrome P450 genes and carboxylesterases were observed in all three populations. Mutations related to pyrethroid resistance in position 1016 and 1534 of the voltage-gated sodium channel gene were also observed. French Guiana and Guadeloupe populations presented a closer profile of resistance mechanisms whereas the New Caledonia population had a more restricted profile. Such differences can be explained by different vector control practices, regional insecticide uses and genetic backgrounds. These results are also compared with others obtained from other parts of the world and are discussed with the perspective of integrative research on vector competence.

The recent escalation in strength of pyrethroid resistance in Anopheles coluzzi in West Africa is linked to increased expression of multiple gene families

Background

Since 2011, the level of pyrethroid resistance in the major malaria mosquito, Anopheles coluzzi, has increased to such an extent in Burkina Faso that none of the long lasting insecticide treated nets (LLINs) currently in use throughout the country kill the local mosquito vectors. We investigated whether this observed increase was associated with transcriptional changes in field-caught Anopheles coluzzi using two independent whole-genome microarray studies, performed in 2011 and 2012.

Results

Mosquitoes were collected from south-west Burkina Faso in 2011 and 2012 and insecticide exposed or non-exposed insects were compared to laboratory susceptible colonies using whole-genome microarrays. Using a stringent filtering process we identified 136 genes, including the well-studied detoxification enzymes (p450 monoxygenases and esterases) and non-detoxification genes (e.g. cell transporters and cuticular components), associated with pyrethroid resistance, whose basal expression level increased during the timeframe of the study. A subset of these were validated by qPCR using samples from two study sites, collected over 3 years and marked increases in expression were observed each year. We hypothesise that these genes are contributing to this rapidly increasing resistance phenotype in An. coluzzi. A comprehensive analysis of the knockdown resistance (kdr) mutations (L1014S, L1014F and N1575Y) revealed that the majority of the resistance phenotype is not explained by target-site modifications.

Conclusions

Our data indicate that the recent and rapid increase in pyrethroid resistance observed in south-west Burkina Faso is associated with gene expression profiles described here. Over a third of these candidates are also overexpressed in multiple pyrethroid resistant populations of An. coluzzi from neighbouring Côte d’Ivoire. This suite of molecular markers can be used to track the spread of the extreme pyrethroid resistance phenotype that is sweeping through West Africa and to determine the functional basis of this trait.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1342-6) contains supplementary material, which is available to authorized users.

Effect of chlorfenapyr on cypermethrin-resistant Culex pipiens pallens Coq mosquitoes

Chlorfenapyr is a promising pyrrole insecticide with a unique mechanism of action that does not confer cross-resistance to neurotoxic insecticides. The effect of chlorfenapyr on pyrethorid-resistant Culex pipiens pallens Coq (Diptera: Culicidae) has not been fully investigated under laboratory conditions. In this study, cypermethrin-resistant C. p. pallens exhibited 376.79-fold and 395.40-fold increase in resistance to cypermethrin compared with susceptible strains after exposure for 24 and 48h, respectively. Larvae and adults were tested for susceptibility using dipping, topical, and impregnated paper methods as recommended by the WHO. No cross-resistance to chlorfenapyr was found. Increased mortality was apparent between 48 and 72h, indicating a slow rate of toxic activity. Synergism experiments with piperonyl butoxide (PBO) showed an antagonistic effect on chlorfenapyr toxicity. Mixtures of chlorfenapyr and cypermethrin could therefore provide additional benefits over either insecticide used alone. Mixtures of 5ng/ml chlorfenapyr and 500ng/ml cypermethrin exhibited a slight synergistic effect on cypermethrin-resistant mosquitoes (3.33, 6.84 and 2.34% after 24, 48 and 72h exposure, respectively. This activity was lost when the chlorfenapyr concentration was increased to 10 or 20ng/ml. Chlorfenapyr showed quite good results for pyrethroid-resistant C. p. pallens, and could improve public health by reducing the occurrence of mosquito bites and subsequently protecting against transmission of lymphatic filariasis and Japanese encephalitis.

Over-transcription of genes in a parathion-resistant strain of mosquito Culex pipiens quinquefasciatus

Insecticide resistance is an evolutionary adaptation that develops quite quickly in mosquitoes because of the high selection pressure of chemical insecticides, rapid generation time and large population size. Identification of genes associated with insecticide resistance is fundamental to understand the complex processes responsible for resistance. We compared the gene transcriptional profiles of parathion-resistant and -susceptible Culex pipiens quinquefasciatus using a combination of suppression subtractive hybridization and complementary DNA (cDNA) microarray techniques. A total of 278 colonies were selected from the resistant-susceptible mosquito subtractive library, 38 of which showed more than two fold stronger immunoblotting signals in the resistant strain than in the susceptible strain using cDNA microarray selection. The sequencing results showed that the 38 colonies can be matched to 12 genes of C. p. quinquefasciatus. Eight genes were confirmed to be overexpressed by more than two fold in the resistant strain. These genes encode chymotrypsin-1, theta glutathione S-transferase, lipase 3, larval serum protein 1 β chain, cytochrome b, mitochondrial ribosomal large subunit, 28S rRNA, and a protein with unknown function. This study serves as a preliminary attempt to identify new genes associated with organophosphate resistance in this mosquito species and provides insights into the complicated physiological phenomenon of insecticide resistance.

Transcriptomic responses to emamectin benzoate in Pacific and Atlantic Canada salmon lice Lepeophtheirus salmonis with differing levels of drug resistance

Salmon lice Lepeophtheirus salmonis are an ecologically and economically important parasite of wild and farmed salmon. In Scotland, Norway, and Eastern Canada, L. salmonis have developed resistance to emamectin benzoate (EMB), one of the few parasiticides available for salmon lice. Drug resistance mechanisms can be complex, potentially differing among populations and involving multiple genes with additive effects (i.e., polygenic resistance). Indicators of resistance development may enable early detection and countermeasures to avoid the spread of resistance. Here, we collect sensitive Pacific L. salmonis and sensitive and resistant Atlantic L. salmonis from salmon farms, propagate in laboratory (F1), expose to EMB in bioassays, and evaluate either baseline (Atlantic only) or induced transcriptomic differences between populations. In all populations, induced responses were minor and a cellular stress response was not identified. Pacific lice did not upregulate any genes in response to EMB, but downregulated degradative enzymes and transport proteins at 50 ppb EMB. Baseline differences between sensitive and now resistant Atlantic lice were much greater than responses to exposures. All resistant lice overexpressed degradative enzymes, and resistant males, the most resistant group, overexpressed collagenases to the greatest extent. These results indicate an accumulation of baseline expression differences related to resistance.

Olive fly transcriptomics analysis implicates energy metabolism genes in spinosad resistance

Background

The olive fly, Bactrocera oleae, is the most devastating pest of cultivated olives. Its control has been traditionally based on insecticides, mainly organophosphates and pyrethroids. In recent years, the naturalyte spinosad is used against the olive fly. As with other insecticides, spinosad is subject to selection pressures that have led to resistance development. Mutations in the α6 subunit of the nicotinic acetylcholine receptor (nAChR) have been implicated in spinosad resistance in several species (e.g., Drosophila melanogaster) but excluded in others (e.g., Musca domestica). Yet, additional mechanisms involving enhanced metabolism of detoxification enzymes (such as P450 monooxygenases or mixed function oxidases) have also been reported. In order to clarify the spinosad resistance mechanisms in the olive fly, we searched for mutations in the α6-subunit of the nAChR and for up-regulated genes in the entire transcriptome of spinosad resistant olive flies.

Results

The olive fly α6-subunit of the nAChR was cloned from the laboratory sensitive strain and a spinosad selected resistant line. The differences reflected silent nucleotide substitutions or conserved amino acid changes. Additionally, whole transcriptome analysis was performed in the two strains in order to reveal any underlying resistance mechanisms. Comparison of over 13,000 genes showed that in spinosad resistant flies nine genes were significantly over-expressed, whereas ~40 were under-expressed. Further functional analyses of the nine over-expressed and eleven under-expressed loci were performed. Four of these loci (Yolk protein 2, ATP Synthase FO subunit 6, Low affinity cationic amino acid transporter 2 and Serine protease 6) showed consistently higher expression both in the spinosad resistant strain and in wild flies from a resistant California population. On the other side, two storage protein genes (HexL1 and Lsp1) and two heat-shock protein genes (Hsp70 and Hsp23) were unfailingly under-expressed in resistant flies.

Conclusion

The observed nucleotide differences in the nAChR-α6 subunit between the sensitive and spinosad resistant olive fly strains did not advocate for the involvement of receptor mutations in spinosad resistance. Instead, the transcriptome comparison between the two strains indicated that several immune system loci as well as elevated energy requirements of the resistant flies might be necessary to lever the detoxification process.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-714) contains supplementary material, which is available to authorized users.

Trypsin-catalyzed deltamethrin degradation

To explore if trypsin could catalyze the degradation of non-protein molecule deltamethrin, we compared in vitro hydrolytic reactions of deltamethrin in the presence and absence of trypsin with ultraviolet-visible (UV/Vis) spectrophotometry and gas chromatography-mass spectrometry (GC/MS). In addition, acute oral toxicity of the degradation products was determined in Wistar rats. The results show that the absorption peak of deltamethrin is around 264 nm, while the absorption peaks of deltamethrin degradation products are around 250 nm and 296 nm. In our GC setting, the retention time of undegraded deltamethrin was 37.968 min, while those of deltamethrin degradation products were 15.289 min and 18.730 min. The LD₅₀ of deltamethrin in Wistar rats is 55 mg/kg, while that of deltamethrin degradation products is 3358 mg/kg in female rats and 1045 mg/kg in male rates (61-fold and 19-fold reductions in toxicity), suggesting that trypsin could directly degrade deltamethrin, which significantly reduces the toxicity of deltamethrin. These results expand people's understanding of the functions of proteases and point to potential applications of trypsin as an attractive agent to control residual pesticides in the environment and on agricultural products.

Genome-wide analysis of genes associated with moderate and high DDT resistance in Drosophila melanogaster

BACKGROUND

Moderate to high DDT resistance in generally associated with overexpression of multiple genes and therefore has been considered to be polygenic. However, very little information is available about the molecular mechanisms that insect populations employ when evolving increased levels of resistance. The presence of common regulatory motifs among resistance-associated genes may help to explain how and why certain suites of genes are preferentially represented in genomic-scale analyses.

RESULTS

A set of commonly differentially expressed genes associated with DDT resistance in the fruit fly was identified on the basis of genome-wide microarray analysis followed by qRT-PCR verification. More genes were observed to be overtranscribed in the highly resistant strain (91-R) than in the moderately resistant strain (Wisconsin) and susceptible strain (Canton-S). Furthermore, possible transcription factor binding sites that occurred in coexpressed resistance-associated genes were discovered by computational motif discovery methods.

CONCLUSION

A glucocorticoid receptor (GR)-like putative transcription factor binding motif (TFBM) was observed to be associated with genes commonly differentially transcribed in both the 91-R and Wisconsin lines of DDT-resistant Drosophila.

Transcriptional characteristics of gene expression in the midgut of domestic silkworms (Bombyx mori) exposed to phoxim

Silkworm (Bombyx mori) is not only an economically important insect but also a model system for lepidoptera. As a vital organ of digestion and nutrient absorption, the midgut of insects also serves as the first physiological barrier to chemical pesticides. In this study, microarray was performed to profile the gene expression changes in the midgut of silkworms exposed to phoxim. After 24h of phoxim exposure (4.0μg/mL), 266 genes displayed at least 2.0-fold changes in expression levels. Among them, 192 genes were up-regulated, and 74 genes were down-regulated. The most significant changes were 14.88-fold up-regulation and 23.36-fold down-regulation. According to gene ontology annotation and pathway analysis, differentially expressed genes were mainly classified into different groups based on their potential involvements in detoxification, immunne response, stress response, energy metabolism and transport. Particularly, the transcription levels of detoxification-related genes were up-regulated, such as cytochrome P450s, esterases and glutathione-S-transferase (GST), indicating increased detoxification activity in the midgut. Our study provides new insights into the molecular mechanism of pesticide metabolism in the midgut of insects, which may promote the development of highly efficient insecticides.

In vitro antiviral activity of an alkaline trypsin from the digestive juice of Bombyx mori larvae against nucleopolyhedrovirus

Alkaline trypsin protein of molecular mass 25,436 Da purified from the digestive juice of Bombyx mori larvae indicated strong antiviral activity against Bombyx mori nucleopolyhedrovirus (BmNPV) under in vitro conditions. Partial N-terminal amino acid sequence of the protein was determined and the cDNA was cloned based on the amino acid sequence. A homology search of the deduced amino acid sequence of the cDNA showed 55% identity with Helicoverpa armigera trypsin and the active site of this protein was completely conserved. Hence, the protein was designated B. mori trypsin (Bmtryp). The results suggest that Bmtryp, an insect digestive enzyme, can be a potential antiviral factor against BmNPV at the initial site of viral infection.

Molecular cloning and preliminary function study of iron responsive element binding protein 1 gene from cypermethrin-resistant Culex pipiens pallens

Background

Insecticide resistance jeopardizes the control of mosquito populations and mosquito-borne disease control, which creates a major public health concern. Two-dimensional electrophoresis identified one protein segment with high sequence homology to part of Aedes aegypti iron-responsive element binding protein (IRE-BP).

Method

RT-PCR and RACE (rapid amplification of cDNA end) were used to clone a cDNA encoding full length IRE-BP 1. Real-time quantitative RT-PCR was used to evaluate the transcriptional level changes in the Cr-IRE strain Aedes aegypti compared to the susceptible strain of Cx. pipiens pallens. The expression profile of the gene was established in the mosquito life cycle. Methyl tritiated thymidine (³H-TdR) was used to observe the cypermethrin resistance changes in C6/36 cells containing the stably transfected IRE-BP 1 gene of Cx. pipiens pallens.

Results

The complete sequence of iron responsive element binding protein 1 (IRE-BP 1) has been cloned from the cypermethrin-resistant strain of Culex pipiens pallens (Cr-IRE strain). Quantitative RT-PCR analysis indicated that the IRE-BP 1 transcription level was 6.7 times higher in the Cr-IRE strain than in the susceptible strain of 4th instar larvae. The IRE-BP 1 expression was also found to be consistently higher throughout the life cycle of the Cr-IRE strain. A protein of predicted size 109.4 kDa has been detected by Western blotting in IRE-BP 1-transfected mosquito C6/36 cells. These IRE-BP 1-transfected cells also showed enhanced cypermethrin resistance compared to null-transfected or plasmid vector-transfected cells as determined by ³H-TdR incorporation.

Conclusion

IRE-BP 1 is expressed at higher levels in the Cr-IRE strain, and may confer some insecticide resistance in Cx. pipiens pallens.

Expression of trypsin-like serine peptidases in pre-imaginal stages of Aedes aegypti (Diptera: Culicidae)

This study reports the biochemical characterization and comparative analyses of highly active serine proteases in the larval and pupal developmental stages of Aedes aegypti (Linnaeus) using substrate-SDS-PAGE. Zymographic analysis of larval stadia detected proteolytic activity in 6-8 bands with apparent molecular masses ranging from 20 to 250 kDa, with activity observed from pH 5.5 to 10.0. The pupal stage showed a complex proteolytic activity in at least 11 bands with apparent Mr ranging from 25 to 250 kDa, and pH optimum at 10.0. The proteolytic activities of both larval and pupal stages were strongly inhibited by phenyl-methyl sulfonyl-fluoride and N-α-Tosyl-L-lysine chloromethyl ketone hydrochloride, indicating that the main proteases expressed by these developmental stages are trypsin-like serine proteases. The enzymes were active at temperatures ranging from 4 to 85°C, with optimal activity between 37 and 60°C, and low activity at 85°C. Comparative analysis between the proteolytic enzymes expressed by larvae and pupae showed that substantial changes in the expression of active trypsin-like serine proteases occur during the developmental cycle of A. aegypti.

Cloning and characterization of myosin regulatory light chain (MRLC) gene from Culex pipiens pallens

Myosin regulatory light chain (MRLC) (GenBank accession no. DQ140391) was cloned from Culex pipiens pallens. An open reading frame (ORF) of 630 bps was found to encode a putative 210 amino acids protein which shows 73% similarity with myosin regulatory light chain of Gryllotalpa orientalis. Real-time quantitative PCR analysis demonstrated that the transcription level of MRLC in deltamethrin-resistant strain (DR-strain) was 4.08-fold higher than in deltamethrin-susceptible strain (DS-strain) of C. pipiens pallens. Over-expression of MRLC in Aedes albopictus C6/36 cells conferred protection against deltamethrin based on tritiated methyl tritiated thymidine ((3)H-TdR) incorporation assay. These results indicate that MRLC may be a potential cause of deltamethrin resistance in C. pipiens pallens.

Cloning and overexpression of ribosomal protein L39 gene from deltamethrin-resistant Culex pipiens pallens

The complete sequence of ribosomal protein L39 has been cloned from deltamethrin-resistant strain of Culex pipiens pallens (DR1 strain). Quantitative RT-PCR analysis indicated that the RPL39 transcription level was 23.4 times higher in DR1 strain than in susceptible strain at 4th instar larvae. The RPL39 expression was also found to be consistently higher throughout the life cycle of DR1 strain. A protein of predicted size 17 kDa has been detected by Western blotting in RPL39-transfected mosquito C6/36 cells. These RPL39-transfected cells also showed enhanced deltamethrin resistance compared to plasmid vector-transfected cells as determined by methyl tritiated thymidine ((3)H-TdR) incorporation. These results indicate that RPL39 is expressed at higher levels in DR1 strain, and may confer some insecticide resistance in Cx. pipiens pallens.

Identification of differentially expressed genes in female Culex pipiens pallens

Culex pipiens pallens is the mosquito vector of a number of human pathogens such as Wuchereria bancrofti, Brugia malayi, and epidemic encephalitis B virus. Female C. pipiens pallens play an important role in transmitting pathogens by sucking blood, which is essential for reproduction. In the present study, a subtractive cDNA library for female C. pipiens pallens was constructed by the suppression subtractive hybridization (SSH) technique and then 100 clones from the female SSH library were sequenced and analyzed. Female-differentially expressed genes in C. pipiens pallens were screened using semiquantitative RT-PCR. The full-length cDNA of an EST sequence (fs68) that was specifically expressed in female C. pipiens pallens was characterized by 3' and 5' rapid amplification of cDNA ends (RACE). The characteristics of the female-specific gene were further analyzed using bioinformatics and Northern blot. It was shown that the female-specific gene was a previously uncharacterized gene and may encode a salivary peptide. This putative salivary peptide could be a very important molecule in the blood feeding of female C. pipiens pallens.

Cloning and characterization of 40S ribosomal protein S4 gene from Culex pipiens pallens

The 40S ribosomal protein S4 gene (RPS4) has been cloned from Culex pipiens pallens. An open reading frame (ORF) of 789 bp was found to encode a putative 262 amino acid protein. The deduced amino acid sequence shares 96% and 91% identity with RPS4 genes from Aedes and Anopheles respectively. Transcript expression of RPS4 was determined by real-time PCR in all life stages of deltamethrin-susceptible and -resistant strains. The results demonstrated that this gene is expressed at all developmental stages. Meanwhile, in pupae and adults, RPS4 is overexpressed in deltamethrin-resistant strain than in -susceptible strain. Our data for the first time suggests that increased expression of the RPS4 gene may play some role in the development of deltamethrin resistance in C. pipiens pallens.