Glutathione S-transferases as antioxidant defence agents confer pyrethroid resistance in Nilaparvata lugens

Transcriptomic and proteomic analysis of resistance to matrine in Oedaleus asiaticus (Orthoptera: Oedipodidae)

In order to determine the specific genes and proteins that are affected by matrine and play a role in regulating metabolism in the locust species Oedaleus asiaticus, we conducted RNA-seq, proteomic sequencing, and bioinformatics analyses on third-instar nymphs. These nymphs were divided into two groups: one group was grown under normal conditions, while the other group was treated with matrine. The purpose of this investigation was to gain insight into the molecular mechanisms underlying matrine resistance. Genes and proteins that exhibited differential expression were identified and subjected to analysis using bioinformatics software. The DESeq analysis revealed a total of 743 transcripts that were differentially expressed (DETs). Among these, 208 transcripts were up-regulated, and 535 were downregulated in ZO/ZCK. The iTRAQ discovered that 34 and 65 proteins were, respectively, up- and down-regulated in ZO/ZCK. Additionally, enrichment studies based on Gene Ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) were performed. The KEGG and GO analysis of the DEGs suggested the essential roles of matrine treatment in the regulation of O. asiaticus, especially via the biosynthesis of amino acids, glycolysis/gluconeogenesis, iInsulin signaling pathway and MAPK signaling pathway. The findings demonstrated that matrine exerted control of the growth of O. asiaticus via modulating the synthesis of metabolism and biosynthesis. Increased expression of detoxifying enzymes was observed, which may be related to matrine detoxification. These findings provide a basis for better comprehension of the molecular mechanism behind the regulation of development mediated by matrine in Asian locust hoppers.

Resistance selection of triflumezopyrim in Laodelphax striatellus (fallén): Resistance risk, cross-resistance and metabolic mechanism

The risk assessment and resistance mechanisms of insecticide resistance are critical for resistance management strategy before a new insecticide is widely used. Triflumezopyrim (TFM) is the first commercialized mesoionic insecticide, which can inhibit nicotinic acetylcholine receptor with high-performance against the small brown planthopper (SBPH), Laodelphax striatellus (Fallén). In our study, the resistance of SBPH to TFM increased 26.29-fold, and the actual heritability of resistance was 0.09 after 21 generations of continuous selection by TFM. After five generations of constant feeding under insecticide-free conditions from F16 generation, the resistance level decreased 2.05-fold, and the average resistance decline rate per generation was 0.01, but there were no statistical decline. The TFM resistant strains had no cross-resistance to imidacloprid, nitenpyram, thiamethoxam, dinotefuran, flonicamid, pymetrozine, and chlorfenapyr. The third and fifth nymphal stage duration, pre-adult stage, adult preoviposition period, longevity, emergence rate, and hatchability of the resistant strain were significantly lower than those of the susceptible strain, while the female-male ratio was considerably increased. The fitness cost was 0.89. Further, cytochrome P450 monooxygenase (P450) and carboxylesterase (CarE) activities were markedly increased, but only the enzyme inhibitor piperonyl butoxide (PBO) had a significant synergistic effect on the resistant strain. The expression of CYP303A1, CYP4CE2, and CYP419A1v2 of P450 genes was significantly increased. SBPH has a certain risk of resistance to TFM with continuous application. The TFM resistance may be due to the increased activity of P450 enzyme regulated by the overexpression of P450 genes.

Pomelo seed oil: Natural insecticide against cowpea aphid

Cowpea aphid (Aphis craccivora Koch) is a plant pest that causes serious damage to vegetable crops. Extensive use of synthetic chemical pesticides causes deleterious effects on consumers as well as the environment. Hence, the search for environmentally friendly insecticides in the management of cowpea aphids is required. The present work aims to investigate the aphicidal activity of pomelo seed oil (PSO) on cowpea aphids, the possible insecticidal mechanisms, its chemical constituent profile, as well as the toxicity of its primary compounds. The results of the toxicity assay showed that PSO had significant insecticidal activity against aphids with a 72-hour LC50 value of 0.09 μg/aphid and 3.96 mg/mL in the contact and residual toxicity assay, respectively. The enzymatic activity of both glutathione S-transferase (GST) and acetyl cholinesterase (AChE) significantly decreased, as well as the total protein content, after PSO treatment, which suggested that the reduction of AChE, GST, and the total protein content in aphids treated with PSO might be responsible for the mortality of A. craccivora. The GC-MS analysis revealed that PSO contained limonene (22.86%), (9Z,12Z)-9,12-octadecadienoic acid (20.21%), n-hexadecanoic acid (15.79%), (2E,4E)-2,4-decadienal (12.40%), and (2E,4Z)-2,4-decadienal (7.77%) as its five major compounds. Furthermore, (9Z,12Z)-9,12-octadecadienoic acid showed higher toxicity to aphids than both PSO and thiamethoxam (positive control). This study emphasized the potential of PSO as a natural plant-derived insecticide in controlling cowpea aphids and also provided a novel approach for the value-added utilization of pomelo seed.

Resistance to Beta-cypermethrin, Azadirachtin, and Matrine, and Biochemical Characterization of Field Populations of Oedaleus asiaticus (Bey-Bienko) in Inner Mongolia, Northern China

Oedaleus asiaticus (Bey-Bienko) is an economically devastating locust species found in grassland and pastoral areas of the Inner Mongolia region of northern China. In this study, resistance to three frequently used insecticides (beta-cypermethrin, matrine, and azadirachtin) was investigated in six field populations of O. asiaticus using the leaf-dip bioassay method. The inhibitory effects of synergists and the activities of detoxification enzyme activities in the different populations were determined to explore potential biochemical resistance mechanisms. The results showed that the field populations SB (resistance ratio [RR] = 7.85), ZB (RR = 5.64), and DB (RR = 6.75) had developed low levels of resistance to beta-cypermethrin compared with a susceptible control strain. Both the SB (RR = 5.92) and XC (RR = 6.38) populations had also developed low levels of resistance against matrine, with the other populations remaining susceptible to both beta-cypermethrin and matrine. All field populations were susceptible to azadirachtin. Synergism analysis showed that triphenyl phosphate (TPP) and diethyl-maleate (DEM) increased the toxicity of beta-cypermethrin significantly in the SB population, while the synergistic effects of TPP, piperonyl butoxide (PBO), and DEM on the toxicity of matrine were higher in SB (SR 3.86, 4.18, and 3.07, respectively) than in SS (SR 2.24, 2.86, and 2.29, respectively), but no synergistic effects of TPP, PBO, and DEM on azadirachtin were found. Biochemical assays showed that the activities of carboxylesterases (CarEs) and glutathione-S-transferases (GSTs) were significantly raised in all field populations of O. asiaticus, with a significant positive correlation observed between beta-cypermethrin resistance and CarE activity. The activities of cytochrome P450 monooxygenases (P450) and multi-function oxidases (MFO) were elevated in all six field populations, and P450 activity displayed strong positive correlations with the three insecticides. Our findings suggest that resistance to beta-cypermethrin in O. asiaticus may be mainly attributed to elevated CarE and GST activities, while P450 plays an important role in metabolizing matrine and azadirachtin. Our study provides insights that will help improve insecticide resistance management strategies.

Tandem duplication of a genomic region encoding glutathione S-transferase epsilon-2 and -4 genes in DDT-resistant Anopheles stephensi strain from India

The glutathione S-transferases (GST) genes are a multigene family of enzymes involved in the metabolism of endogenous and xenobiotic compounds by catalysing the conjugation of the reduced form of glutathione to the substrate. The epsilon class of GST (GSTe), unique to arthropods, is known to be involved in the detoxification process of several classes of insecticides, and GSTe2 in particular is known to have DDT dehydrochlorinase activity. This communication reports a tandem duplication of a genomic region encoding GSTe2 and GSTe4 genes in a laboratory-colonized DDT-resistant Anopheles stephensi. We identified duplication breakpoints and the organization of gene duplication through Sanger sequencing performed on long-PCR products. Manual annotation of sequences revealed a tandemly-arrayed duplication of a 3.62 kb segment of GST epsilon gene clusters comprised of five genes: a partial GSTe1, GSTe2, GSTe2-pseudogene, GSTe4 and partial GSTe5, interconnected by a conserved 2.42 kb DNA insert segment major part of which is homologous to a genomic region located on a different chromosome. The tandemly duplicated array contained a total of two GSTe2 and three GSTe4 functional paralog genes. Read-depth coverage and split-read analysis of Illumina-based whole-genome sequence reads confirmed the presence of duplication in the corresponding region of the genome. The increased gene dose in mosquitoes as a result of the GSTe gene-duplication may be an adaptive process to increase levels of detoxifying enzymes to counter insecticide pressure.

Functional and Structural Diversity of Insect Glutathione S-transferases in Xenobiotic Adaptation

As a superfamily of multifunctional enzymes that is mainly associated with xenobiotic adaptation, glutathione S-transferases (GSTs) facilitate insects' survival under chemical stresses in their environment. GSTs confer xenobiotic adaptation through direct metabolism or sequestration of xenobiotics, and/or indirectly by providing protection against oxidative stress induced by xenobiotic exposure. In this article, a comprehensive overview of current understanding on the versatile functions of insect GSTs in detoxifying chemical compounds is presented. The diverse structures of different classes of insect GSTs, specifically the spatial localization and composition of their amino acid residues constituted in their active sites are also summarized. Recent availability of whole genome sequences of numerous insect species, accompanied by RNA interference, X-ray crystallography, enzyme kinetics and site-directed mutagenesis techniques have significantly enhanced our understanding of functional and structural diversity of insect GSTs.

The Cross-Resistance Pattern and the Metabolic Resistance Mechanism of Acetamiprid in the Brown Planthopper, Nilaparvata lugens (Stål)

Acetamiprid is widely used in paddy fields for controlling Nilaparvata lugens (Stål). However, the risk of resistance development, the cross-resistance pattern and the resistance mechanism of acetamiprid in this pest remain unclear. In this study, an acetamiprid-resistant strain (AC-R) was originated from a field strain (UNSEL) through successive selection with acetamiprid for 30 generations, which reached 60.0-fold resistance when compared with a laboratory susceptible strain (AC-S). The AC-R strain (G₃₀) exhibited cross-resistance to thiamethoxam (25.6-fold), nitenpyram (21.4-fold), imidacloprid (14.6-fold), cycloxaprid (11.8-fold), dinotefuran (8.7-fold), sulfoxaflor (7.6-fold) and isoprocarb (8.22-fold), while there was no cross-resistance to etofenprox, buprofezin and chlorpyrifos. Acetamiprid was synergized by the inhibitor piperonyl butoxide (2.2-fold) and the activity of cytochrome P450 monooxygenase was significantly higher in the AC-R strain compared with the AC-S strain, suggesting the critical role of P450. The gene expression results showed that the P450 gene CYP6ER1 was significantly overexpressed in AC-R compared with the AC-S and UNSEL strains. In addition, the RNA interference (RNAi) of CYP6ER1 significantly increased the susceptibility of AC-R to acetamiprid. Molecular docking predicted that acetamiprid and CYP6ER1 had close binding sites, and the nitrogen atoms had hydrogen bond interactions with CYP6ER1. These results demonstrated that the overexpression of CYP6ER1 contributed to acetamiprid resistance in N. lugens.

Response of xenobiotic biodegradation and metabolic genes in Tribolium castaneum following eugenol exposure

Eugenol, a plant-derived component possessing small side effects, has an insecticidal activity to Tribolium castaneum; however, the underlying molecular mechanisms of eugenol acting on T. castaneum are currently unclear. Here, a nerve conduction carboxylesterase and a detoxifying glutathione S-transferase were significantly inhibited after eugenol exposure, resulting in the paralysis or death of beetles. Then, RNA-sequencing of eugenol-exposed and control samples identified 362 differentially expressed genes (DEGs), containing 206 up-regulated and 156 down-regulated genes. RNA-seq data were validated further by qRT-PCR. GO analysis revealed that DEGs were associated with 1308 GO terms of which the most enriched GO terms were catalytic activity, and integral component of membrane; KEGG pathway analysis showed that these DEGs were distributed in 151 different pathways, of which some pathways associated with metabolism of xenobiotics or drug were significantly enriched, which indicated that eugenol most likely disturbed the processes of metabolism, and detoxication. Moreover, several DEGs including Hexokinase type 2, Isocitrate dehydrogenase, and Cytochrome b-related protein, might participate in the respiratory metabolism of eugenol-exposed beetles. Some DEGs encoding CYP, UGT, GST, OBP, CSP, and ABC transporter were involved in the xenobiotic or drug metabolism pathway, which suggested that these genes of T. castaneum participated in the response to eugenol exposure. Additionally, TcOBPC11/ TcGSTs7, detected by qRT-PCR and RNA-interference against these genes, significantly increased the mortality of eugenol-treated T. castaneum, providing further evidence for the involvement of OBP/GST in eugenol metabolic detoxification in T. castaneum. These results aid eugenol insecticidal mechanisms and provide the basis of insect control.

A scaffold-level genome assembly of a minute pirate bug, Orius laevigatus (Hemiptera: Anthocoridae), and a comparative analysis of insecticide resistance-related gene families with hemipteran crop pests

BACKGROUND

Orius laevigatus, a minute pirate bug, is a highly effective beneficial predator of crop pests including aphids, spider mites and thrips in integrated pest management (IPM) programmes. No genomic information is currently available for O. laevigatus, as is the case for the majority of beneficial predators which feed on crop pests. In contrast, genomic information for crop pests is far more readily available. The lack of publicly available genomes for beneficial predators to date has limited our ability to perform comparative analyses of genes encoding potential insecticide resistance mechanisms between crop pests and their predators. These mechanisms include several gene/protein families including cytochrome P450s (P450s), ATP binding cassette transporters (ABCs), glutathione S-transferases (GSTs), UDP-glucosyltransferases (UGTs) and carboxyl/cholinesterases (CCEs).

METHODS AND FINDINGS

In this study, a high-quality scaffold level de novo genome assembly for O. laevigatus has been generated using a hybrid approach with PacBio long-read and Illumina short-read data. The final assembly achieved a scaffold N50 of 125,649 bp and a total genome size of 150.98 Mb. The genome assembly achieved a level of completeness of 93.6% using a set of 1658 core insect genes present as full-length genes. Genome annotation identified 15,102 protein-coding genes - 87% of which were assigned a putative function. Comparative analyses revealed gene expansions of sigma class GSTs and CYP3 P450s. Conversely the UGT gene family showed limited expansion. Differences were seen in the distributions of resistance-associated gene families at the subfamily level between O. laevigatus and some of its targeted crop pests. A target site mutation in ryanodine receptors (I4790M, PxRyR) which has strong links to diamide resistance in crop pests and had previously only been identified in lepidopteran species was found to also be present in hemipteran species, including O. laevigatus.

CONCLUSION AND SIGNIFICANCE

This assembly is the first published genome for the Anthocoridae family and will serve as a useful resource for further research into target-site selectivity issues and potential resistance mechanisms in beneficial predators. Furthermore, the expansion of gene families often linked to insecticide resistance may be an indicator of the capacity of this predator to detoxify selective insecticides. These findings could be exploited by targeted pesticide screens and functional studies to increase effectiveness of IPM strategies, which aim to increase crop yields by sustainably, environmentally-friendly and effectively control pests without impacting beneficial predator populations.

Effects of Aluminium Contamination on the Nervous System of Freshwater Aquatic Vertebrates: A Review

Aluminium (Al) is the most common natural metallic element in the Earth’s crust. It is released into the environment through natural processes and human activities and accumulates in aquatic environments. This review compiles scientific data on the neurotoxicity of aluminium contamination on the nervous system of aquatic organisms. More precisely, it helps identify biomarkers of aluminium exposure for aquatic environment biomonitoring in freshwater aquatic vertebrates. Al is neurotoxic and accumulates in the nervous system of aquatic vertebrates, which is why it could be responsible for oxidative stress. In addition, it activates and inhibits antioxidant enzymes and leads to changes in acetylcholinesterase activity, neurotransmitter levels, and in the expression of several neural genes and nerve cell components. It also causes histological changes in nerve tissue, modifications of organism behaviour, and cognitive deficit. However, impacts of aluminium exposure on the early stages of aquatic vertebrate development are poorly described. Lastly, this review also poses the question of how accurate aquatic vertebrates (fishes and amphibians) could be used as model organisms to complement biological data relating to the developmental aspect. This “challenge” is very relevant since freshwater pollution with heavy metals has increased in the last few decades.

The Key Glutathione S-Transferase Family Genes Involved in the Detoxification of Rice Gramine in Brown Planthopper Nilaparvata lugens

Phytochemical toxins are considered a defense measure for herbivore invasion. To adapt this defensive strategy, herbivores use glutathione S-transferases (GSTs) as an important detoxification enzyme to cope with toxic compounds, but the underlying molecular basis for GST genes in this process remains unclear. Here, we investigated the basis of how GST genes in brown planthopper (BPH, Nilaparvata lugens (Stål)) participated in the detoxification of gramine by RNA interference. For BPH, the LC25 and LC50 concentrations of gramine were 7.11 and 14.99 μg/mL at 72 h after feeding, respectively. The transcriptions of seven of eight GST genes in BPH were induced by a low concentration of gramine, and GST activity was activated. Although interferences of seven genes reduced BPH tolerance to gramine, only the expression of NlGST1-1, NlGSTD2, and NlGSTE1 was positively correlated with GST activities, and silencing of these three genes inhibited GST activities in BPH. Our findings reveal that two new key genes, NlGSTD2 and NlGSTE1, play an essential role in the detoxification of gramine such as NlGST1-1 does in BPH, which not only provides the molecular evidence for the coevolution theory, but also provides new insight into the development of an environmentally friendly strategy for herbivore population management.

Resistance in the Genus Spodoptera: Key Insect Detoxification Genes

The genus Spodoptera (Lepidoptera: Noctuidae) includes species that are among the most important crop pests in the world. These polyphagous species are able to feed on many plants, including corn, rice and cotton. In addition to their ability to adapt to toxic compounds produced by plants, they have developed resistance to the chemical insecticides used for their control. One of the main mechanisms developed by insects to become resistant involves detoxification enzymes. In this review, we illustrate some examples of the role of major families of detoxification enzymes such as cytochromes P450, carboxyl/cholinesterases, glutathione S-transferases (GST) and transporters such as ATP-binding cassette (ABC) transporters in insecticide resistance. We compare available data for four species, Spodoptera exigua, S. frugiperda, S. littoralis and S. litura. Molecular mechanisms underlying the involvement of these genes in resistance will be described, including the duplication of the CYP9A cluster, over-expression of GST epsilon or point mutations in acetylcholinesterase and ABCC2. This review is not intended to be exhaustive but to highlight the key roles of certain genes.

Transposon-mediated insertional mutagenesis unmasks recessive insecticide resistance in the aphid Myzus persicae

The evolution of resistance to insecticides threatens the sustainable control of many of the world's most damaging insect crop pests and disease vectors. To effectively combat resistance, it is important to understand its underlying genetic architecture, including the type and number of genetic variants affecting resistance and their interactions with each other and the environment. While significant progress has been made in characterizing the individual genes or mutations leading to resistance, our understanding of how genetic variants interact to influence its phenotypic expression remains poor. Here, we uncover a mechanism of insecticide resistance resulting from transposon-mediated insertional mutagenesis of a genetically dominant but insecticide-susceptible allele that enables the adaptive potential of a previously unavailable recessive resistance allele to be unlocked. Specifically, we identify clones of the aphid pest Myzus persicae that carry a resistant allele of the essential voltage-gated sodium channel (VGSC) gene with the recessive M918T and L1014F resistance mutations, in combination with an allele lacking these mutations but carrying a Mutator-like element transposon insertion that disrupts the coding sequence of the VGSC. This results in the down-regulation of the dominant susceptible allele and monoallelic expression of the recessive resistant allele, rendering the clones resistant to the insecticide bifenthrin. These findings are a powerful example of how transposable elements can provide a source of evolutionary potential that can be revealed by environmental and genetic perturbation, with applied implications for the control of highly damaging insect pests.

Genome-Wide Transcriptional Analysis and Functional Validation Linked a Cluster of Epsilon Glutathione S-Transferases with Insecticide Resistance in the Major Malaria Vector Anopheles funestus across Africa

Resistance is threatening the effectiveness of insecticide-based interventions in use for malaria control. Pinpointing genes associated with resistance is crucial for evidence-based resistance management targeting the major malaria vectors. Here, a combination of RNA-seq based genome-wide transcriptional analysis and RNA-silencing in vivo functional validation were used to identify key insecticide resistance genes associated with DDT and DDT/permethrin cross-resistance across Africa. A cluster of glutathione-S-transferase from epsilon group were found to be overexpressed in resistant populations of Anopheles funestus across Africa including GSTe1 [Cameroon (fold change, FC: 2.54), Ghana (4.20), Malawi (2.51)], GSTe2 [Cameroon (4.47), Ghana (7.52), Malawi (2.13)], GSTe3 [Cameroon (2.49), Uganda (2.60)], GSTe4 in Ghana (3.47), GSTe5 [Ghana (2.94), Malawi (2.26)], GSTe6 [Cameroun (3.0), Ghana (3.11), Malawi (3.07), Uganda (3.78)] and GSTe7 (2.39) in Ghana. Validation of GSTe genes expression profiles by qPCR confirmed that the genes are differentially expressed across Africa with a greater overexpression in DDT-resistant mosquitoes. RNAi-based knock-down analyses supported that five GSTe genes are playing a major role in resistance to pyrethroids (permethrin and deltamethrin) and DDT in An. funestus, with a significant recovery of susceptibility observed when GSTe2, 3, 4, 5 and GSTe6 were silenced. These findings established that GSTe3, 4, 5 and 6 contribute to DDT resistance and should be further characterized to identify their specific genetic variants, to help design DNA-based diagnostic assays, as previously done for the 119F-GSTe2 mutation. This study highlights the role of GSTes in the development of resistance to insecticides in malaria vectors and calls for actions to mitigate this resistance.

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.

Cloning and expression studies on glutathione S-transferase like-gene in honey bee for its role in oxidative stress

Glutathione S-transferases (GSTs) constitute an important multifunctional enzyme family that plays vital roles in cellular detoxification and protecting organisms against oxidative stress caused by reactive oxygen species (ROS). In this study, we isolated a GST-like gene from Apis cerana cerana (AccGSTL) and investigated its antioxidant functions under stress conditions. We found that AccGSTL belongs to the Sigma class of GSTs. Real-time quantitative PCR and western blotting analyses showed that the mRNA and protein levels of AccGSTL were altered in response to oxidative stress caused by various external stimuli. In addition, a heterologous expression analysis showed that AccGSTL overexpression in Escherichia coli (E. coli) cells enhanced resistance to oxidative stress. After AccGSTL silencing with RNA interference (RNAi) technology, the expression of some antioxidant genes was inhibited, and the enzymatic activities of POD, CAT, and SOD were decreased. In conclusion, these data suggest that AccGSTL may be involved in antioxidant defense under adverse conditions in A. cerana cerana.

Function Analysis of P450 and GST Genes to Imidacloprid in Aphis craccivora (Koch)

Aphis craccivora (Koch) is an economically important pest that affects legumes in worldwide. Chemical control is still the primary efficient method for A. craccivora management. However, the mechanism underlying insecticide resistance in A. craccivora has not been elucidated. A previous study observed that piperonyl butoxide (PBO) and diethyl maleate (DEM) significantly synergized imidacloprid in A. craccivora field populations, indicating that cytochrome P450 (P450) and glutathione S-transferase (GST) genes may play pivotal roles in imidacloprid resistance. In this study, 38 P450 genes and 10 GST genes were identified in A. craccivora through transcriptomic analysis. The expression levels of these P450 and GST genes were measured in susceptible (SUS) strains of A. craccivora under imidacloprid treatment with LC₁₅, LC₅₀, and LC₈₅ doses. The expression levels of CYP18A1, CYP6CY21, CYP6DA1, CYP6DA2, CYP4CJ1, CYP4CJ2, and CYP380C6 were up-regulated in the three treatments. Most of these genes belong to CYP3 and CYP4 Clans. In addition, the expression levels of all P450 and GST genes in A. craccivora were also measured in the Juye (JY) and Linqing (LQ) field populations. The expression levels of CYP6DA2, CYP4CJ1, and CYP380C6 were up-regulated in the SUS strain after imidacloprid treatment at three doses, and these genes were overexpressed in the JY population. Furthermore, the sensitivity of A. craccivora to imidacloprid was significantly increased after knockdown of CYP380C6 and CYP6DA2 through RNA interference. These results may help to elucidate the mechanisms underlying of imidacloprid resistance in A. craccivora.

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.

Investigating the molecular basis of multiple insecticide resistance in a major malaria vector Anopheles funestus (sensu stricto) from Akaka-Remo, Ogun State, Nigeria

Background

Understanding the mechanisms used by Anopheles mosquitoes to survive insecticide exposure is key to manage existing insecticide resistance and develop more suitable insecticide-based malaria vector control interventions as well as other alternative integrated tools. To this regard, the molecular basis of permethrin, DDT and dieldrin resistance in Anopheles funestus (sensu stricto) at Akaka-Remo was investigated.

Methods

Bioassays were conducted on 3–5-day-old adult An. funestus (s.s.) mosquitoes for permethrin, DDT and dieldrin susceptibility test. The molecular mechanisms of mosquito resistance to these insecticides were investigated using microarray and reverse transcriptase PCR techniques. The voltage-gated sodium channel region of mosquitoes was also screened for the presence of knockdown resistance mutations (kdr west and east) by sequencing method.

Results

Anopheles funestus (s.s.) population was resistant to permethrin (mortality rate of 68%), DDT (mortality rate of 10%) and dieldrin (mortality rate of 8%) insecticides. Microarray and RT-PCR analyses revealed the overexpression of glutathione S-transferase genes, cytochrome P450s, esterase, trypsin and cuticle proteins in resistant mosquitoes compared to control. The GSTe2 was the most upregulated detoxification gene in permethrin-resistant (FC = 44.89), DDT-resistant (FC = 57.39) and dieldrin-resistant (FC = 41.10) mosquitoes compared to control population (FC = 22.34). The cytochrome P450 gene, CYP6P9b was also upregulated in both permethrin- and DDT-resistant mosquitoes. The digestive enzyme, trypsin (hydrolytic processes) and the cuticle proteins (inducing cuticle thickening leading to reduced insecticides penetration) also showed high involvement in insecticide resistance, through their overexpression in resistant mosquitoes compared to control. The kdr east and west were absent in all mosquitoes analysed, suggesting their non-involvement in the observed mosquito resistance.

Conclusions

The upregulation of metabolic genes, especially the GSTe2 and trypsin, as well as the cuticle proteins is driving insecticide resistance of An. funestus (s.s.) population. However, additional molecular analyses, including functional metabolic assays of these genes as well as screening for a possible higher cuticular hydrocarbon and lipid contents, and increased procuticle thickness in resistant mosquitoes are needed to further describe their distinct roles in mosquito resistance.

Evolution of insecticide resistance and its mechanisms in Anopheles stephensi in the WHO Eastern Mediterranean Region

BACKGROUND

While Iran is on the path to eliminating malaria, the disease with 4.9 million estimated cases and 9300 estimated deaths in 2018 remains a serious health problem in the World Health Organization (WHO) Eastern Mediterranean Region. Anopheles stephensi is the main malaria vector in Iran and its range extends from Iraq to western China. Recently, the vector invaded new territories in Sri Lanka and countries in the Horn of Africa. Insecticide resistance in An. stephensi is a potential issue in controlling the spread of this vector.

METHODS

Data were collated from national and international databases, including PubMed, Google Scholar, Scopus, ScienceDirect, SID, and IranMedex using appropriate search terms.

RESULTS

Indoor residual spaying (IRS) with DDT was piloted in Iran in 1945 and subsequently used in the malaria eradication programme. Resistance to DDT in An. stephensi was detected in Iran, Iraq, Pakistan, and Saudi Arabia in the late 1960s. Malathion was used for malaria control in Iran in 1967, then propoxur in 1978, followed by pirimiphos-methyl from 1992 to 1994. The pyrethroid insecticide lambda-cyhalothrin was used from 1994 to 2003 followed by deltamethrin IRS and long-lasting insecticidal nets (LLINs). Some of these insecticides with the same sequence were used in other malaria-endemic countries of the region. Pyrethroid resistance was detected in An. stephensi in Afghanistan in 2010, in 2011 in India and in 2012 in Iran. The newly invaded population of An. stephensi in Ethiopia was resistant to insecticides of all four major insecticide classes. Different mechanisms of insecticide resistance, including metabolic and insecticide target site insensitivity, have been developed in An. stephensi. Resistance to DDT was initially glutathione S-transferase based. Target site knockdown resistance was later selected by pyrethroids. Esterases and altered acetylcholinesterase are the underlying cause of organophosphate resistance and cytochrome p450s were involved in pyrethroid metabolic resistance.

CONCLUSIONS

Anopheles stephensi is a major malaria vector in Iran and many countries in the region and beyond. The species is leading in terms of development of insecticide resistance as well as developing a variety of resistance mechanisms. Knowledge of the evolution of insecticide resistance and their underlying mechanisms, in particular, are important to Iran, considering the final steps the country is taking towards malaria elimination, but also to other countries in the region for their battle against malaria. This systematic review may also be of value to countries and territories newly invaded by this species, especially in the Horn of Africa, where the malaria situation is already dire.

Insecticidal Activity of Artemisia vulgaris Essential Oil and Transcriptome Analysis of Tribolium castaneum in Response to Oil Exposure

Red flour beetle (Tribolium castaneum) is one of the most destructive pests of stored cereals worldwide. The essential oil (EO) of Artemisia vulgaris (mugwort) is known to be a strong toxicant that inhibits the growth, development, and reproduction of T. castaneum. However, the molecular mechanisms underlying the toxic effects of A. vulgaris EO on T. castaneum remain unclear. Here, two detoxifying enzymes, carboxylesterase (CarEs) and cytochrome oxidase P450 (CYPs), were dramatically increased in red flour beetle larvae when they were exposed to A. vulgaris EO. Further, 758 genes were differentially expressed between EO treated and control samples. Based on Gene Ontology (GO) analysis, numerous differentially expressed genes (DEGs) were enriched for terms related to the regulation of biological processes, response to stimulus, and antigen processing and presentation. Our results indicated that A. vulgaris EO disturbed the antioxidant activity in larvae and partially inhibited serine protease (SP), cathepsin (CAT), and lipase signaling pathways, thus disrupting larval development and reproduction as well as down-regulating the stress response. Moreover, these DEGs showed that A. vulgaris indirectly affected the development and reproduction of beetles by inducing the expression of genes encoding copper-zinc-superoxide dismutase (CuZnSOD), heme peroxidase (HPX), antioxidant enzymes, and transcription factors. Moreover, the majority of DEGs were mapped to the drug metabolism pathway in the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. Notably, the following genes were detected: 6 odorant binding proteins (OBPs), 5 chemosensory proteins (CSPs), 14 CYPs, 3 esterases (ESTs), 5 glutathione S-transferases (GSTs), 6 UDP-glucuronosyltransferases (UGTs), and 2 multidrug resistance proteins (MRPs), of which 8 CYPs, 2 ESTs, 2 GSTs, and 3 UGTs were up-regulated dramatically after exposure to A. vulgaris EO. The residual DEGs were significantly down-regulated in EO exposed larvae, implying that partial compensation of metabolism detoxification existed in treated beetles. Furthermore, A. vulgaris EO induced overexpression of OBP/CYP, and RNAi against these genes significantly increased mortality of larvae exposed to EO, providing further evidence for the involvement of OBP/CYP in EO metabolic detoxification in T. castaneum. Our results provide an overview of the transcriptomic changes in T. castaneum in response to A. vulgaris EO.

Monitoring Insecticide Susceptibility in Aedes Aegypti Populations from the Two Biggest Cities, Ouagadougou and Bobo-Dioulasso, in Burkina Faso: Implication of Metabolic Resistance

In West Africa, Aedes aegypti remains the major vector of dengue virus. Since 2013, dengue fever has been reemerging in Burkina Faso with annual outbreaks, thus becoming a major public health problem. Its control relies on vector control, which is unfortunately facing the problem of insecticide resistance. At the time of this study, although data on phenotypic resistance were available, information related to the metabolic resistance in Aedes populations from Burkina Faso remained very scarce. Here, we assessed the phenotypic and the metabolic resistance of Ae. aegypti populations sampled from the two main urban areas (Ouagadougou and Bobo-Dioulasso) of Burkina Faso. Insecticide susceptibility bioassays to chlorpyriphos-methyl 0.4%, bendiocarb 0.1% and deltamethrin 0.05% were performed on natural populations of Ae. aegypti using the WHO protocol. The activity of enzymes involved in the rapid detoxification of insecticides, especially non-specific esterases, oxidases (cytochrome P450) and glutathione-S-transferases, was measured on individual mosquitos. The mortality rates for deltamethrin 0.05% were low and ranged from 20.72% to 89.62% in the Bobo-Dioulasso and Ouagadougou sites, respectively. When bendiocarb 0.1% was tested, the mortality rates ranged from 7.73% to 71.23%. Interestingly, in the two urban areas, mosquitoes were found to be fully susceptible to chlorpyriphos-methyl 0.4%. Elevated activity of non-specific esterases and glutathione-S-transferases was reported, suggesting multiple resistance mechanisms involved in Ae. aegypti populations from Bobo-Dioulasso and Ouagadougou (including cytochrome P450). This update to the insecticide resistance status within Ae. aegypti populations in the two biggest cities is important to better plan dengue vectors control in the country and provides valuable information for improving vector control strategies in Burkina Faso, West Africa.

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.

Conidiobolus coronatus induces oxidative stress and autophagy response in Galleria mellonella larvae

Cell homeostasis requires the correct levels of reactive oxygen species (ROS) to be maintained as these regulate the proliferation and differentiation of cells, and control the immune response and inflammation. High levels of ROS can cause oxidative stress, leading to protein, lipid and DNA damage, or even cell death. Under physiological conditions, the rate of autophagy remains stable; however, it can be accelerated by a number of exogenous stimuli such as oxidative stress, starvation or hypoxia, leading to cell death. The present paper examines the effect of Conidiobolus coronatus infection on the immune response, oxidative stress processes and autophagy in the greater wax moth, Galleria mellonella. Fungal infection was found to result in the disorganization of the cytoskeleton of the larval immune cells and the enhancement of oxidative defense processes. Lipid peroxidation and autophagy were also induced in the hemocytes. Our findings show that G. mellonella is an ideal model for exploring immune mechanisms.

Combining genetic crosses and pool targeted DNA-seq for untangling genomic variations associated with resistance to multiple insecticides in the mosquito Aedes aegypti

In addition to combating vector-borne diseases, studying the adaptation of mosquitoes to insecticides provides a remarkable example of evolution-in-action driving the selection of complex phenotypes. Actually, most resistant mosquito populations show multi-resistance phenotypes as a consequence of the variety of insecticides employed and of the complexity of selected resistance mechanisms. Such complexity makes the identification of alleles conferring resistance to specific insecticides challenging and prevents the development of molecular assays to track them in the field. Here we showed that combining simple genetic crosses with pool targeted DNA-seq can enhance the specificity of resistance allele's detection while maintaining experimental work and sequencing effort at reasonable levels. A multi-resistant population of the mosquito Aedes aegypti was exposed to three distinct insecticides (deltamethrin, bendiocarb and fenitrothion), and survivors to each insecticide were crossed with a susceptible strain to generate three distinct lines. F2 individuals from each line were then segregated based on their survival to two insecticide doses. Hundreds of genes covering all detoxifying enzymes and insecticide targets together with more than 7,000 intergenic regions equally spread over mosquito genome were sequenced from pools of F0 and F2 individuals unexposed or surviving insecticide. Differential coverage analysis identified 39 detoxification enzymes showing an increased gene copy number in association with resistance. Combining an allele frequency filtering approach with a Bayesian F ST-based genome scan identified multiple genomic regions showing strong selection signatures together with 50 nonsynonymous variations associated with resistance. This study provides a simple and cost-effective approach to improve the specificity of resistance allele's detection in multi-resistant populations while reducing false positives frequently arising when comparing populations showing divergent genetic backgrounds. The identification of novel DNA resistance markers opens new opportunities for improving the tracking of insecticide resistance in the field.

Transcriptome Sequencing and Analysis of Changes Associated with Insecticide Resistance in the Dengue Mosquito (Aedes aegypti) in Vietnam

The mosquito Aedes aegypti is a transmission vector for dangerous epidemic diseases in humans. Insecticides have been used as the most general vector control method in the world. However, Ae. aegypti have developed many resistant mechanisms such as reduced neuronal sensitivity to insecticides (target-site resistance), enhanced insecticide metabolism (metabolic resistance), altered transport, sequestration, and other mechanisms. It has become a major problem for vector control programs. Transcriptome sequencing and bioinformatic analysis were used to compare transcription levels between a susceptible strain (Bora7) and a resistant strain (KhanhHoa7) collected from the field. A total of 161 million Illumina reads, including 66,076,678 reads from the Bora7 strain and 69,606,654 reads from the KhanhHoa7 strain, were generated and assembled into 11,174 genes. A comparison of the KhanhHoa7 transcriptome to that of Bora7 showed 672 upregulated genes and 488 downregulated genes. We identified the highly upregulated genes: cytochrome P450 4C1, 4C3, 4C21, 4D1, 4D1 isoform X2, 4D2, 4D2 isoform X2, 4G15, 6A2, 6A8, 6D3, and 9E2; Glutathione S transferase (GST1), UGT1-3, 1-7, 2B15, and 2B37; binding cassette transporter (ABC) transporter F family member 4 and ABC transporter G family member 20. Interestingly, there was a significant increase in the expression of the genes such as CYP9E2 (8.3-fold), CYP6A8 (5.9-fold), CYP6D3 (5.4-fold), CYP4C21 (5.4-fold), CYP4G15 (5.2-fold), GST1 (3.5-fold), and ABC transporter 4 (2.1-fold). Our results suggested a potential relationship between the expression of the genes in metabolic processes and insecticide resistance in the studied strain. These results may contribute to the understanding of the mechanisms of insecticide resistance in Ae. aegypti.

Functional genetic validation of key genes conferring insecticide resistance in the major African malaria vector, Anopheles gambiae

Insecticide resistance in Anopheles gambiae mosquitoes can derail malaria control programs, and to overcome it, we need to discover the underlying molecular basis. Here, we characterize 3 genes most often associated with insecticide resistance directly by their overproduction in genetically modified An. gambiae. We show that overexpression of each gene confers resistance to representatives of at least 1 insecticide class, and taken together, the 3 genes provide cross-resistance to all 4 major insecticide classes currently used in public health. These data validate the candidate genes as markers to monitor the spread of resistance in mosquito populations. The modified mosquitoes produced are also valuable tools to prescreen the efficacy of new insecticides against existing resistance mechanisms.

Resistance in Anopheles gambiae to members of all 4 major classes (pyrethroids, carbamates, organochlorines, and organophosphates) of public health insecticides limits effective control of malaria transmission in Africa. Increase in expression of detoxifying enzymes has been associated with insecticide resistance, but their direct functional validation in An. gambiae is still lacking. Here, we perform transgenic analysis using the GAL4/UAS system to examine insecticide resistance phenotypes conferred by increased expression of the 3 genes—Cyp6m2, Cyp6p3, and Gste2—most often found up-regulated in resistant An. gambiae. We report evidence in An. gambiae that organophosphate and organochlorine resistance is conferred by overexpression of GSTE2 in a broad tissue profile. Pyrethroid and carbamate resistance is bestowed by similar Cyp6p3 overexpression, and Cyp6m2 confers only pyrethroid resistance when overexpressed in the same tissues. Conversely, such Cyp6m2 overexpression increases susceptibility to the organophosphate malathion, presumably due to conversion to the more toxic metabolite, malaoxon. No resistant phenotypes are conferred when either Cyp6 gene overexpression is restricted to the midgut or oenocytes, indicating that neither tissue is involved in insecticide resistance mediated by the candidate P450s examined. Validation of genes conferring resistance provides markers to guide control strategies, and the observed negative cross-resistance due to Cyp6m2 gives credence to proposed dual-insecticide strategies to overcome pyrethroid resistance. These transgenic An. gambiae-resistant lines are being used to test the “resistance-breaking” efficacy of active compounds early in their development.

Identification and Functional Analysis of a Delta Class Glutathione S-Transferase Gene Associated with Insecticide Detoxification in Bradysia odoriphaga

A delta class glutathione S-transferase gene (BoGSTd2) is identified from Bradysia odoriphaga for the first time. Developmental expression analysis showed that expression of BoGSTd2 is significantly higher in the fourth instar larval stage and the adult stage. Tissue-specific expression analysis found that BoGSTd2 was expressed predominantly in the midgut and Malpighian tubules in the fourth instar larvae and the abdomen of adults. Expression of BoGSTd2 was significantly upregulated following exposure to chlorpyrifos and clothianidin. In vitro inhibition and metabolic assays indicated that recombinant BoGSTd2 could not directly metabolize chlorpyrifos and clothianidin. Nevertheless, disk diffusion assays indicated that BoGSTd2 plays an important role in protection against oxidative stress. RNAi assays showed that BoGSTd2 participates in the elimination of reactive oxygen species induced by chlorpyrifos and clothianidin. These results strongly suggest that BoGSTd2 plays an important role in chlorpyrifos and clothianidin detoxification in B. odoriphaga by protecting tissues from oxidative stress induced by these insecticides.

Status of insecticide resistance and its biochemical and molecular mechanisms in Anopheles stephensi (Diptera: Culicidae) from Afghanistan

Background

Insecticide resistance of Anopheles stephensi, the main malaria vector in eastern Afghanistan, has been reported previously. This study describes the biochemical and molecular mechanisms of resistance to facilitate effective vector control and insecticide resistance management.

Methods

Mosquito larvae were collected from the provinces of Kunar, Laghman and Nangarhar from 2014 to 2017. The susceptibility of the reared 3–4 days old adults was tested with deltamethrin 0.05%, bendiocarb 0.1%, malathion 5%, permethrin 0.75% and DDT 4%. Cytochrome P450 content and general esterase, glutathione S-transferase (GST) and acetylcholinesterase (AChE) activities were measured in the three field populations and the results were compared with those of the laboratory susceptible An. stephensi Beech strain. Two separate allele-specific PCR assays were used to identify L1014, L1014F and L1014S mutations in the voltage gated sodium channel gene of An. stephensi. Probit analysis, ANOVA and Hardy–Weinberg equilibrium were used to analyse bioassay, biochemical assay and gene frequency data respectively.

Results

The population of An. stephensi from Kunar was susceptible to bendiocarb, apart from this, all populations were resistant to all the other insecticides tested. The differences between all values for cytochrome P450s, general esterases, GSTs and AChE inhibition rates in the Kunar, Laghman and Nangarhar populations were statistically significant when compared to the Beech strain, excluding GST activities between Kunar and Beech due to the high standard deviation in Kunar. The three different sodium channel alleles [L1014 (wild type), L1014F (kdr west) and L1014S (kdr east)] were all segregated in the Afghan populations. The frequencies of kdr east mutation were 22.9%, 32.7% and 35% in Kunar, Laghman and Nangarhar populations respectively. Kdr west was at the lowest frequency of 4.44%.

Conclusions

Resistance to different groups of insecticides in the field populations of An. stephensi from Kunar, Laghman and Nangarhar Provinces of Afghanistan is caused by a range of metabolic and site insensitivity mechanisms, including esterases, cytochrome P450s and GSTs combined with AChE and sodium channel target site insensitivity. The intensity and frequency of these mechanisms are increasing in these populations, calling for urgent reorientation of vector control programmes and implementation of insecticide resistance management strategies.

Mosquitoes cloak their legs to resist insecticides

Malaria incidence has halved since the year 2000, with 80% of the reduction attributable to the use of insecticides. However, insecticide resistance is now widespread, is rapidly increasing in spectrum and intensity across Africa, and may be contributing to the increase of malaria incidence in 2018. The role of detoxification enzymes and target site mutations has been documented in the major malaria vector Anopheles gambiae; however, the emergence of striking resistant phenotypes suggests the occurrence of additional mechanisms. By comparing legs, the most relevant insect tissue for insecticide uptake, we show that resistant mosquitoes largely remodel their leg cuticles via enhanced deposition of cuticular proteins and chitin, corroborating a leg-thickening phenotype. Moreover, we show that resistant female mosquitoes seal their leg cuticles with higher total and different relative amounts of cuticular hydrocarbons, compared with susceptible ones. The structural and functional alterations in Anopheles female mosquito legs are associated with a reduced uptake of insecticides, substantially contributing to the resistance phenotype.

Enzymatic and molecular characterization of insecticide resistance mechanisms in field populations of Aedes aegypti from Selangor, Malaysia

BACKGROUND

Dengue is a serious public health problem worldwide, including in Selangor, Malaysia. Being an important vector of dengue virus, Aedes aegypti are subjected to control measures which rely heavily on the usage of insecticides. Evidently, insecticide resistance in Ae. aegypti, which arise from several different point mutations within the voltage-gated sodium channel genes, has been documented in many countries. Thus, this robust study was conducted in all nine districts of Selangor to understand the mechanisms of resistance to various insecticides in Ae. aegypti. Mosquitoes were collected from dengue epidemic and non-dengue outbreak areas in Selangor.

METHODS

Using the Center for Disease Control and Prevention (CDC) bottle assays, the insecticide resistance status of nine different Ae. aegypti strains from Selangor was accessed. Synergism tests and biochemical assays were conducted to further understand the metabolic mechanisms of insecticide resistance. Polymerase chain reaction (PCR) amplification and sequencing of the IIP-IIS6 as well as IIIS4-IIIS6 regions of the sodium channel gene were performed to enable comparisons between susceptible and resistant mosquito strains. Additionally, genomic DNA was used for allele-specific PCR (AS-PCR) genotyping of the gene to detect the presence of F1534C, V1016G and S989P mutations.

RESULTS

Adult female Ae. aegypti from various locations were susceptible to malathion and propoxur. However, they exhibited different levels of resistance against dichlorodiphenyltrichloroethane (DDT) and pyrethroids. The results of synergism tests and biochemical assays indicated that the mixed functions of oxidases and glutathione S-transferases contributed to the DDT and pyrethroid resistance observed in the present study. Besides detecting three single kdr mutations, namely F1534C, V1016G and S989P, co-occurrence of homozygous V1016G/S989P (double allele) and F1534C/V1016G/S989P (triple allele) mutations were also found in Ae. aegypti. As per the results, the three kdr mutations had positive correlations with the expressions of resistance to DDT and pyrethroids.

CONCLUSIONS

In view of the above outcomes, it is important to seek new tools for vector management instead of merely relying on insecticides. If the latter must be used, regular monitoring of insecticide resistance should also be carried out at all dengue epidemic areas. Since the eggs of Ae. aegypti can be easily transferred from one location to another, it is probable that insecticide-resistant Ae. aegypti can be found at non-dengue outbreak sites as well.

Knock-Down of Gossypol-Inducing Cytochrome P450 Genes Reduced Deltamethrin Sensitivity in Spodoptera exigua (Hübner)

Plants employ an intricate and dynamic defense system that includes physiological, biochemical, and molecular mechanisms to counteract the effects of herbivorous attacks. In addition to their tolerance to phytotoxins, beet armyworm has quickly developed resistance to deltamethrin; a widely used pyrethroid insecticide in cotton fields. The lethal concentration (LC₅₀) required to kill 50% of the population of deltamethrin to gossypol-fed Spodoptera exigua larvae was 2.34-fold higher than the control group, suggesting a reduced sensitivity as a consequence of the gossypol diet. Piperonyl butoxide (PBO) treatment was found to synergize with deltamethrin in gossypol-fed S. exigua larvae. To counteract these defensive plant secondary metabolites, beet armyworm elevates their production of detoxification enzymes, including cytochrome P450 monooxygenases (P450s). Gossypol-fed beet armyworm larvae showed higher 7-ethoxycoumarin-O-deethylase (ECOD) activities and exhibited enhanced tolerance to deltamethrin after 48 and 72 h when compared to the control. Moreover, gossypol pretreated S. exigua larvae showed faster weight gain than the control group after transferring to a deltamethrin-supplemented diet. Meanwhile, gossypol-induced P450s exhibited high divergence in the expression level of two P450 genes: CYP6AB14 and CYP9A98 in the midgut and fat bodies contributed to beet armyworm tolerance to deltamethrin. Knocking down of CYP6AB14 and CYP9A98, via double-stranded RNAs (dsRNA) in a controlled diet, rendered the larvae more sensitive to the insecticide. These data demonstrate that generalist insects can exploit secondary metabolites from host plants to enhance their defense systems against other toxic chemicals. Impairing this defense pathway by RNA interference (RNAi) holds a potential to eliminate the pest’s tolerance to insecticides and, therefore, reduce the required dosages of agrochemicals in pest control.

A marker of glutathione S-transferase-mediated resistance to insecticides is associated with higher Plasmodium infection in the African malaria vector Anopheles funestus

Metabolic resistance to insecticides is threatening malaria control in Africa. However, the extent to which it impacts malaria transmission remains unclear. Here, we investigated the association between a marker of glutathione S-transferase mediated metabolic resistance and Plasmodium infection in field population of Anopheles funestus s.s. in comparison to the A296S-RDL target site mutation. The 119F-GSTe2 resistant allele was present in southern (Obout) (56%) and central (Mibellon) (25%) regions of Cameroon whereas the 296S-RDL resistant allele was detected at 98.5% and 15% respectively. The whole mosquito Plasmodium and sporozoite infection rates were 57% and 14.8% respectively in Obout (n = 508) and 19.7% and 5% in Mibellon (n = 360). No association was found between L119F-GSTe2 genotypes and whole mosquito infection status. However, when analyzing oocyst and sporozoite infection rates separately, the resistant homozygote 119F/F genotype was significantly more associated with Plasmodium infection in Obout than both heterozygote (OR = 2.5; P = 0.012) and homozygote susceptible (L/L119) genotypes (OR = 2.10; P = 0.013). In contrast, homozygote RDL susceptible mosquitoes (A/A296) were associated more frequently with Plasmodium infection than other genotypes (OR = 4; P = 0.03). No additive interaction was found between L119F and A296S. Sequencing of the GSTe2 gene showed no association between the polymorphism of this gene and Plasmodium infection. Glutathione S-transferase metabolic resistance is potentially increasing the vectorial capacity of resistant An. funestus mosquitoes. This could result in a possible exacerbation of malaria transmission in areas of high GSTe2-based metabolic resistance to insecticides.

Exposure to the insecticide-treated bednet PermaNet 2.0 reduces the longevity of the wild African malaria vector Anopheles funestus but GSTe2-resistant mosquitoes live longer

BACKGROUND

Despite the increased report of insecticide resistance in malaria vectors, its impact on mosquito's life-traits after exposure to insecticide-treated nets remains under investigated. Here, we assessed the effects of exposure to PermaNet 2.0 on several life traits of An. gambiae s.l. and An. funestus s.l. field mosquitoes in Cameroon.

METHODOLOGY

Female Anopheles mosquitoes were collected indoor using electric aspirators in southern Cameroon (Obout) in 2016. After assessing the resistance status of F1 from the field collected-mosquitoes, progeny of the first generation (An. funestus s.l.) and seventh generation (An. gambiae s.l.) were used to assess the long-term effect of exposure to PermaNet 2.0 on several life-traits of these vectors (longevity, blood feeding ability, fecundity and fertility) in comparison to untreated net. In addition, the L119F-GSTe2 mutation associated with DDT/pyrethroids resistance in An. funestus was genotyped to assess its association with increased life-span post-exposure.

PRINCIPAL FINDINGS

Both An. funestus and An. gambiae were resistant to pyrethroids and DDT with a greater level in the latter. Pyrethroid-only nets PermaNet 2.0 (17.5% mortality) and Olyset (0% mortality) exhibited a significantly reduced efficacy against An. funestus in contrast to a greater efficacy for PBO-based Nets Olyset Plus (65% mortality), PermaNet 3.0 top (100% mortality). In both species, mosquitoes that survived exposure to PermaNet 2.0 exhibited a significantly reduced longevity than those non-exposed (6.95 days vs 12.46 for An. funestus P<0.001; 8.87 vs 11.25 days for An. gambiae; P<0.001). However, no significant difference was observed for blood feeding and fecundity in both species. In addition, molecular analysis of the L119F-GSTe2 mutation revealed that this mutation is associated with an increase in the chance of surviving after exposure to this net in An. funestus.

CONCLUSIONS

These results show that although the PermaNet 2.0 presents a reduced efficacy against resistant populations, it remains efficient after exposure by reducing the life expectancy of the vectors which could contribute in the reduction of malaria incidence.

The interaction between permethrin exposure and malaria infection affects the host-seeking behaviour of mosquitoes

Background

Insecticide-treated bed nets (ITNs) help to control malaria by mechanically impeding the biting of mosquitoes, by repelling and irritating them and by killing them. In contrast to spatial repellency, irritancy implies that mosquitoes contact the ITN and are exposed to at least a sub-lethal dose of insecticide, which impedes their further blood-seeking. This would weaken the transmission of malaria, if mosquitoes are infectious.

Methods

It was therefore tested whether sub-lethal exposure to permethrin impedes blood-feeding differently in uninfected mosquitoes and in mosquitoes carrying the non-transmissible stage (oocysts) or the infectious stage (sporozoites) of the malaria parasite Plasmodium berghei. In addition, as the degree of irritancy determines the dose of insecticide the mosquitoes may receive, the irritancy to permethrin of infected and uninfected mosquitoes was compared.

Results

In this laboratory setting, sub-lethal exposure to permethrin inhibited the blood-seeking behaviour of Anopheles gambiae mosquitoes for almost 48 h. Although infection by malaria did not affect the irritancy of the mosquitoes to permethrin at either the developmental stage or the infectious stage, both stages of infection shortened the duration of inhibition of blood-seeking.

Conclusions

The results suggest that the impact of ITNs may be weaker for malaria-infected than for uninfected mosquitoes. This will help to understand the global impact of ITNs on the transmission of malaria and gives a more complete picture of the effectiveness of that vector control measure.

Electronic supplementary material

The online version of this article (10.1186/s12936-019-2718-x) contains supplementary material, which is available to authorized users.

Effects of icariside II on brain tissue oxidative stress and Nrf2/HO-1 expression in rats with cerebral ischemia-reperfusion injury1

PURPOSE

To investigate the effects of icariside II on brain tissue oxidative stress and Nrf2/HO-1 expression in rats with cerebral ischemia-reperfusion injury (CIRI).

METHODS

One hundred SD rats were randomly divided into sham-operated, model, and 5, 10 and 20 mg/kg icariside II groups, 20 rats in each group. The middle cerebral artery occlusion model (ischemia for 2 h followed by reperfusion for 24 h) was established in the later 4 groups. In later 3 groups, at reperfusion beginning, the rats were intragastrically administrated with 5, 10 and 20 mg/kg icariside II, respectively. After 24 h of reperfusion, the neurological severity score, cerebral water content and cerebral infarction volume, brain tissue oxidative stress indexes and Nrf2 and HO-1 protein expressions were determined.

RESULTS

Compared with model group, in 20 mg/kg icariside II group the neurological severity score, cerebral water content and cerebral infarction volume, brain tissue ROS content and MDA level were significantly decreased (P<0.05), and the brain tissue SOD, GSH-Px and catalase levels and Nrf2 and HO-1 protein levels were significantly increased (P<0.05).

CONCLUSION

Icariside II can alleviate the CIRI in rats through reducing brain tissue oxidative stress and improving Nrf2/HO-1 expression.

Fitness Costs of the Glutathione S-Transferase Epsilon 2 (L119F-GSTe2) Mediated Metabolic Resistance to Insecticides in the Major African Malaria Vector Anopheles Funestus

Metabolic resistance to insecticides threatens malaria control. However, little is known about its fitness cost in field populations of malaria vectors, thus limiting the design of suitable resistance management strategies. Here, we assessed the association between the glutathione S-transferase GSTe2-mediated metabolic resistance and life-traits of natural populations of Anopheles funestus. A total of 1200 indoor resting blood-fed female An. funestus (F₀) were collected in Mibellon, Cameroon (2016/2017), and allowed to lay eggs individually. Genotyping of F1 mosquitoes for the L119F-GSTE2 mutation revealed that L/L119-homozygote susceptible (SS) mosquitoes significantly laid more eggs than heterozygotes L119F-RS (odds ratio (OR) = 2.06; p < 0.0001) and homozygote resistant 119F/F-RR (OR = 2.93; p < 0.0001). L/L119-SS susceptible mosquitoes also showed the higher ability for oviposition than 119F/F-RR resistant (OR = 2.68; p = 0.0002) indicating a reduced fecundity in resistant mosquitoes. Furthermore, L119F-RS larvae developed faster (nine days) than L119F-RR and L119F-SS (11 days) (X² = 11.052; degree of freedom (df) = 4; p = 0.02) suggesting a heterozygote advantage effect for larval development. Interestingly, L/L119-SS developed faster than 119F/F-RR (OR = 5.3; p < 0.0001) revealing an increased developmental time in resistant mosquitoes. However, genotyping and sequencing revealed that L119F-RR mosquitoes exhibited a higher adult longevity compared to RS (OR > 2.2; p < 0.05) and SS (OR > 2.1; p < 0.05) with an increased frequency of GSTe2-resistant haplotypes in mosquitoes of D30 after adult emergence. Additionally, comparison of the expression of GSTe2 revealed a significantly increased expression from D1-D30 after emergence of adults (Anova test (F) = 8; df= 3; p = 0.008). The negative association between GSTe2 and some life traits of An. funestus could facilitate new resistance management strategies. However, the increased longevity of GSTe2-resistant mosquitoes suggests that an increase in resistance could exacerbate malaria transmission.

Insecticide resistance status, frequency of L1014F Kdr and G119S Ace-1 mutations, and expression of detoxification enzymes in Anopheles gambiae (s.l.) in two regions of northern Benin in preparation for indoor residual spraying

BACKGROUND

This study aims to provide baseline data on the resistance status to insecticides, the frequency of mechanisms involved and the impact of the association with the synergist piperonyl butoxide (PBO) on resistant Anopheles gambiae (s.l.) populations in two regions of northern Benin, prior to an indoor residual spraying campaign and introduction of next generation long-lasting insecticidal nets (LLINs) incorporating PBO.

METHODS

Adult Anopheles gambiae (s.l.) originating from larvae collected in two study regions (Alibori within the Kandi-Gogounou-Segbana districts and Donga within the Djougou-Copargo-Ouake districts) were tested with impregnated papers (bendiocarb 0.1%, pirimiphos-methyl 0.25%, permethrin 0.75% and deltamethrin 0.05%). The synergist PBO was used to check for the involvement of detoxification enzymes in pyrethroid resistant populations. Molecular analyses were performed for the identification of species within the Anopheles gambiae (s.l.) complex and kdr L1014F and G119S Ace-1 mutations. Biochemical assays assessed the activity of detoxification enzymes.

RESULTS

Anopheles gambiae (s.l.) was resistant to pyrethroids, with a mortality range of 25-83% with deltamethrin and 6-55% with permethrin. A significant increase in mortality was observed after pre-exposure to PBO for both deltamethrin (63-99%) and permethrin (56-99%). With bendiocarb, An. gambiae (s.l.) were susceptible in Kandi (99% mortality), with possible resistance (92-95%) recorded in Djougou, Copargo, Gogounou, Ouake and Segbana. All study populations were fully susceptible to pirimiphos-methyl. The frequencies of resistant mutations varied according to species and sites: 0.67-0.88 for L1014F kdr and 0-0.06 for G119S Ace-1. Three study locations (Djougou, Gogounou and Kandi) showed high oxidase activity and four sites (Djougou, Ouake, Copargo and Kandi) showed elevated esterase activity.

CONCLUSIONS

This study confirms resistance to pyrethroids and suggests emerging bendiocarb resistance in An. gambiae (s.l.) populations in northern Benin. However, recovery of susceptibility to pyrethroids after PBO exposure, and susceptibility to organophosphates in the An. gambiae (s.l.) populations indicate that next generation LLINs incorporating PBO synergist combined with an indoor residual spraying (IRS) campaign with organophosphate insecticides may be regarded as alternative control tools.

Biodegradation of chlorpyrifos and 3,5,6-trichloro-2-pyridinol by the epiphytic yeasts Rhodotorula glutinis and Rhodotorula rubra

The possible involvement of the epiphytic yeasts Rhodotorula glutinis and Rhodotorula rubra in the biodegradation of the insecticide chlorpyrifos and its metabolite 3,5,6-trichloro-2-pyridinol (TCP), in pure cultures and in plant surfaces (tomato fruits) was investigated. Higher biodegradation rates were observed as the concentration of chlorpyrifos and the inoculum of the microorganisms were increased, while the yeasts proved to be more active at 25 and 15 °C. The presence of glucose in the mineral nutrient medium, as an extra source of carbon, delayed the biodegradation by Rhodotorula glutinis, while Rhodotorula rubra proved to be more active. The detection and quantification of the parent compound and TCP was successfully achieved using a LC/MS/MS chromatographic system. The in vitro enzymatic assays applied suggested that esterases may be involved in the biodegradation of chlorpyrifos, a fact that was further enhanced after the addition of the synergists triphenyl phosphate, diethyl maleate and piperonyl butoxide in the biodegradation trials. The decrease of chlorpyrifos residues on tomato fruits confirmed the corresponding on pure cultures, resulting in the suggestion that the yeasts R. glutinis and R. rubra can possibly be used successfully for the removal or detoxification of chlorpyrifos residues on tomatoes.

Mitotype Interacts With Diet to Influence Longevity, Fitness, and Mitochondrial Functions in Adult Female Drosophila

Mitochondrial DNA (mtDNA) and the dietary macronutrient ratio are known to influence a wide range of phenotypic traits including longevity, fitness and energy production. Commonly mtDNA mutations are posited to be selectively neutral or reduce fitness and, to date, no selectively advantageous mtDNA mutations have been experimentally demonstrated in adult female Drosophila. Here we propose that a ND V161L mutation interacted with diets differing in their macronutrient ratios to influence organismal physiology and mitochondrial traits, but further studies are required to definitively show no linked mtDNA mutations are functionally significant. We utilized two mtDNA types (mitotypes) fed either a 1:2 Protein: Carbohydrate (P:C) or 1:16 P:C diet. When fed the former diet, Dahomey females harboring the V161L mitotype lived longer than those with the Alstonville mitotype and had higher climbing, basal reactive oxygen species (ROS) and elevated glutathione S-transferase E1 expression. The short lived Alstonville females ate more, had higher walking speed and elevated mitochondrial functions as suggested by respiratory control ratio (RCR), mtDNA copy number and expression of mitochondrial transcription termination factor 3. In contrast, Dahomey females fed 1:16 P:C were shorter lived, had higher fecundity, walking speed and mitochondrial functions. They had reduced climbing. This result suggests that mtDNA cannot be assumed to be a strictly neutral evolutionary marker when the dietary macronutrient ratio of a species varies over time and space and supports the hypothesis that mtDNA diversity may reflect the amount of time since the last selective sweep rather than strictly demographic processes.

Molecular basis of permethrin and DDT resistance in an Anopheles funestus population from Benin

BACKGROUND

Insecticide resistance in Anopheles mosquitoes is threatening the success of malaria control programmes. In order to implement suitable insecticide resistance management strategies, it is necessary to understand the underlying mechanisms involved. To achieve this, the molecular basis of permethrin and DDT resistance in the principal malaria vector, Anopheles funestus from inland Benin (Kpome), was investigated.

RESULTS

Here, using a microarray-based genome-wide transcription and qRT-PCR analysis, we showed that metabolic resistance mechanisms through over-expression of cytochrome P450 and glutathione S-transferase genes (GSTs) are a major contributor to DDT and permethrin resistance in Anopheles funestus from Kpome. The GSTe2 gene was the most upregulated detoxification gene in both DDT- [fold-change (FC: 16.0)] and permethrin-resistant (FC: 18.1) mosquitoes suggesting that upregulation of this gene could contribute to DDT resistance and cross-resistance to permethrin. CYP6P9a and CYP6P9b genes that have been previously associated with pyrethroid resistance were also significantly overexpressed with FC 5.4 and 4.8, respectively, in a permethrin resistant population. Noticeably, the GSTs, GSTd1-5 and GSTd3, were more upregulated in DDT-resistant than in permethrin-resistant Anopheles funestus suggesting these genes are more implicated in DDT resistance. The absence of the L1014F or L1014S kdr mutations in the voltage-gated sodium channel gene coupled with the lack of directional selection at the gene further supported that knockdown resistance plays little role in this resistance.

CONCLUSIONS

The major role played by metabolic resistance to pyrethroids in this An. funestus population in Benin suggests that using novel control tools combining the P450 synergist piperonyl butoxide (PBO), such as PBO-based bednets, could help manage the growing pyrethroid resistance in this malaria vector in Benin.

Diversity and convergence of mechanisms involved in pyrethroid resistance in the stored grain weevils, Sitophilus spp

Target-site mutations and changes in insect metabolism or behavior are common mechanisms in insecticide-resistant insects. The co-occurrence of such mechanisms in a pest strain is a prominent threat to their management, particularly when alternative compounds are scarce. Pyrethroid resistance among stored grain weevils (i.e., Sitophilus spp.) is an example of a long-standing concern, for which reports of resistance generally focus on a single mechanism in a single species. Here, we investigated pyrethroid resistance in maize and rice weevils (i.e., Sitophilus zeamais and S. oryzae), exploring potential knockdown resistance (kdr) mutations in their sodium channels (primary site for pyrethroid actions) and potential changes in their detoxification and walking processes. Resistance in pyrethroid-resistant rice weevils was associated with the combination of a kdr mutation (L1014F) and increases in walking and detoxification activities, while another kdr mutation (T929I) combined with increases in walking activity were the primary pyrethroid resistance mechanisms in maize weevils. Our results suggest that the selection of pyrethroid-resistant individuals in these weevil species may result from multiple and differential mechanisms because the L1014F mutation was only detected in Latin American rice weevils (e.g., Brazil, Argentina and Uruguay), not in Australian and Turkish rice weevils or Brazilian maize weevils.

Laboratory selection of Aedes aegypti field populations with the organophosphate malathion: Negative impacts on resistance to deltamethrin and to the organophosphate temephos

Background

Resistance to pyrethroids and to the organophosphate temephos is widespread in Brazilian populations of the dengue vector, Aedes aegypti. Thereof, since 2009 Insect Growth Regulators are employed as larvicides, and malathion is used against adults.

Methodology/Principal findings

We performed laboratory selection with malathion of two A. aegypti field populations initially susceptible to this organophosphate but resistant to temephos and deltamethrin. A fixed malathion dose inducing at least 80% mortality in the first generation, was used throughout the selection process, interrupted after five generations, when the threshold of 20% mortality was reached. For each population, three experimental and two control groups, not exposed to insecticides, were kept independently. For both populations, quantitative bioassays revealed, in the selected groups, acquisition of resistance to malathion and negative impact of malathion selection on deltamethrin and temephos resistance levels. In the control groups resistance to all evaluated insecticides decreased except, unexpectedly, to deltamethrin. Analysis of the main resistance mechanisms employed routine methodologies: biochemical and molecular assays for, respectively, metabolic resistance and quantification of the Na_V pyrethroid target main kdr mutations at positions 1016 and 1534. No diagnostic alteration could be specifically correlated with malathion selection, neither with the unusual deltamethrin increase in resistance levels observed in the control groups.

Conclusions/Significance

Our results confirm the multifactorial character of insecticide resistance and point to the need of high throughput methodologies and to the study of additional field vector populations in order to unravel resistance mechanisms.

Dengue, Zika and chikungunya viruses affect millions of people worldwide. Due to the lack of specific antivirals or to the limited supply of vaccines, focus remains on the control of the main vector, Aedes aegypti. Although the importance of social participation in the elimination of A. aegypti breeding sites is increasingly recognized, chemical control is still an important component of vector control. The exaggerated use of insecticides results in the spread of resistance and, consequently, in the loss of their effectiveness. In Brazil, malathion is the last adulticide available to the control of A. aegypti, due to the widespread resistance to pyrethroids. In order to anticipate what could occur in the field, we exposed two vector populations to selection with malathion. Both malathion and temephos, a larvicide largely employed, are organophosphates; however, they are structurally distinct molecules and seem to elicit different resistance mechanisms. We confirmed this issue: selection with malathion had a negative impact on temephos resistance compared to groups reared without any insecticide. Indeed, the variety of responses of both vector populations to the various insecticides points to the participation of multiple resistance mechanisms and confirms previous assumptions regarding the difficulty of identifying diagnostic insecticide resistance mechanisms.

Overexpression of cytochrome P450s in a lambda-cyhalothrin resistant population of Apolygus lucorum (Meyer-Dür)

The mirid bug, Apolygus lucorum Meyer-Dür, has been an important pest of cotton crop in China, and is primarily controlled with insecticides, such as pyrethroids. To elucidate the potential resistant mechanisms of A. lucorum to lambda-cyhalothrin, a series of biological, biochemical, and molecular assays were conducted in the reference (AL-S) and lambda-cyhalothrin-resistant (AL-R) populations. Comparison of the molecular target of pyrethroid insecticides, voltage-gated sodium channel, revealed that there were no mutation sites in the resistant population, indicating target insensitivity is not responsible for increased resistance of AL-R to lambda-cyhalothrin. Furthermore, the synergism assays and the activities of detoxification enzymes were performed to determine detoxification mechanism conferring the lambda-cyhalothrin resistance. In the tested synergists, the piperonyl butoxide had the highest synergism ratio against lambda-cyhalothrin, which was up to five-fold in both populations. In addition, the result also showed that only cytochrome P450 had significantly higher O-deethylase activity with 7-ethoxycoumarin (1.78-fold) in AL-R population compared with AL-S population. Seven cytochrome P450 genes were found to be significantly overexpressed in the resistant AL-R population compared with AL-S population. Taken together, these results demonstrate that multiple over-transcribed cytochrome P450 genes would be involved in the development of lambda-cyhalothrin resistance in AL-R population.

Cloning and Expression of the Organophosphate Pesticide-Degrading α-β Hydrolase Gene in Plasmid pMK-07 to Confer Cross-Resistance to Antibiotics

Pesticide residual persistence in agriculture soil selectively increases the pesticide-degrading population and transfers the pesticide-degrading gene to other populations, leading to cross-resistance to a wide range of antibiotics. The enzymes that degrade pesticides can also catabolize the antibiotics by inducing changes in the gene or protein structure through induced mutations. The present work focuses on the pesticide-degrading bacteria isolated from an agricultural field that develop cross-resistance to antibiotics. This cross-resistance is developed through catabolic gene clusters present in an extrachromosomal plasmid. A larger plasmid (236.7 Kbp) isolated from Bacillus sp. was sequenced by next-generation sequencing, and important features such as α-β hydrolase, DNA topoisomerase, DNA polymerase III subunit beta, reverse transcriptase, plasmid replication rep X, recombination U, transposase, and S-formylglutathione hydrolase were found in this plasmid. Among these, the α-β hydrolase enzyme is known for the degradation of organophosphate pesticides. The cloning and expression of the α-β hydrolase gene imply nonspecific cleavage of antibiotics through a cross-resistance phenomenon in the host. The docking of α-β hydrolase with a spectrum of antibiotics showed a high G-score against chloramphenicol (-3.793), streptomycin (-2.865), cefotaxime (-5.885), ampicillin (-4.316), and tetracycline (-3.972). This study concludes that continuous exposure to pesticide residues may lead to the emergence of multidrug-resistant strains among the wild microbial flora.

Differentially expressed microRNAs associated with changes of transcript levels in detoxification pathways and DDT-resistance in the Drosophila melanogaster strain 91-R

Dichloro-diphenyl-trichloroethane (DDT) resistance among arthropod species is a model for understanding the molecular adaptations in response to insecticide exposures. Previous studies reported that DDT resistance may involve one or multiple detoxification genes, such as cytochrome P450 monooxygenases (P450s), glutathione S-transferases (GSTs), esterases, and ATP binding cassette (ABC) transporters, or changes in the voltage-sensitive sodium channel. However, the possible involvement of microRNAs (miRNAs) in the post-transcriptional regulation of genes associated with DDT resistance in the Drosophila melanogaster strain 91-R remains poorly understood. In this study, the majority of the resulting miRNAs discovered in small RNA libraries from 91-R and the susceptible control strain, 91-C, ranged from 16-25 nt, and contained 163 precursors and 256 mature forms of previously-known miRNAs along with 17 putative novel miRNAs. Quantitative analyses predicted the differential expression of ten miRNAs between 91-R and 91-C, and, based on Gene Ontology and pathway analysis, these ten miRNAs putatively target transcripts encoding proteins involved in detoxification mechanisms. RT-qPCR validated an inverse correlation between levels of differentially-expressed miRNAs and their putatively targeted transcripts, which implies a role of these miRNAs in the differential regulation of detoxification pathways in 91-R compared to 91-C. This study provides evidence associating the differential expression of miRNAs in response to multigenerational DDT selection in Drosophila melanogaster and provides important clues for understanding the possible roles of miRNAs in mediating insecticide resistance traits.

Antioxidant Role of PcGSTd1 in Fenpropathrin Resistant Population of the Citrus Red Mite, Panonychus citri (McGregor)

The citrus red mite, Panonychus citri, a major citrus pest distributed worldwide, has evolved severe resistance to various classes of chemical acaricides/insecticides including pyrethroids. It is well known that the resistance to pyrethroids is mainly caused by point mutations of voltage-gated sodium channel gene in a wide range of pests. However, increasing number of evidences support that pyrethroids resistance might also be resulted from the integrated mechanisms including metabolic mechanisms. In this study, firstly, comparative analysis of RNA-seq data showed that multiple detoxification genes, including a GSTs gene PcGSTd1, were up-regulated in a fenpropathrin-resistant population compared with the susceptible strain (SS). Quantitative real time-PCR results showed that the exposure of fenpropathrin had an induction effect on the transcription of PcGSTd1 in a time-dependent manner. In vitro inhibition and metabolic assay of recombinant PcGSTd1 found that fenpropathrin might not be metabolized directly by this protein. However, its antioxidant role in alleviating the oxidative stress caused by fenpropathrin was demonstrated via the reversely genetic experiment. Our results provide a list of candidate genes which may contribute to a multiple metabolic mechanisms implicated in the evolution of fenpropathrin resistance in the field population of P. citri. Furthermore, during the detoxification process, PcGSTd1 plays an antioxidant role by detoxifying lipid peroxidation products induced by fenpropathrin.

Viral infection induces different detoxification enzyme activities in insecticide-resistant and -susceptible brown planthopper Nilaparvata lugens strains

This study aimed to describe the relationship between viral infection in Nilaparvata lugens (Stål), the brown planthopper (BPH), and different insecticide susceptibilities. BPH-resistant strains were selected using fenthion (an organophosphate) or etofenprox (a pyrethroid); a susceptible strain was used as the baseline colony before insecticide selection. All strains were infected with rice ragged stunt virus (RRSV) or rice grassy stunt virus (RGSV), after which the activities of three detoxification enzymes, cytochrome-P450-monooxygenase (P450), glutathione S-transferase (GST), and carboxylesterase (CE), were compared. Males of the strains selected for both insecticides showed high P450 and GST-CDNB activities. The activity of all enzymes was higher in males than in females, as a whole. However, males of the susceptible strain infected with RRSV showed decreased CE and GST-CDNB activities. BPH with low susceptibility to etofenprox showed a marked increase in P450 activity after RRSV infection; the GST-CDNB activity of females in the insecticide-resistant strain increased. RGSV infection induced high CE and P450 activities in etofenprox-selected females. The RRSV infection rate, but not the RGSV, decreased in etofenprox-selected strains.

The effect of metal pollution on the life history and insecticide resistance phenotype of the major malaria vector Anopheles arabiensis (Diptera: Culicidae)

Metal exposure is one of the commonest anthropogenic pollutants mosquito larvae are exposed to, both in agricultural and urban settings. As members of the Anopheles gambiae complex, which contains several major malaria vector species including An. arabiensis, are increasingly adapting to polluted environments, this study examined the effects of larval metal exposure on various life history traits of epidemiological importance. Two laboratory strains of An. arabiensis, SENN (insecticide susceptible) and SENN DDT (insecticide resistant), were reared in maximum acceptable toxicity concentrations, (MATC-the highest legally accepted concentration) of cadmium chloride, lead nitrate and copper nitrate. Following these exposures, time to pupation, adult size and longevity were determined. Larvae reared in double the MATC were assessed for changes in malathion and deltamethrin tolerance, measured by lethal time bottle bioassay, as well as changes in detoxification enzyme activity. As defence against oxidative stress has previously been demonstrated to affect the expression of insecticide resistance, catalase, glutathione peroxidase and superoxide dismutase activity was assessed. The relative metal toxicity to metal naïve larvae was also assessed. SENN DDT larvae were more tolerant of metal pollution than SENN larvae. Pupation in SENN larvae was significantly reduced by metal exposure, while adult longevity was not affected. SENN DDT showed decreased adult size after larval metal exposure. Adult insecticide tolerance was increased after larval metal exposure, and this effect appeared to be mediated by increased β-esterase, cytochrome P450 and superoxide dismutase activity. These data suggest an enzyme-mediated positive link between tolerance to metal pollutants and insecticide resistance in adult mosquitoes. Furthermore, exposure of larvae to metal pollutants may have operational consequences under an insecticide-based vector control scenario by increasing the expression of insecticide resistance in adults.