Permethrin induction of multiple cytochrome P450 genes in insecticide resistant mosquitoes, Culex quinquefasciatus

The role of P450 enzymes in malaria and other vector-borne infectious diseases

Vector-borne infectious diseases are still an important global health problem. Malaria is the most important among them, mainly pediatric, life-threatening disease. Malaria and other vector-borne disorders caused by parasites, bacteria, and viruses have a strong impact on public health and significant economic costs. Most vector-borne diseases could be prevented by vector control, with attention to the ecological and biodiversity conservation aspects. Chemical control with pesticides and insecticides is widely used as a measure of prevention although increasing resistance to insecticides is a serious issue in vector control. Metabolic resistance is the most common mechanism and poses a big challenge. Insect enzyme systems, including monooxygenase CYP P450 enzymes, are employed by vectors mainly to metabolize insecticides thus causing resistance. The discovery and application of natural specific inhibitors/blockers of vector P450 enzymes as synergists for commonly used pesticides will contribute to the "greening" of insecticides. Besides vector CYPs, host CYP enzymes could also be exploited to fight against vector-borne diseases: using mostly their detoxifying properties and involvement in the immune response. Here, we review published research data on P450 enzymes from all players in vector-borne infections, that is, pathogens, vectors, and hosts, regarding the potential role of CYPs in disease. We discuss strategies on how to exploit cytochromes P450 in vector-borne disease control.

A masked gene concealed hand in glove in the forkhead protein crocodile regulates the predominant detoxification CYP6DA1 in Aphis gossypii Glover

Cytochrome P450-mediated metabolism is an important mechanism of insecticide resistance, most studies show upregulated transcript levels of P450s in resistant insect strains. Our previous studies illustrated that some upregulated P450s were associated with cyantraniliprole resistance, and it is more comprehensive to use the tissue specificity of transcriptomes to compare resistant (CyR) and susceptible (SS) strains. In this study, the expression profiles of P450s in a CyR strain compared with a SS strain in remaining carcass or midgut were investigated by RNA sequencing, and candidate genes were selected for functional study. Drosophila melanogaster bioassays suggested that ectopic overexpression of CYP4CK1, CYP6CY5, CYP6CY9, CYP6CY19, CYP6CZ1 and CYP6DA1 in flies was sufficient to confer cyantraniliprole resistance, among which CYP6DA1 was the predominant contributor to resistance (12.24-fold). RNAi suppression of CYP4CK1, CYP6CY5, CYP6CY9 and CYP6DA1 significantly increased CyR aphid sensitivity to cyantraniliprole. The CYP6DA1 promoter had two predicted binding sites for crocodile (CROC), an intron-free ORF with bidirectional transcription yielding CROC (+) and CROC (-). Y1H, RNAi and EMSA found that CROC (-) was a transcription factor directly regulating CYP6DA1 expression. In conclusion, P450 genes contribute to cyantraniliprole resistance, and the transcription factor CROC (-) regulates the expression of CYP6DA1 in A. gossypii.

Time-series analysis of transcriptomic changes due to permethrin exposure reveals that Aedes aegypti undergoes detoxification metabolism over 24 h

Insecticide resistance is a multifaceted response and an issue across taxa. Aedes aegypti, the mosquito that vectors Zika, dengue, chikungunya, and yellow fever, demonstrates high levels of pyrethroid resistance across the globe, presenting a challenge to public health officials. To examine the transcriptomic shifts across time after exposure to permethrin, a 3'Tag-Seq analysis was employed on samples 6, 10, and 24 h after exposure along with controls. Differential expression analysis revealed significant shifts in detoxifying enzymes and various energy-producing metabolic processes. These findings indicate significant alterations in gene expression associated with key energy mobilization pathways within the system. These changes encompass a coordinated response involving lipolysis, beta-oxidation, and the citric acid cycle, required for the production of energetic molecules such as ATP, NADH, NADPH, and FADH. These findings highlight a complex interplay of metabolic processes that may have broader implications for understanding insect physiology and response to environmental stimuli. Among the upregulated detoxifying enzymes are cytochrome P450s, glutathione s-transferases and peroxidases, and ATP-binding cassette transporters. Additionally, eight heat shock genes or genes with heat shock domains exhibit the highest fold change across time. Twenty-four hours after exposure, samples indicate a global downregulation of these processes, though principal component analysis suggests lasting signatures of the response. Understanding the recovery response to insecticide exposure provides information on possible new genetic and synergist targets to explore.

Lack of fitness costs in dsRNA-resistant Leptinotarsa decemlineata ([Coleoptera]: [Chrysomelidae])

The Colorado potato beetle, Leptinotarsa decemlineata (Say) ([Coleoptera]: [Chrysomelidae]), is the most important defoliator of solanaceous plants worldwide. This insect displays a notorious ability in adapting to biological and synthetic insecticides, although in some cases this adaptation carries relevant fitness costs. Insecticidal gene silencing by RNA interference is a novel mode of action pesticide against L. decemlineata that is activated by ingestion of a double stranded RNA (dsRNA) targeting a vital L. decemlineata gene. We previously reported laboratory selection of a > 11,000-fold resistant strain of L. decemlineata to a dsRNA delivered topically to potato leaves. In this work, we tested the existence of fitness costs in this dsRNA-resistant colony by comparing biological parameters to the parental strain and an additional susceptible reference strain. Biological parameters included length of egg incubation period, number of eggs per clutch, egg viability, larval viability, length of larval and pupal periods, adult emergence, number of eggs laid per day, sex ratio, and adult longevity. Comparisons between the 3 beetle strains detected no fitness costs associated with resistance to dsRNA. This information is important to guide effective insect resistance management plans for dsRNA insecticides against L. decemlineata applied topically to potato leaves.

Selection for insecticide resistance can promote Plasmodium falciparum infection in Anopheles

Insecticide resistance is under strong selective pressure in Anopheles mosquitoes due to widespread usage of insecticides in vector control strategies. Resistance mechanisms likely cause changes that profoundly affect mosquito physiology, yet it remains poorly understood how selective pressures imposed by insecticides may alter the ability of the mosquito to host and transmit a Plasmodium infection. From pyrethroid-resistant field-derived Anopheles gambiae s.l. mosquitoes, we established resistant (RES) and susceptible (SUS) colonies by either selection for, or loss of insecticide resistance. We show increased oocyst intensity and growth rate as well as increased sporozoite prevalence and intensity in RES compared to SUS females infected with Plasmodium falciparum. The increase in infection intensity in RES females was not associated with the presence of the kdrL1014F mutation and was not impacted by inhibition of Cytochrome P450s. The lipid transporter lipophorin (Lp), which was upregulated in RES compared to SUS, was at least partly implicated in the increased intensity of P. falciparum but not directly involved in the insecticide resistance phenotype. Interestingly, we observed that although P. falciparum infections were not affected when RES females were exposed to permethrin, these females had decreased lipid abundance in the fat body following exposure, pointing to a possible role for lipid mobilization in response to damage caused by insecticide challenge. The finding that selection for insecticide resistance can increase P. falciparum infection intensities and growth rate reinforces the need to assess the overall impact on malaria transmission dynamics caused by selective pressures mosquitoes experience during repeated insecticide challenge.

Resistance monitoring and mechanism in the fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) for chlorantraniliprole from Sichuan Province, China

The fall armyworm, Spodoptera frugiperda (Noctuidae: Lepidoptera), is a wide-reaching notorious insect pest of important cereal crops, which has developed resistance to multiple classes of insecticides. It invaded the Sichuan Province of China in 2019. In this study, we performed resistance monitoring of insecticides for 11 field-collected populations from Sichuan, and all the populations were susceptible to emamectin benzoate and chlorpyrifos. The variations in resistance level to indoxacarb (resistance ratio (RR), 9.23-45.53-fold), spinetoram (RR, 4.32-18.05-fold), and chlorantraniliprole (RR, 2.02-10.39-fold) were observed among these populations. To investigate the resistance mechanism of chlorantraniliprole, synergism tests were performed and showed that piperonyl butoxide had a slight synergistic effect on chlorantraniliprole for the QJ-20 population (1.43-fold) in moderate resistance (RR, 10.39-fold) compared with the treatment group without synergist. Furthermore, the expression scanning for resistance-related genes showed that five P450 genes (CYP6AE43, CYP321A8, CYP305A1, CYP49A1, and CYP306A1) and the ryanodine receptor gene (Ryr, chlorantraniliprole target) were overexpressed in the QJ-20 population. These results indicated that the fall armyworm in Sichuan has exhibited diverse susceptibilities to several classes of insecticides, and the overexpression of Ryr and several P450 genes may contribute to the development of resistance in S. frugiperda to chlorantraniliprole.

Potential key genes involved in metabolic resistance to malathion in the southern house mosquito, Culex quinquefasciatus, and functional validation of CYP325BC1 and CYP9M12 as candidate genes using RNA interference

BACKGROUND

Metabolic detoxification is one of the major mechanisms contributing to the development of resistance in mosquitoes, including the southern house mosquito, Culex quinquefasciatus. The three major detoxification supergene families, cytochrome P450s, glutathione S-transferases and general esterases, have been demonstrated to play an important role in metabolic resistance. In this study, we performed differential gene expression analysis based on high-throughput transcriptome sequencing on samples from four experimental groups to give insight into key genes involved in metabolic resistance to malathion in Cx. quinquefasciatus. We conducted a whole transcriptome analysis of field captured wild Cx. quinquefasciatus from Harris County (WI), Texas and a malathion susceptible laboratory-maintained Sebring colony (CO) to investigate metabolic insecticide resistance. Field captured mosquitoes were also phenotypically classified into the malathion resistant and malathion susceptible groups following a mortality response measure conducted using a Centers for Disease Control and Prevention (CDC) bottle assay. The live (MR) and dead (MS) specimens from the bottle assay, along with an unselected WI sample and a CO sample were processed for total RNA extraction and subjected to whole-transcriptome sequencing.

RESULTS

We demonstrated that the genes coding for detoxification enzymes, particularly cytochrome P450s, were highly up-regulated in the MR group compared to the MS group with similar up-regulation observed in the WI group compared to the CO group. A total of 1,438 genes were differentially expressed in comparison between MR and MS group, including 614 up-regulated genes and 824 down-regulated genes. Additionally, 1,871 genes were differentially expressed in comparison between WI and CO group, including 1,083 up-regulated genes and 788 down-regulated genes. Further analysis on differentially expressed genes from three major detoxification supergene families in both comparisons resulted in 16 detoxification genes as candidates potentially associated with metabolic resistance to malathion. Knockdown of CYP325BC1 and CYP9M12 using RNA interference on the laboratory-maintained Sebring strain significantly increased the mortality of Cx. quinquefasciatus after exposure to malathion.

CONCLUSION

We generated substantial transcriptomic evidence on metabolic detoxification of malathion in Cx. quinquefasciatus. We also validated the functional roles of two candidate P450 genes identified through DGE analysis. Our results are the first to demonstrate that knockdown of CYP325BC1 and CYP9M12 both significantly increased malathion susceptibility in Cx. quinquefasciatus, indicating involvement of these two genes in metabolic resistance to malathion.

Multiple-P450 Gene Co-Up-Regulation in the Development of Permethrin Resistance in the House Fly, Musca domestica

This paper reports a study conducted at the whole transcriptome level to characterize the P450 genes involved in the development of pyrethroid resistance, utilizing expression profile analyses of 86 cytochrome P450 genes in house fly strains with different levels of resistance to pyrethroids/permethrin. Interactions among the up-regulated P450 genes and possible regulatory factors in different autosomes were examined in house fly lines with different combinations of autosomes from a resistant house fly strain, ALHF. Eleven P450 genes that were significantly up-regulated, with levels > 2-fold those in the resistant ALHF house flies, were in CYP families 4 and 6 and located on autosomes 1, 3 and 5. The expression of these P450 genes was regulated by trans- and/or cis-acting factors, especially on autosomes 1 and 2. An in vivo functional study indicated that the up-regulated P450 genes also conferred permethrin resistance in Drosophila melanogaster transgenic lines. An in vitro functional study confirmed that the up-regulated P450 genes are able to metabolize not only cis- and trans-permethrin, but also two metabolites of permethrin, PBalc and PBald. In silico homology modeling and the molecular docking methodology further support the metabolic capacity of these P450s for permethrin and substrates. Taken together, the findings of this study highlight the important function of multi-up-regulated P450 genes in the development of insecticide resistance in house flies.

Assessing pyrethroid resistance status in the Culex pipiens complex (Diptera: Culicidae) from the northwest suburbs of Chicago, Illinois using Cox regression of bottle bioassays and other detection tools

Culex pipiens complex is an important vector of epizootic and zoonotic pathogens, including West Nile virus. Chicago, Illinois and its suburbs have suffered high incidence of human West Nile virus infections in the past. This makes abatement programs in and around the Chicago area an essential service. The control of Cx. pipiens is often complicated by rapidly evolving resistance to pyrethroids, which are the most widely used chemical class in US mosquito abatement programs. The present study assessed Sumithrin® resistance in Cx. pipiens collected from five locations around Cook County, Illinois, neighboring the city limits of Chicago. According to CDC guidelines, samples from all five locations demonstrated some resistance to Sumithrin®. When assessed with Anvil®, a formulated product made of Sumithrin® synergized with piperonyl butoxide, susceptibility was rescued in mosquitoes from three out of the five locations, suggesting involvement of mixed-function oxidases and/or carboxylesterases in Sumithrin® resistance at these locations. Not all locations had susceptibility rescued by Anvil®, but these locations had relatively low knockdown resistance allele frequencies, suggesting that mechanisms other than knockdown resistance may be involved. Enzyme activities did not reveal any marked trends that could be related back to mortality in the bottle bioassays, which highlights the need for multiple types of assays to infer enzymatic involvement in resistance. Future directions in pyrethroid resistance management in Chicago area Cx. pipiens are discussed.

Ambivalent Roles of Oxidative Stress in Triangular Relationships among Arthropod Vectors, Pathogens and Hosts

Blood-feeding arthropods, particularly ticks and mosquitoes are considered the most important vectors of arthropod-borne diseases affecting humans and animals. While feeding on blood meals, arthropods are exposed to high levels of reactive oxygen species (ROS) since heme and other blood components can induce oxidative stress. Different ROS have important roles in interactions among the pathogens, vectors, and hosts. ROS influence various metabolic processes of the arthropods and some have detrimental effects. In this review, we investigate the various roles of ROS in these arthropods, including their innate immunity and the homeostasis of their microbiomes, that is, how ROS are utilized to maintain the balance between the natural microbiota and potential pathogens. We elucidate the mechanism of how ROS are utilized to fight off invading pathogens and how the arthropod-borne pathogens use the arthropods’ antioxidant mechanism to defend against these ROS attacks and their possible impact on their vector potentials or their ability to acquire and transmit pathogens. In addition, we describe the possible roles of ROS in chemical insecticide/acaricide activity and/or in the development of resistance. Overall, this underscores the importance of the antioxidant system as a potential target for the control of arthropod and arthropod-borne pathogens.

High pyrethroid-resistance intensity in Culex quinquefasciatus (Say) (Diptera: Culicidae) populations from Jigawa, North-West, Nigeria

This study examined pyrethroid resistance intensity and mechanisms in Culex quinquefasciatus (Say) (Diptera: Culicidae) populations from Jigawa, North-West Nigeria. Resistance statuses to permethrin, lambda-cyhalothrin and alphacypermethrin were determined with both WHO and CDC resistance bioassays. Synergist assay was conducted by pre-exposing the populations to Piperonyl butoxide (PBO) using the WHO method. Resistance intensities to 2x, 5x and 10x of diagnostic concentrations were determined with the CDC bottle method. Species analysis and presence of knockdown mutation (Leu-Phe) were done using Polymerase Chain Reaction (PCR). Results showed that Cx. quinquefasciatus was the only Culex spp. present and “Kdr-west” mutation was not detected in all analyzed samples. Using WHO method, Cx. quinquefasciatus resistance to permethrin was detected in Dutse (12.2%) and Kafin-Hausa (77.78%). Lambda-cyhalothrin resistance was recorded only in Kafin-Hausa (83.95%) with resistance suspected in Ringim (90%). Resistance to alphacypermethrin was recorded in all locations. Pre-exposure to PBO led to 100% mortality to alphacypermethrin and lambda-cyhalothrin in Ringim while mortality to permethrin and alphacypermethrin in Dutse increased from 12.2% to 97.5% and 64.37% to 79.52% respectively. Using CDC bottle bioassay, resistance was also recorded in all populations and the result shows a significant positive correlation (R² = 0.728, p = 0.026) with the result from the WHO bioassay. Results of resistance intensity revealed a very high level of resistance in Kafin-Hausa with susceptibility to lambda-cyhalothrin and alphacypermethrin not achieved at 10x of diagnostic doses. Resistance intensity was also high in Dutse with susceptibility to all insecticides not achieved at 5x of diagnostic doses. Widespread and high intensity of resistance in Cx. quinquefasciatus from North-West Nigeria is a major threat to the control of diseases transmitted by Culex and other mosquito species. It is a challenge that needs to be adequately addressed so as to prevent the failure of pyrethroid-based vector control tools.

Development of resistance to insecticide by mosquitoes has been identified to be a major challenge in the prevention and control of diseases transmitted by mosquitoes. This informs this study that investigated the level of resistance of Culex mosquitoes from Jigawa, North-West Nigeria to Pyrethroids. The main type of Culex mosquitoes found in the sampled area was Cx. quinquefasciatus. The Cx. quinquefasciatus populations were found to be resistant to permethrin, lambda-cyhalothrin and alphacypermethrin. Resistance in Cx. quinquefasciatus from the three LGAs is more pronounced to alphacypermethrin. The methods employed by the mosquitoes in developing resistance involve detoxification of the insecticides by metabolic enzymes. Cx. quinquefasciatus from the three LGAs were observed to be highly resistant and can withstand multiple of the recommended doses. This development whereby Cx. quinquefasciatus populations were highly resistant to these recommended insecticides is of serious concern as it can lead to failure of all efforts geared towards prevention and control of diseases transmitted by Culex mosquitoes in North-West Nigeria.

Defining the mechanisms of action and mosquito larva midgut response to a yeast-encapsulated orange oil larvicide

BACKGROUND

Yeast-encapsulated orange oil (YEOO) is a novel, ingestible larvicide that combines the benefits of a low-cost essential oil with yeast, an attractive food source for mosquito larvae. In this work, we investigated the underlying mechanisms of action associated with YEOO ingestion by Aedes aegypti larvae.

METHODS

Aedes aegypti third-stage larvae (L3) were treated with sublethal or lethal concentrations of YEOO. Genes associated with apoptosis, autophagy and innate immune responses were investigated by RT-qPCR in guts and carcasses dissected from treated and control larvae. Differential expression of cytochrome P450 genes in the CYP6 and CYP9 families were also investigated. Confocal and transmission electron microscopy were used to assess damage caused by YEOO throughout the larval alimentary canal. TUNEL was used to assess apoptosis via DNA fragmentation.

RESULTS

The apoptosis genes IAP1 and IAP2 in larvae displayed opposing effects following exposure to lethal doses of YEOO, with a 26-fold induction of IAP1 at 8 h post YEOO ingestion. The effector caspase CASPS8 displayed a 6.7-fold induction in the gut and concomitant 70-fold induction in the carcass at 8 h post YEOO ingestion. The midgut epithelia regenerator, Vein, had an 11-fold induction in the gut after 4 h and was repressed 7.6-fold in the carcass at 24 h. Sublethal concentrations (< LC₅₀) led to significant differential expression of CYP6 and CYP9 genes. Midgut epithelial damage was highlighted by the destruction of microvilli, vacuolization of midgut cells and damage to cell junctions and basal lamina as early as 30 min. Larval type 2 peritrophic matrix structural integrity and porosity remain unchanged.

CONCLUSION

Our results strongly suggest that the robust larvicidal activity of YEOO is due to a generalized broad-acting mechanism combining epithelial damage and apoptosis, with concomitant expression of multiple innate response genes involved in epithelial regeneration and detoxification. YEOO's amenability for use as part of an integrated vector management program makes this novel larvicide a practical approach for mosquito larval control in the future.

Comparative transcriptome and RNA interference reveal CYP6DC1 and CYP380C47 related to lambda-cyhalothrin resistance in Rhopalosiphum padi

The bird-cherry-oat aphid, Rhopalosiphum padi, is a serious agricultural pest of Triticeae crops, and pyrethroids are the most widely used chemical pesticides for the control of the aphid. Our previous studies found that some R. padi field populations have developed resistance against pyrethroids; an M918L target-site mutation of the voltage gated sodium channel was present in the pyrethroid resistant individuals, while the high-level resistance to lambda-cyhalothrin revealed the presence of other mechanisms in the pest. Here, we conducted genome-wide transcriptional analysis for the lambda-cyhalothrin susceptible (SS) and resistant (LC-RR) strains of R. padi. Results indicated that 2457 genes were differently expressed between the SS and LC-RR strains. In the LC-RR, a total of 1265 and 1192 genes were up- and down-regulated, respectively. KEGG analysis implicated enrichment of P450 involved in insecticide metabolic pathways in the resistant transcriptome. qRT-PCR results confirmed that two P450 genes (CYP6DC1 and CYP380C47) were significantly overexpressed in the LC-RR individuals. Furthermore, RNA interference (RNAi) of CYP6DC1 or CYP380C47 significantly increased mortality of R. padi exposure to lambda-cyhalothrin. These results suggest that the overexpression of CYP6DC1 and CYP380C47 contributed to the lambda-cyhalothrin resistance in the pest. This study provides knowledge for further analyzing the molecular mechanism of resistance to pyrethroids in R. padi.

Sublethal doses of broflanilide prevents molting in the fall armyworm, Spodoptera frugiperda via altering molting hormone biosynthesis

Broflanilide is a novel insecticide with a unique mode of action on the insect GABA receptor and is registered worldwide for the control of agricultural pests. It shows high efficacy in controlling the fall armyworm (FAW) Spodoptera frugiperda, which is a destructive pest to various crops. FAW was exposed to sublethal concentrations of broflanilide to determine its impact on insect development. Sublethal doses (LD₁₀ and LD₃₀) caused failure of ecdysis, reduced body length of larvae, malformation of pupae, and vestigial wing formation in adults. Also, broflanilide at LD₃₀ significantly reduced the amount of molting hormone (MH). After exposure to LD₁₀ or LD₃₀ broflanilide, expression of five Halloween genes, which participate in MH biosynthesis, were found to be altered. Specifically, the transcript levels of SfrCYP307A1 (Spook), SfrCYP314A1 (Shade) and SfrCYP315A1 (Shadow) in 3rd day larvae were significantly decreased as well as SfrCYP302A1 (Disembodied) and SfrCYP306A1 (Phantom) in 5th day pupae. In contrast, the transcript levels of SfrCYP302A1 in 3rd day larvae, SfrCYP307A1 and SfrCYP314A1 in 5th day pupae, and SfrCYP306A1, SfrCYP307A1 and SfrCYP315A1 in 0.5th day adults were significantly increased. Our results demonstrate that broflanilide caused the failure of ecdysis in FAW possibly by influencing the intake of cholesterol through inhibition of feeding and also via altering expression of genes important for MH biosynthesis.

The Central Role of Multiple P450 Genes and Their Co-factor CPR in the Development of Permethrin Resistance in the Mosquito Culex quinquefasciatus

Mosquitoes’ increasing resistance to insecticides is becoming a major threat for control efforts worldwide. Multiple P450 genes that are up-regulated in permethrin resistant strains of Culex quinquefasciatus have been linked to the development of resistance. In the current study, we characterized the function of six P450 genes, CYP6P14, CYP6BZ2, CYP9J33, CYP9J34, CYP9J40, and CYP9J45, that are overexpressed in the permethrin resistant Culex mosquitoes and showed their capability in metabolism of permethrin. These six P450 genes can convert 3-phenoxybenzoic alcohol (PBCHO) to a less toxic product, 3-phenoxybenzoic acid (PBCOOH), indicating that these P450s play an important role in permethrin degradation pathways. Although we know multiple P450 genes are over-expressed in permethrin resistant Culex mosquitoes, it remains to be seen whether cytochrome P450-reductase (CPR) gene that are co-overexpressed with P450 genes in permethrin resistant mosquitoes do indeed serve as a resistance mechanism. An in-depth investigation of the expression of CPR gene in resistant mosquitoes was conducted in permethrin resistant mosquitoes. The finding of CPR gene overexpression in permethrin resistant mosquitoes suggested the importance of co-overexpression of multiple P450 genes with their obligatory electron donor CPR in the complex detoxification system, boosting the metabolism of permethrin and hence the development of permethrin resistance in Cx. quinquefasciatus.

Inductions of a CYP6 cluster conferring deltamethrin resistance in colonized and field-collected Culex pipiens pallens

Mosquitoes transmit many damaging vector-borne diseases. Unfortunately, the rise of insecticide resistance has become a major obstacle to mosquito control. A preliminary study showed that a CYP6 cluster is significant for deltamethrin resistance in colonized Culex pipiens pallens. Here, several field strains were collected to explore the association of the cluster in deltamethrin tolerance. We examined the effect of deltamethrin treatment on the cluster expression at a deltamethrin concentration of LC₅₀ in these strains using five time points. As a result, both P450 induction and constitutive overexpression were associated with deltamethrin resistance. Deltamethrin could stimulate different expression sets in the P450 cluster in different strains, predominately correlated with the resistance level of the strain. Our results will offer more insight into working with the characterization of P450s related to insecticide resistance.

Host-pathogen interaction between Asian citrus psyllid and entomopathogenic fungus (Cordyceps fumosorosea) is regulated by modulations in gene expression, enzymatic activity and HLB-bacterial population of the host

The host-pathogen interaction has been explored by several investigations, but the impact of fungal pathogens against insect resistance is still ambiguous. Therefore, we assessed the enzymatic activity and defense-related gene expression of Asian citrus psyllid (ACP) nymphal and adult populations on Huanglongbing-diseased citrus plants under the attack of Cordyceps fumosorosea. Overall, five enzymes viz. superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione S-transferase (GST), carboxylesterase (CarE), and four genes, namely SOD, 16S, CYP4C68, CYP4BD1, were selected for respective observations from ACP populations. Enzymatic activity of four enzymes (SOD, POD, GST, CarE) was significantly decreased after 5-days post-treatment (dpt) and 3-dpt fungal exposure in fungal treated ACP adult and nymphal populations, respectively, whereas the activity of CAT was boosted substantially post-treatment time schedule. Besides, we recorded drastic fluctuations in the expression of CYP4 genes among fungal treated ACP populations. After 24 hours post-treatment (hpt), expression of both CYP4 genes was boosted in fungal treated populations than controlled populations (adult and nymph). After 3-dpt, however, the expression of CYP4 genes was declined in the given populations. Likewise, fungal attack deteriorated the resistance of adult and nymphal of ACP population, as SOD expression was down-regulated in fungal-treated adult and nymphs after 5-dpt and 3-dpt exposure, respectively. Moreover, bacterial expression via the 16S gene was significantly increased in fungal-treated adult and nymphal ACP populations with increasing post-treatment time. Overall, our data illustrate that the fungal application disrupted the insect defense system. The expression of these genes and enzymes suppress the immune function of adult and nymphal ACP populations. As it is reported first time that the applications of C. fumosorosea against ACP reduce insect resistance by interfering with the CYP4 and SOD system. Therefore, we propose new strategies to discover the role of certain toxic compounds from fungus, which can reduce insect resistance, focusing on resistance-related genes and enzymes.

Transcriptomic analysis reveals pronounced changes in gene expression due to sub-lethal pyrethroid exposure and ageing in insecticide resistance Anopheles coluzzii

BACKGROUND

Malaria control is heavily reliant on the use of insecticides that target and kill the adult female Anopheline vector. The intensive use of insecticides of the pyrethroid class has led to widespread resistance in mosquito populations. The intensity of pyrethroid resistance in some settings in Africa means mosquitoes can contact bednets treated with this insecticide class multiple times with minimal mortality effects. Furthermore, both ageing and diel cycle have been shown to have large impacts on the resistance phenotype. Together, these traits may affect other aspects of vector biology controlling the vectorial capacity or fitness of the mosquito.

RESULTS

Here we show that sublethal exposure of a highly resistant Anopheles coluzzii population originally from Burkina Faso to the pyrethroid deltamethrin results in large and sustained changes to transcript expression. We identify five clear patterns in the data showing changes to transcripts relating to: DNA repair, respiration, translation, behaviour and oxioreductase processes. Further, we highlight differential regulation of transcripts from detoxification families previously linked with insecticide resistance, in addition to clear down-regulation of the oxidative phosphorylation pathway both indicative of changes in metabolism post-exposure. Finally, we show that both ageing and diel cycle have major effects on known insecticide resistance related transcripts.

CONCLUSION

Sub-lethal pyrethroid exposure, ageing and the diel cycle results in large-scale changes in the transcriptome of the major malaria vector Anopheles coluzzii. Our data strongly supports further phenotypic studies on how transcriptional changes such as reduced expression of the oxidative phosphorylation pathway or pyrethroid induced changes to redox state might impact key mosquito traits, such as vectorial capacity and life history traits.

Multiple cytochrome P450 genes: conferring high levels of permethrin resistance in mosquitoes, Culex quinquefasciatus

Insecticides, especially pyrethroids, are the most important in the insect pest control and preventing insect vector-borne human diseases. However, insect pests, including mosquitoes, have developed resistance in the insecticides that used against them. Cytochrome P450s are associated with insecticide resistance through overexpression and detoxification mechanisms in insect species. In this study, we utilized a powerful tool, the RNAi technique, to determine the roles of key P450 genes overexpressed in permethrin resistant mosquitoes that confer insecticide resistance to unravel the molecular basis of resistance mechanisms in the mosquito Culex quinquefasciatus. The results showed that knockdown of 8 key P450 genes using RNAi techniques significantly decreased resistance to permethrin in resistant mosquitoes. In silico modeling and docking analysis further revealed the potential metabolic function of overexpressed P450 genes in the development of insecticide resistance in mosquitoes. These findings not only highlighted the functional importance of these P450 genes in insecticide resistance, but also revealed that overexpression of multiple P450 genes was responsible for the high levels of insecticide resistance in a mosquito population of Culex quinquefasciatus.

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.

Sub-lethal aquatic doses of pyriproxyfen may increase pyrethroid resistance in malaria mosquitoes

BACKGROUND

Pyriproxyfen (PPF), an insect growth hormone mimic is widely used as a larvicide and in some second-generation bed nets, where it is combined with pyrethroids to improve impact. It has also been evaluated as a candidate for auto-dissemination by adult mosquitoes to control Aedes and Anopheles species. We examined whether PPF added to larval habitats of pyrethroid-resistant malaria vectors can modulate levels of resistance among emergent adult mosquitoes.

METHODOLOGY

Third-instar larvae of pyrethroid-resistant Anopheles arabiensis (both laboratory-reared and field-collected) were reared in different PPF concentrations, between 1×10-9 milligrams active ingredient per litre of water (mgAI/L) and 1×10-4 mgAI/L, or no PPF at all. Emergent adults escaping these sub-lethal exposures were tested using WHO-standard susceptibility assays on pyrethroids (0.75% permethrin and 0.05% deltamethrin), carbamates (0.1% bendiocarb) and organochlorides (4% DDT). Biochemical basis of pyrethroid resistance was investigated by pre-exposure to 4% PBO. Bio-efficacies of long-lasting insecticide-treated nets, Olyset® and PermaNet 2.0 were also examined against adult mosquitoes with or without previous aquatic exposure to PPF.

RESULTS

Addition of sub-lethal doses of PPF to larval habitats of pyrethroid-resistant An. arabiensis, consistently resulted in significantly reduced mortalities of emergent adults when exposed to pyrethroids, but not to bendiocarb or DDT. Mortality rates after exposure to Olyset® nets, but not PermaNet 2.0 were also reduced following aquatic exposures to PPF. Pre-exposure to PBO followed by permethrin or deltamethrin resulted in significant increases in mortality, compared to either insecticide alone.

CONCLUSIONS

Partially-resistant mosquitoes exposed to sub-lethal aquatic concentrations of PPF may become more resistant to pyrethroids than they already are without such pre-exposures. Studies should be conducted to examine whether field applications of PPF, either by larviciding or other means actually exacerbates pyrethroid-resistance in areas where signs of such resistance already exist in wild the vector populations. The studies should also investigate mechanisms underlying such magnification of resistance, and how this may impact the potential of PPF-based interventions in areas with pyrethroid resistance.

MiR-4448 is involved in deltamethrin resistance by targeting CYP4H31 in Culex pipiens pallens

Background

Culex pipiens (Cx. pipiens) complex, which acts as a vector of viruses and is widespread and abundant worldwide, including West Nile virus, Japanese encephalitis virus, and Sindbis virus, can cause serious vector-borne diseases affecting human health. Unfortunately, mosquitoes have developed deltamethrin resistance because of its long-term overuse, representing a major challenge to mosquito control. Understanding the molecular regulatory mechanisms of resistance is vital to control mosquitoes. MicroRNAs (miRNAs) are short non-coding RNAs that have been demonstrated to be important regulators of gene expression across a wide variety of organisms, which might function in mosquito deltamethrin resistance. In the present study, we aimed to investigate the regulatory functions of miR-4448 and CYP4H31 in the formation of insecticidal resistance in mosquito Culex pipiens pallens.

Methods

We used quantitative real-time reverse transcription PCR to measure miR-4448 and CYP4H31 (encoding a cytochrome P450) expression levels. The regulatory functions of miR-4448 and CYP4H31 were assessed using dual-luciferase reporter assays. Then, oral feeding, RNA interference, and the American Centers for Disease Control and Prevention bottle bioassay were used to determine miR-4448’s association with deltamethrin resistance by targeting CYP4H31in vivo. Cell Counting Kit-8 (CCK-8) was also used to detect the viability of pIB/V5-His-CYP4H31-transfected C6/36 cells after deltamethrin treatment in vitro.

Results

MiR-4448 was downregulated in the deltamethrin-resistant strain (DR strain), whereas CYP4H31 was downregulated in deltamethrin-susceptible strain. CYP4H31 expression was downregulated by miR-4448 recognizing and binding to its 3′ untranslated region. Functional verification experiments showed that miR-4448 overexpression resulted in lower expression of CYP4H31. The mortality of miR-4448 mimic-injected DR strain mosquitoes was higher than that of the controls. CCK-8 assays showed that CYP4H31 decreased cellular resistance to deltamethrin in vitro and the mortality of the DR strain increased when CYP4H31 was knocked down in vivo.

Conclusions

In mosquitoes, miR-4448 participates in deltamethrin resistance by targeting CYP4H31. The results of the present study increase our understanding of deltamethrin resistance mechanisms.

Knockdown of CYP301B1 and CYP6AX1v2 increases the susceptibility of the brown planthopper to beta-asarone, a potential plant-derived insecticide

The cytochrome P450 monooxygenases of insects play crucial roles in the metabolic detoxification of insecticides. Our previous finding showed that two cytochrome P450 genes, both CYP301B1 and CYP6AX1v2, in the BPH underwent overexpression due to β-asarone. In this study, we investigated the molecular characteristics, expression patterns and functions of these two cytochrome P450 genes. The results showed that CYP301B1 had the highest expression level in the eggs, while CYP6AX1v2 was expressed in macropterous female adults. Moreover, the expression level of CYP301B1 in the head was higher than that in the integument, fat body and gut. The expression level of CYP6AX1v2 in the fat body and gut was higher than that in head and integument. Importantly, silencing CYP301B1 and CYP6AX1v2 separately could increase the sensitivity, resulting in significant higher mortality of BPH following treatment with β-asarone. Our findings indicated that CYP301B1 and CYP6AX1v2 could contribute to the resistance of BPH to β-asarone, and these two genes may be involved in the detoxification metabolism of β-asarone in BPH.

Permethrin exposure affects neurobehavior and cellular characterization in rats' brain

This study investigated the neurotoxic effects of permethrin on the cerebellum, hippocampus and prefrontal cortex of Wistar rats and its effects on some behavioral patterns. Fifteen adult male Wistar rats were grouped into three categories: Group A received 0.1 mL normal saline (control), and Groups B and C received mixed feed with 500 mg/kg and 1,000 mg/kg of 0.6% permethrin, respectively, for 14 days. The animals were assessed for memory, anxiety and exploratory locomotion and thereafter anesthetized and transcardially perfused with normal saline and 4% paraformaldehyde (PFA). Cerebellum, hippocampus and prefrontal cortex were excised from the whole brain and processed for tissue histology, histochemistry and immunohistochemistry. Oxidative status and lipid peroxidation were also assessed using catalase, glutathione peroxidase, superoxide dismutase and malondialdehyde as biomarkers. Results revealed dosedependent decrease in body weights but increase in cerebellar and prefrontal weights, depletion of endogenous antioxidant markers, cognitive deficits, reduced locomotor activities, degenerative changes in the microarchitecture at high doses and presence of chromatolytic cells at both low and high doses of permethrin. Astrocytes were activated while synaptophysin expression was downregulated. Permethrin causes dose-dependent neurotoxicity on the morphology, neurochemistry and oxidative status of different brain regions, and these could affect behavioral performance and other neurologic functions.

A differential expression of pyrethroid resistance genes in the malaria vector Anopheles funestus across Uganda is associated with patterns of gene flow

BACKGROUND

Insecticide resistance is challenging the effectiveness of insecticide-based control interventions to reduce malaria burden in Africa. Understanding the molecular basis of insecticides resistance and patterns of gene flow in major malaria vectors such as Anopheles funestus are important steps for designing effective resistance management strategies. Here, we investigated the association between patterns of genetic structure and expression profiles of genes involved in the pyrethroid resistance in An. funestus across Uganda and neighboring Kenya.

METHODS

Blood-fed mosquitoes An. funestus were collected across the four localities in Uganda and neighboring Kenya. A Microarray-based genome-wide transcription analysis was performed to identify the set of genes associated with permethrin resistance. 17 microsatellites markers were genotyped and used to establish patterns of genetic differentiation.

RESULTS

Microarray-based genome-wide transcription profiling of pyrethroid resistance in four locations across Uganda (Arua, Bulambuli, Lira, and Tororo) and Kenya (Kisumu) revealed that resistance was mainly driven by metabolic resistance. The most commonly up-regulated genes in pyrethroid resistance mosquitoes include cytochrome P450s (CYP9K1, CYP6M7, CYP4H18, CYP4H17, CYP4C36). However, expression levels of key genes vary geographically such as the P450 CYP6M7 [Fold-change (FC) = 115.8 (Arua) vs 24.05 (Tororo) and 16.9 (Kisumu)]. In addition, several genes from other families were also over-expressed including Glutathione S-transferases (GSTs), carboxylesterases, trypsin, glycogenin, and nucleotide binding protein which probably contribute to insecticide resistance across Uganda and Kenya. Genotyping of 17 microsatellite loci in the five locations provided evidence that a geographical shift in the resistance mechanisms could be associated with patterns of population structure throughout East Africa. Genetic and population structure analyses indicated significant genetic differentiation between Arua and other localities (FST>0.03) and revealed a barrier to gene flow between Arua and other areas, possibly associated with Rift Valley.

CONCLUSION

The correlation between patterns of genetic structure and variation in gene expression could be used to inform future interventions especially as new insecticides are gradually introduced.

Detection of the Nav channel kdr-like mutation and modeling of factors affecting survivorship of Culex quinquefasciatus mosquitoes from six areas of Harris County (Houston), Texas, after permethrin field-cage tests

Culex quinquefasciatus is one of the most important mosquito vectors of arboviruses. Currently, the fastest approach to control disease transmission is the application of synthetic adulticide insecticides. However, in highly populated urban centers the development of insecticide resistance in mosquito populations could impair insecticide efficacy and therefore, disease control. To assess the effect of resistance on vector control, females of Cx. quinquefasciatus collected from six mosquito control operational areas in Harris County, Texas, were treated in field cage tests at three different distances with the pyrethroid Permanone® 31-66 applied at the operational rate. Females were analyzed by sequencing and/or diagnostic PCR using de novo designed primers for detecting the kdr-like mutation in the voltage-gated sodium channel (L982F; TTA to TTT) (house fly kdr canonical mutation L1014F). Females from the Cx. quinquefasciatus susceptible Sebring strain and those from the six operational areas placed at 30.4 m from the treatment source were killed in the tests, while 14% of field-collected mosquitoes survived at 60.8 m, and 35% at 91.2 m from the source. The diagnostic PCR had a with 97.5% accuracy to detect the kdr-like mutation. Pyrethroid resistant mosquitoes carrying the L982F mutation were broadly distributed in Harris County at high frequency. Among mosquitoes analyzed (n = 1,028), the kdr-kdr genotype was prevalent (81.2%), the kdr-s genotype was 18%, and s-s mosquitoes were less than 1% (n = 8). A logistic regression model estimated an equal probability of survival for the genotypes kdr-kdr and kdr-s in all areas analyzed. Altogether, our results point to a high-risk situation for the pyrethroid-based arboviral disease control in Harris County.

Sublethal effects of chlorantraniliprole on molting hormone levels and mRNA expressions of three Halloween genes in the rice stem borer, Chilo suppressalis

While sublethal effects of insecticide on insect development have been widely studied, the underlying mechanisms remain elusive. Our previous studies revealed that sublethal concentrations of chlorantraniliprole significantly increased the juvenile hormone levels and resulted in both prolonged developmental time and reduced fecundity in Chilo suppressalis. In the present study, we evaluated the sublethal effects of chlorantraniliprole on molting hormone (MH) levels and mRNA expressions of three Halloween genes including CsCYP307A1, CsCYP306A1 and CsCYP314A1 in C. suppressalis. The results showed that the MH levels in different developmental stages of C. suppressalis were decreased after exposure to LC₁₀ and LC₃₀ of chlorantraniliprole. However, analysis of temporal expression profiles revealed that the mRNA levels of three Halloween genes were not closely correlated with the ecdysteroid titers in C. suppressalis. Notably, the transcript levels of CsCYP307A1, CsCYP306A1 and CsCYP314A1 were induced after treatment with sublethal concentrations of chlorantraniliprole in specific developmental stages. These results indicated that chlorantraniliprole had adverse effects on insect MH biosynthesis, and in addition to the involvement in MH biosynthesis, CsCYP307A1, CsCYP306A1 and CsCYP314A1 may also play important roles in the detoxification metabolism of chlorantraniliprole in C. suppressalis.

CYP4BN6 and CYP6BQ11 mediate insecticide susceptibility and their expression is regulated by Latrophilin in Tribolium castaneum

BACKGROUND

Many insect cytochrome P450 proteins (CYPs) are involved in the metabolic detoxification of exogenous compounds such as plant toxins and insecticides. Tribolium castaneum, the red flour beetle, is a major agricultural pest that damages stored grains and cereal products. With the completion of the sequencing of its genome, two T. castaneum species-specific CYP genes, CYP4BN6, and CYP6BQ11, were identified. However, it is unknown whether the functions of most CYPs are shared by TcCYP4BN6 and TcCYP6BQ11, and the upstream regulatory mechanism of these two CYPs remains elusive.

RESULTS

QRT-PCR analysis indicated that TcCYP4BN6 and TcCYP6BQ11 were both most highly expressed at the late pupal stage and were mainly observed in the head and gut, respectively, of adults. Moreover, the transcripts of these two CYPs were significantly induced by dichlorvos and carbofuran, and RNA interference (RNAi) targeting of each of them enhanced the susceptibility of beetles to these two insecticides. Intriguingly, knockdown of the latrophilin (lph) gene, which has been reported to be related to the insecticide susceptibility, reduced the expression of TcCYP4BN6 and TcCYP6BQ11 after insecticide treatment, suggesting that these two CYP genes are regulated by lph to participate in insecticide susceptibility in T. castaneum.

CONCLUSION

These results shed new light on the function and mechanism of CYP genes associated with insecticide susceptibility and could facilitate research on appropriate and sustainable pest control management. © 2019 Society of Chemical Industry.

Effects of Propoxur Exposure on Insecticidal Susceptibility and Developmental Traits in Culex pipiens quinquefasciatus

Propoxur-sel strains of Culex pipiens quinquefasciatus were derived from a lab-bred strain following 16 generations of propoxur exposure under sublethal concentrations of LC₂₅ (lethal concentration of 25%) and LC₅₀ (lethal concentration of 50%), respectively. This resulted in resistance development in F16 with ratios of 8.8× and 6.3×, respectively, compared with F0. The fecundity, longevity, sex ratio (F/M), and hatchability of the propoxur-exposed Cx. quinquefasciatus adult survivors and their offspring were decreased, with no effect on the emergence ratio and pupa survival rate. In addition, the intrinsic rates of increase (r), the net reproduction (R₀), and the finite rate of increase (λ) of the Cx. quinquefasciatus offspring generations were also decreased significantly compared to F0. Correspondingly, the mean generation time (T) and the population double time (DT) in propoxur-sels were increased. Enhanced activities of cytochrome P450 monooxygenase and esterase were also observed in propoxur-sels, indicating that a detoxification mechanism might be responsible for resistance development in Cx. quinquefasciatus. Except for the three genes cyp4d42v1, cyp4c52v1, and cyp6aa9 which displayed a coincidence in some degree in different treatments, induction by different doses of propoxur and constitutive expression in different generations of propoxur-sel strains resulted in an inconsistent identification of the P450 genes probably related with resistance.

Cytochrome P450s Cyp4p1 and Cyp4p2 associated with the DDT tolerance in the Drosophila melanogaster strain 91-R

Cytochrome P450s are part of a super-gene family that has undergone gene duplication, divergence, over-expression and, in some cases, loss of function. One such case is the 91-R and 91-C strains of common origin, in Drosophila melanogaster, whereby 91-R (DDT resistant strain) overexpresses Cyp4p1 and Cyp4p2 and both genes are lost in 91-C (DDT susceptible strain). In this study, we used a comparative approach to demonstrate that transcription of Cyp4p1 and Cyp4p2 were constitutively up-regulated in the Drosophila melanogaster strain 91-R as compared to another DDT susceptible strain Canton-S which does not have a loss of function of these genes. Furthermore, significantly increased expression of Cyp4p1 and Cyp4p2 was induced in 91-R in response to sublethal DDT exposure, however, such induction did not occur in the DDT treated Canton-S. Additionally, fixed nucleotide variation within putative transcription factor binding sites of Cyp4p1 and Cyp4p2 promoters were observed between 91-R and Canton-S, however, their impact on transcription remains to be determined. Two GAL4/UAS transgenic strains with integrated heat shock-inducible Cyp4p1- or Cyp4p2-RNAi constructs within wild-type genetic backgrounds were developed. Following heat shock induction of Cyp4p1 and Cyp4p2 knockdown, these transgenic lines showed increased DDT mortality as compared to their corresponding non-heat shock controls. These results provide a functional link of Cyp4p1 and Cyp4p2 in conferring tolerance to DDT exposure.

Using targeted next-generation sequencing to characterize genetic differences associated with insecticide resistance in Culex quinquefasciatus populations from the southern U.S

Resistance to insecticides can hamper the control of mosquitoes such as Culex quinquefasciatus, known to vector arboviruses such as West Nile virus and others. The strong selective pressure exerted on a mosquito population by the use of insecticides can result in heritable genetic changes associated with resistance. We sought to characterize genetic differences between insecticide resistant and susceptible Culex quinquefasciatus mosquitoes using targeted DNA sequencing. To that end, we developed a panel of 122 genes known or hypothesized to be involved in insecticide resistance, and used an Ion Torrent PGM sequencer to sequence 125 unrelated individuals from seven populations in the southern U.S. whose resistance phenotypes to permethrin and malathion were known from previous CDC bottle bioassay testing. Data analysis consisted of discovering SNPs (Single Nucleotide Polymorphism) and genes with evidence of copy number variants (CNVs) statistically associated with resistance. Ten of the seventeen genes found to be present in higher copy numbers were experimentally validated with real-time PCR. Of those, six, including the gene with the knock-down resistance (kdr) mutation, showed evidence of a ≥ 1.5 fold increase compared to control DNA. The SNP analysis revealed 228 unique SNPs that had significant p-values for both a Fisher’s Exact Test and the Cochran-Armitage Test for Trend. We calculated the population frequency for each of the 64 nonsynonymous SNPs in this group. Several genes not previously well characterized represent potential candidates for diagnostic assays when further validation is conducted.

Insecticide Exposure Triggers a Modulated Expression of ABC Transporter Genes in Larvae of Anopheles gambiae s.s

Insecticides remain a main tool for the control of arthropod vectors. The urgency to prevent the insurgence of insecticide resistance and the perspective to find new target sites, for the development of novel molecules, are fuelling the study of the molecular mechanisms involved in insect defence against xenobiotic compounds. In this study, we have investigated if ATP-binding cassette (ABC) transporters, a major component of the defensome machinery, are involved in defence against the insecticide permethrin, in susceptible larvae of the malaria vector Anopheles gambiae sensu stricto. Bioassays were performed with permethrin alone, or in combination with an ABC transporter inhibitor. Then we have investigated the expression profiles of five ABC transporter genes at different time points following permethrin exposure, to assess their expression patterns across time. The inhibition of ABC transporters increased the larval mortality by about 15-fold. Likewise, three genes were up-regulated after exposure to permethrin, showing different patterns of expression across the 48 h. Our results provide the first evidences of ABC transporters involvement in defence against a toxic in larvae of An. gambiae s.s. and show that the gene expression response is modulated across time, being continuous, but stronger at the earliest and latest times after exposure.

Impacts of Sub-lethal DDT Exposures on microRNA and Putative Target Transcript Expression in DDT Resistant and Susceptible Drosophila melanogaster Strains

Ten constitutively differentially expressed miRNAs were previously described between DDT-resistant 91-R and -susceptible control Drosophila melanogaster strains, and among their predicted target genes were those associated with metabolic DDT resistance mechanisms. The present study evaluated the inducibility of miRNA expression and putative downstream regulation of cytochrome P450s in response to DDT exposure in a time-dependent manner in 91-R and the susceptible Canton-S strain. Specifically, RT-qPCR analysis showed that DDT exposures led to the significant down-regulation (repression) of miR-310-3p, miR-311-3p, miR-312-3p, miR-313-3p, and miR-92a-3p levels in Canton-S. This is contrasted with the lack of significant changes in 91-R at most time-points following DDT exposure. The levels of expression among miRNAs exhibited opposite expression patterns compared to their corresponding putative target cytochrome P450s at the same time points after DDT exposure. Collectively, results from this study suggest that miR-310-3p, miR-311-3p, miR-312-3p, miR-313-3p, and miR-92a-3p might have a potential role in the control of DDT detoxification through the post-transcriptional regulation of target cytochrome P450s in Canton-S. Conversely, the lack of significant changes of these same miRNAs in 91-R following DDT-exposure suggests a possible adaptive mutation that removes repressive control mechanisms. These data are important for the understanding impact of adaptive changes in miRNA expression on post-transcriptional regulatory mechanism involved in the evolution of DDT resistance in 91-R.

Mechanisms of resistance to thiamethoxam and dinotefuran compared to imidacloprid in the brown planthopper: Roles of cytochrome P450 monooxygenase and a P450 gene CYP6ER1

The brown planthopper (BPH, Nilaparvata lugens) has developed high resistance to the first-generation neonicotinoids (imidacloprid). With commercialization and widespread field use of the second-(thiamethoxam) and third-(dinotefuran) generation neonicotinoids, resistance to these insecticides is also reported. We investigated the cytochrome P450 monooxygenase-mediated detoxification in thiamethoxam- and dinotefuran- resistant in comparison to imidacloprid-resistant strains of BPH. In the three moderately resistant BPH strains selected separately with the three insecticides from a same susceptible strain, P450 activities were significantly enhanced over the susceptible control. Seven of 26 tested P450 genes were up-regulated and CYP6ER1 was a strongly over-expressed gene in all the three resistant strains. Knockdown of CYP6ER1 in the susceptible insects reduced P450 activity, retarded nymph growth and significantly increased sensitivity to each one of the three neonicotinoids. Taken together, we show that enhanced P450 activity and over-expression of CYP6ER1 gene are involved in BPH resistance to thiamethoxam and dinotefuran as to imidacloprid. These findings are of significance in management thiamethoxam and dinotefuran resistance in the BPH, especially in the management of potential cross-resistance to the three generations of neonicotinoids.

Glutathione S-transferases play a role in the detoxification of flumethrin and chlorpyrifos in Haemaphysalis longicornis

Background

Haemaphysalis longicornis is a tick of importance to health, as it serves as a vector of several pathogens, including Theileria orientalis, Babesia ovata, Rickettsia japonica and the severe fever with thrombocytopenia syndrome virus (SFTSV). Presently, the major method of control for this tick is the use of chemical acaricides. The glutathione S-transferase (GST) system is one mechanism through which the tick metabolizes these acaricides. Two GSTs from H. longicornis (HlGST and HlGST2) have been previously identified.

Results

Enzyme kinetic studies were performed to determine the interaction of acaricides with recombinant H. longicornis GSTs. Recombinant HlGST activity was inhibited by flumethrin and cypermethrin, while recombinant HlGST2 activity was inhibited by chlorpyrifos and cypermethrin. Using real-time RT-PCR, the upregulation of the HlGST gene was observed upon exposure to sublethal doses of flumethrin, while the HlGST2 gene was upregulated when exposed to sublethal doses of chlorpyrifos. Sex and strain dependencies in the induction of GST gene expression by flumethrin were also observed. Knockdown of the HlGST gene resulted in the increased susceptibility of larvae and adult male ticks to sublethal doses of flumethrin and the susceptibility of larvae against sublethal doses of chlorpyrifos was increased upon knockdown of HlGST2.

Conclusions

HlGST could be vital for the metabolism of flumethrin in larvae and adult male ticks, while HlGST2 is important in the detoxification of chlorpyrifos in larval ticks.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3044-9) contains supplementary material, which is available to authorized users.

Molecular identification of four novel cytochrome P450 genes related to the development of resistance of Spodoptera exigua (Lepidoptera: Noctuidae) to chlorantraniliprole

BACKGROUND

The beet armyworm, Spodoptera exigua, is an omnivorous insect that damages a variety of crops worldwide. Chlorantraniliprole is a new diamide insecticide that acts on the ryanodine receptors in insects. The aim of this study was to explore key genes related to the development of chlorantraniliprole resistance in S. exigua.

RESULTS

Transcriptomes were compared between beet armyworms from a susceptible laboratory strain (Sus-Lab) and Sus-Lab screened with LC₂₅ sublethal doses of chlorantraniliprole for six generations (SE-Sel). Ten of 11 cytochrome P450 genes with upregulated expression verified by quantitative reverse transcription polymerase chain reaction (qRT-PCR) in SE-Sel strains were detected in SE-PZ16 and SE-ZY16, two extremely resistant field populations. In addition, expression of four new cytochrome P450 genes, CYP9A21v1, CYP9A21v2, CYP9A21v3 and CYP9A21v4, was shown in the two field populations and was significantly higher in the SE-Sel strain than in the Sus-Lab strain (P < 0.05). Their full-length and protein tertiary structures were also cloned and predicted. The function of CYP9A21v3 was analysed by RNA interference, and the relative expression of CYP9A21v3 in the SE-ZY16 population after feeding on dsRNA was lower than in the control group. Moreover, mortality rates in insects treated at the LC₅₀ of chlorantraniliprole after dsRNA feeding were significantly higher than in the control group 24 h after treatment (P < 0.05).

CONCLUSION

Overexpression of CYP9A21v3 may be a primary factor in the development of chlorantraniliprole resistance in beet armyworms. © 2018 Society of Chemical Industry.

Multiple insecticide resistance in Culex quinquefasciatus populations from Guadeloupe (French West Indies) and associated mechanisms

West Nile (WN) virus has been detected in Guadeloupe since 2002. Even if no WN human cases have been detected so far, mosquitoes from Culex genus especially Culex quinquefasciatus are recognized as potential WN vectors in Guadeloupe. To evaluate the impact of local vector control activities on this mosquito species we assessed the resistance levels of Cx. quinquefasciatus populations from three different sites from Guadeloupe (Abymes, Saint François and Gourbeyre) to malathion, temephos and deltamethrin. In addition, the frequencies of the L1014F kdr and the G119S ace-1 mutations were established in Cx. quinquefasciatus populations, as well as the constitutive expressions of five cytochrome P450 genes. Mosquito populations tested displayed high resistance to deltamethrin, moderate resistance to malathion (Abymes, Gourbeyre) and low resistance to temephos (Abymes et Gourbeyre). Molecular analyses revealed high frequencies of both L1014F kdr and G119S ace-1 mutations in Cx. quinquefasciatus populations, as well as overexpression of cytochrome P450 genes CYP9J45, CYP9J40 and CYP6AA7. Finally, deltamethrin resistance and knock-down rates were strongly correlated with the frequency of the resistant kdr and ace-1 alleles, as well as with CYP9J40 overexpression. These results should be taken into account to refine the current vector control strategies to prevent the appearance of Cx. quinquefasciatus-borne diseases in Guadeloupe.

Gene expression profiles of the Southern house mosquito Culex quinquefasciatus during exposure to permethrin

Insecticide resistance is a major obstacle to the management of disease-vectoring mosquitoes worldwide. The genetic changes and detoxification genes involved in insecticide resistance have been extensively studied in populations of insecticide-resistant mosquitoes, however few studies have focused on the resistance genes upregulated upon insecticide exposure and the possible regulation pathways involved in insecticide resistance. To characterize the changes in gene expression during insecticide exposure, and to investigate the possible connection of known regulation pathways with insecticide resistance, we conducted RNA-Seq analysis of a highly permethrin-resistant strain of Culex quinquefasciatus following permethrin exposure. Gene expression profiles revealed a total of 224 upregulated and 146 downregulated genes when compared to a blank acetone carrier treated control, respectively, suggesting that there were multiple, but specific genes involved in permethrin resistance. Functional enrichment analysis showed that the upregulated genes contained multiple detoxification genes including a glutathione S-transferase and multiple cytochrome P450 genes, as well as several immune-related genes, while the downregulated genes consisted primarily of proteases and carbohydrate metabolism and transport. Further analysis showed that permethrin exposure resulted in a decrease in the expression of serum storage proteins and likely represented a delay in the development of the fourth instar possibly due to a decrease in feeding. This effect was more pronounced in an insecticide-resistant strain than in an insecticide-susceptible strain and may represent a behavioral mechanism of insecticide resistance in Culex mosquitoes.

Pyrethroid resistance associated with M918 L mutation and detoxifying metabolism in Aphis gossypii from Bt cotton growing regions of China

BACKGROUND

The cotton aphid, Aphis gossypii (Glover), is a destructive pest that affects numerous crops throughout the world. Pyrethroid resistance has become endemic in A. gossypii populations in the cotton growing areas of China due to wide- spread application of insecticides. To assess the extent of pyrethroid resistance, bioassays were conducted on field populations collected from several cotton cultivation areas from 2010 to 2015. The frequency of a known resistance-associated sodium channel mutation (M918 L) in A. gossypii was evaluated and the bioassay of bifenthrin with or without the synergist was performed to illuminate the mechanisms underlying resistance to pyrethroids.

RESULTS

The field populations exhibited very high levels of resistance to both beta-cypermethrin and deltamethrin. Pretreatment with synergists, DEF and PBO, significantly increased the toxicity of bifenthrin to cotton aphid populations collected from Bt cotton fields in China. Further, 96.8-100% of individuals with the M918 L mutation (including both RR and RS individuals) were observed in various populations, and only 2.8-3.2% of individuals with wild-type homozygotes (SS) were detected.

CONCLUSION

The mutation M918 L in the voltage-gated sodium channel along with detoxifying metabolism was contributed to the pyrethroid resistance in the field populations of Aphis gossypii from cotton growing regions of China. And insecticides with different modes of action should be recommended for the control of A. gossypii in the future. © 2017 Society of Chemical Industry.

Gene expression modulation of ABC transporter genes in response to permethrin in adults of the mosquito malaria vector Anopheles stephensi

Living organisms have evolved an array of genes coding for detoxifying enzymes and efflux protein pumps, to cope with endogenous and xenobiotic toxic compounds. The study of the genes activated during toxic exposure is relevant to the area of arthropod vector control, since these genes are one of the targets upon which natural selection acts for the evolution of insecticide resistance. ATP-binding cassette (ABC) transporters participate to insecticide detoxification acting as efflux pumps, that reduce the intracellular concentration of toxic compounds, or of their metabolic derivatives. Here we analyzed the modulation of the expression of six genes coding for ABC transporters, after the exposure of adult females and males of the mosquito Anopheles stephensi, a major malaria vector in Asia, to permethrin. Male and female mosquitoes were exposed to insecticide for one hour, then the expression profiles of the ABC transporter genes AnstABCB2, AnstABCB3, AnstABCB4, AnstABCBmember6, AnstABCC11, and AnstABCG4 were analysed after one and 24h. Our results showed that three genes (AnstABCB2, AnstABCBmember6, AnstABCG4) were up-regulated in both sexes; two of these (AnstABCBmember6 and AnstABCG4) have previously been shown to be up-regulated also in larval stages of An. stephensi, supporting a role for these genes in permethrin defence in larvae as well as in adults. Finally, the same ABC transporter genes were activated both in females and males; however, the timing of gene induction was different, with a prompter induction in females than in males.

The function of two P450s, CYP9M10 and CYP6AA7, in the permethrin resistance of Culex quinquefasciatus

Cytochrome P450 monooxygenases play a critical role in insecticide resistance by allowing resistant insects to metabolize insecticides. Previous studies revealed that two P450 genes, CYP9M10 and CYP6AA7, are not only up-regulated but also induced in resistant Culex mosquitoes. In this study, CYP9M10 and CYP6AA7 were separately co-expressed with cytochrome P450 reductase (CPR) in insect Spodoptera frugiperda (Sf9) cells using a baculovirus-mediated expression system and the enzymatic activity and metabolic ability of CYP9M10/CPR and CYP6AA7/CPR to permethrin and its metabolites, including 3-phenoxybenzoic alcohol (PBOH) and 3-phenoxybenzaldehyde (PBCHO), characterized. PBOH and PBCHO, both of which are toxic to Culex mosquito larvae, can be further metabolized by CYP9M10/CPR and CYP6AA7/CPR, with the ultimate metabolite identified here as PBCOOH, which is considerably less toxic to mosquito larvae. A cell-based MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) cytotoxicity assay revealed that Sf9 cells expressing CYP9M10/CPR or CYP6AA7/CPR increased the cell line's tolerance to permethrin, PBOH, and PBCHO. This study confirms the important role played by CYP9M10 and CYP6AA7 in the detoxification of permethrin and its metabolites PBOH and PBCHO.

The choreography of the chemical defensome response to insecticide stress: insights into the Anopheles stephensi transcriptome using RNA-Seq

Animals respond to chemical stress with an array of gene families and pathways termed “chemical defensome”. In arthropods, despite many defensome genes have been detected, how their activation is arranged during toxic exposure remains poorly understood. Here, we sequenced the transcriptome of Anopheles stephensi larvae exposed for six, 24 and 48 hours to the LD₅₀ dose of the insecticide permethrin to monitor transcriptional changes of defensome genes across time. A total of 177 genes involved in insecticide defense were differentially expressed (DE) in at least one time-point, including genes encoding for Phase 0, I, II, III and antioxidant enzymes and for Heat Shock and Cuticular Proteins. Three major patterns emerged throughout time. First, most of DE genes were down-regulated at all time-points, suggesting a reallocation of energetic resources during insecticide stress. Second, single genes and clusters of genes turn off and on from six to 48 hours of treatment, showing a modulated response across time. Third, the number of up-regulated genes peaked at six hours and then decreased during exposure. Our results give a first picture of how defensome gene families respond against toxicants and provide a valuable resource for understanding how defensome genes work together during insecticide stress.

Current knowledge of detoxification mechanisms of xenobiotic in honey bees

The western honey bee Apis mellifera is the most important managed pollinator species in the world. Multiple factors have been implicated as potential causes or factors contributing to colony collapse disorder, including honey bee pathogens and nutritional deficiencies as well as exposure to pesticides. Honey bees' genome is characterized by a paucity of genes associated with detoxification, which makes them vulnerable to specific pesticides, especially to combinations of pesticides in real field environments. Many studies have investigated the mechanisms involved in detoxification of xenobiotics/pesticides in honey bees, from primal enzyme assays or toxicity bioassays to characterization of transcript gene expression and protein expression in response to xenobiotics/insecticides by using a global transcriptomic or proteomic approach, and even to functional characterizations. The global transcriptomic and proteomic approach allowed us to learn that detoxification mechanisms in honey bees involve multiple genes and pathways along with changes in energy metabolism and cellular stress response. P450 genes, is highly implicated in the direct detoxification of xenobiotics/insecticides in honey bees and their expression can be regulated by honey/pollen constitutes, resulting in the tolerance of honey bees to other xenobiotics or insecticides. P450s is also a key detoxification enzyme that mediate synergism interaction between acaricides/insecticides and fungicides through inhibition P450 activity by fungicides or competition for detoxification enzymes between acaricides. With the wide use of insecticides in agriculture, understanding the detoxification mechanism of insecticides in honey bees and how honeybees fight with the xenobiotis or insecticides to survive in the changing environment will finally benefit honeybees' management.

A G-protein-coupled receptor regulation pathway in cytochrome P450-mediated permethrin-resistance in mosquitoes, Culex quinquefasciatus

Rhodopsin-like G protein-coupled receptors (GPCRs) are known to be involved in the GPCR signal transduction system and regulate many essential physiological processes in organisms. This study, for the first time, revealed that knockdown of the rhodopsin-like GPCR gene in resistant mosquitoes resulted in a reduction of mosquitoes’ resistance to permethrin, simultaneously reducing the expression of two cAMP-dependent protein kinase A genes (PKAs) and four resistance related cytochrome P450 genes. The function of rhodopsin-like GPCR was further confirmed using transgenic lines of Drosophila melanogaster, in which the tolerance to permethrin and the expression of Drosophila resistance P450 genes were both increased. The roles of GPCR signaling pathway second messenger cyclic adenosine monophosphate (cAMP) and downstream effectors PKAs in resistance were investigated using cAMP production inhibitor Bupivacaine HCl and the RNAi technique. Inhibition of cAMP production led to significant decreases in both the expression of four resistance P450 genes and two PKA genes and mosquito resistance to permethrin. Knockdown of the PKA genes had shown the similar effects on permethrin resistance and P450 gene expression. Taken together, our studies revealed, for the first time, the role of the GPCR/cAMP/PKA-mediated regulatory pathway governing P450 gene expression and P450-mediated resistance in Culex mosquitoes.

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

Background

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

Methods

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

Results

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

Conclusions

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

Electronic supplementary material

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

Cytochrome P450s--Their expression, regulation, and role in insecticide resistance

P450s are known to be critical for the detoxification and/or activation of xenobiotics such as drugs and pesticides and overexpression of P450 genes can significantly affect the disposition of xenobiotics in the tissues of organisms, altering their pharmacological/toxicological effects. In insects, P450s play an important role in detoxifying exogenous compounds such as insecticides and plant toxins and their overexpression can result in increased levels of P450 proteins and P450 activities. This has been associated with enhanced metabolic detoxification of insecticides and has been implicated in the development of insecticide resistance in insects. Multiple P450 genes have been found to be co-overexpressed in individual insect species via several constitutive overexpression and induction mechanisms, which in turn are co-responsible for high levels of insecticide resistance. Many studies have also demonstrated that the transcriptional overexpression of P450 genes in resistant insects is regulated by trans and/or cis regulatory genes/factors. Taken together, these earlier findings suggest not only that insecticide resistance is conferred via multi-resistance P450 genes, but also that it is mediated through the interaction of regulatory genes/factors and resistance genes. This chapter reviews our current understanding of how the molecular mechanisms of P450 interaction/gene regulation govern the development of insecticide resistance in insects and our progress along the road to a comprehensive characterization of P450 detoxification-mediated insecticide resistance.

In silico models for predicting vector control chemicals targeting Aedes aegypti

Human arboviral diseases have emerged or re-emerged in numerous countries worldwide due to a number of factors including the lack of progress in vaccine development, lack of drugs, insecticide resistance in mosquitoes, climate changes, societal behaviours, and economical constraints. Thus, Aedes aegypti is the main vector of the yellow fever and dengue fever flaviviruses and is also responsible for several recent outbreaks of the chikungunya alphavirus. As for the other mosquito species, the A. aegypti control relies heavily on the use of insecticides. However, because of increasing resistance to the different families of insecticides, reduction of Aedes populations is becoming increasingly difficult. Despite the unquestionable utility of insecticides in fighting mosquito populations, there are very few new insecticides developed and commercialized for vector control. This is because the high cost of the discovery of an insecticide is not counterbalanced by the 'low profitability' of the vector control market. Fortunately, the use of quantitative structure-activity relationship (QSAR) modelling allows the reduction of time and cost in the discovery of new chemical structures potentially active against mosquitoes. In this context, the goal of the present study was to review all the existing QSAR models on A. aegypti. The homology and pharmacophore models were also reviewed. Specific attention was paid to show the variety of targets investigated in Aedes in relation to the physiology and ecology of the mosquito as well as the diversity of the chemical structures which have been proposed, encompassing man-made and natural substances.

Permethrin resistance variation and susceptible reference line isolation in a field population of the mosquito, Culex quinquefasciatus (Diptera: Culicidae)

This study examines the genetic variations and mechanisms involved in the development of permethrin resistance in individual mosquitoes from a field population of Culex quinquefasciatus, HAmCq(G0) , and characterizes susceptible reference lines of mosquitoes with a similar genetic background to the field HAmCq(G0) strain. Six upregulated cytochrome P450 genes, CYP9M10, CYP9J34, CYP6P14, CYP9J40, CYP6AA7, and CYP4C52v1, previously identified as being upregulated in the larvae of resistant HAmCq(G8) mosquitoes were examined in the larvae of 3 strains (susceptible S-Lab, parental HAmCq(G0) and permethrin-selected highly resistant HAmCq(G8) ) and 8 HAmCq(G0) single-egg raft colonies, covering a range of levels of susceptibility/resistance to permethrin and exhibiting different variations in the expression of A and/or T alleles at the L-to-F kdr locus of the sodium channel. The 2 lines with the lowest tolerance to permethrin and bearing solely the susceptible A allele at the L-to-F kdr locus of the sodium channels, from colonies Cx_SERC5 and Cx_SERC8, showed lower or similar levels of all 6 of the P450 genes tested compared with the S-Lab strain, suggesting that these 2 lines could be used as the reference mosquitoes in future studies characterizing insecticide resistance in HAmCq mosquitoes. This study also provides a detailed investigation of the mechanisms involved in insecticide resistance in individuals within a population: individuals with elevated levels of resistance to permethrin all displayed one or more potential resistance mechanisms-either elevated levels of P450 gene expression, or L-to-F mutations in the sodium channel, or both.

Transcriptional analysis of four family 4 P450s in a Puerto Rico strain of Aedes aegypti (Diptera: Culicidae) compared with an Orlando strain and their possible functional roles in permethrin resistance

A field strain of Aedes aegypti (L.) was collected from Puerto Rico in October 2008. Based on LD50 values by topical application, the Puerto Rico strain was 73-fold resistant to permethrin compared with a susceptible Orlando strain. In the presence of piperonyl butoxide, the resistance of Puerto Rico strain of Ae. aegypti was reduced to 15-fold, suggesting that cytochrome P450-mediated detoxification is involved in the resistance of the Puerto Rico strain to permethrin. To determine the cytochrome P450s that might play a role in the resistance to permethrin, the transcriptional levels of 164 cytochrome P450 genes in the Puerto Rico strain were compared with that in the Orlando strain. Of the 164 cytochrome P450s, 33 were significantly (P < 0.05) up-regulated, including cytochrome P450s in families four, six, and nine. Multiple studies have investigated the functionality of family six and nine cytochrome P450s, therefore, we focused on the up-regulated family 4 cytochrome P450s. To determine whether up-regulation of the four cytochrome P450s had any functional role in permethrin resistance, transgenic Drosophila melanogaster Meigen lines overexpressing the four family 4 P450 genes were generated, and their ability to survive exposure to permethrin was evaluated. When exposed to 5 microg per vial permethrin, transgenic D. melanogaster expressing CYP4D24, CYP4H29, CYP4J15v1, and CYP4H33 had a survival rate of 60.0 +/- 6.7, 29.0 +/- 4.4, 64.4 +/- 9.7, and 11.0 +/- 4.4%, respectively. However, none of the control flies survived the permethrin exposure at the same concentration. Similarly, none of the transgenic D. melanogaster expressing CYP4J15v1 or CYP4H33 ?5 survived when they were exposed to permethrin at 10 microg per vial. However, transgenic D. melanogaster expressing CYP4D24 and CYP4H29 had a survival rate of 37.8 +/- 4.4 and 2.2 +/- 2.2%, respectively. Taken together, our results suggest that CYP4D24 might play an important role in cytochrome P450-mediated resistance to permethrin.