Differential expression of the detoxification genes in the different life stages of the malaria vector Anopheles gambiae

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.

Transcriptomic and functional genetic evidence for distinct ecophysiological responses across complex life cycle stages

Organisms with complex life cycles demonstrate a remarkable ability to change their phenotypes across development, presumably as an evolutionary adaptation to developmentally variable environments. Developmental variation in environmentally sensitive performance, and thermal sensitivity in particular, has been well documented in holometabolous insects. For example, thermal performance in adults and juvenile stages exhibit little genetic correlation (genetic decoupling) and can evolve independently, resulting in divergent thermal responses. Yet, we understand very little about how this genetic decoupling occurs. We tested the hypothesis that genetic decoupling of thermal physiology is driven by fundamental differences in physiology between life stages, despite a potentially conserved Cellular Stress Response. We used RNAseq to compare transcript expression in response to a cold stressor in Drosophila melanogaster larvae and adults and used RNAi (RNA interference) to test whether knocking down nine target genes differentially affected larval and adult cold tolerance. Transcriptomic responses of whole larvae and adults during and following exposure to -5°C were largely unique both in identity of responding transcripts and in temporal dynamics. Further, we analyzed the tissue-specificity of differentially-expressed transcripts from FlyAtlas 2 data, and concluded that stage-specific differences in transcription were not simply driven by differences in tissue composition. In addition, RNAi of target genes resulted in largely stage-specific and sometimes sex-specific effects on cold tolerance. The combined evidence suggests that thermal physiology is largely stage-specific at the level of gene expression, and thus natural selection may be acting on different loci during the independent thermal adaptation of different life stages.

Emerging Midges Transport Pesticides from Aquatic to Terrestrial Ecosystems: Importance of Compound- and Organism-Specific Parameters

Emerging aquatic insects have the potential to retain aquatic contaminants after metamorphosis, potentially transporting them into adjacent terrestrial food webs. It is unknown whether this transfer is also relevant for current-use pesticides. We exposed larvae of the nonbiting midge, Chironomus riparius, to a sublethal pulse of a mixture of nine moderately polar fungicides and herbicides (logK_ow 2.5-4.7) at three field relevant treatment levels (1.2-2.5, 17.5-35.0, or 50.0-100.0 μg/L). We then assessed the pesticide bioaccumulation and bioamplification over the full aquatic-terrestrial life cycle of both sexes including the egg laying of adult females. By applying sensitive LC-MS/MS analysis to small sample volumes (∼5 mg, dry weight), we detected all pesticides in larvae from all treatment levels (2.8-1019 ng/g), five of the pesticides in the adults from the lowest treatment level and eight in the higher treatment levels (1.5-3615 ng/g). Retention of the pesticides through metamorphosis was not predictable based solely on pesticide lipophilicity. Sex-specific differences in adult insect pesticide concentrations were significant for five of the pesticides, with greater concentrations in females for four of them. Over the duration of the adults' lifespan, pesticide concentrations generally decreased in females while persisting in males. Our results suggest that a low to moderate daily dietary exposure to these pesticides may be possible for tree swallow nestlings and insectivorous bats.

Understanding molt control switches: Transcriptomic and expression analysis of the genes involved in ecdysteroidogenesis and cholesterol uptake pathways in the Y-organ of the blue crab, Callinectes sapidus

The crustacean molting process is regulated by an interplay of hormones produced by the eyestalk ganglia and Y-organs (YO). Molt-inhibiting hormone and crustacean hyperglycemic hormone released by the sinus gland of the eyestalk ganglia (EG) inhibit the synthesis and secretion of ecdysteroid by the YO, hence regulating hemolymph levels during the molt cycle. The purpose of this study is to investigate the ecdysteroidogenesis pathway, specifically genes linked to changes in ecdysteroid levels occurring at early premolt (ePM). To this end, a reference transcriptome based on YO, EG, and hepatopancreas was de novo assembled. Two genes (cholesterol 7-desaturase Neverland and cytochrome p450 307a1-like Spook) involved in ecdysteroidogenesis were identified from the YO transcriptome using sequence comparisons and transcript abundance. Two other candidates, Hormone receptor 4 and probable cytochrome p450 49a1 potentially involved in ecdysteroidogenesis were also identified. Since cholesterol is the ecdysteroid precursor, a putative cholesterol carrier (Apolipoprotein D-like) was also examined to understand if cholesterol uptake coincided with the increase in the ecdysteroid levels at the ePM stage. The expression level changes of the five candidate genes in the YO were compared between intermolt (IM) and induced ePM (iePM) stages using transcriptomic analysis. Expression analysis using qPCR were carried out at IM, iePM, and normal ePM. The increase in Spook and Neverland expression in the YO at the ePM was accompanied by a concomitant rise in ecdysteroid levels. The data obtained from iePM stage were congruent with those obtained from the normal ePM stage of intact control animals. The present findings support the role of Halloween genes in the ecdysteroidogenesis and molt cycle in the blue crab, Callinectes sapidus.

Combined Effects of Pesticides and Electromagnetic-Fields on Honeybees: Multi-Stress Exposure

Honeybee and general pollinator decline is extensively reported in many countries, adding new concern to the general biodiversity loss. Many studies were addressed to assess the causes of pollinator decline, concluding that in most cases multi-stress effects were the most probable ones. In this research, the combined effects of two possible stress sources for bees, pesticides and electromagnetic fields (multi-stress conditions), were analyzed in the field. Three experimental sites were chosen: a control one far from direct anthropogenic stress sources, a pesticide-stress site and multi-stress one, adding to the same exposure to pesticides the presence of an electromagnetic field, coming from a high-voltage electric line. Experimental apiaries were monitored weekly for one year (from April 2017 to April 2018) by means of colony survival, queen activity, storage and brood amount, parasites and pathogens, and several biomarkers in young workers and pupae. Both exposure and effect biomarkers were analysed: among the first, acetylcholinesterase (AChE), catalase (CAT), glutathione S-transferase (GST) and alkaline phosphatase (ALP) and Reactive Oxygen Species (ROS); and among the last, DNA fragmentation (DNAFRAGM) and lipid peroxidation (LPO). Results showed that bee health conditions were the worst in the multi-stress site with only one colony alive out of the four ones present at the beginning. In this site, a complex picture of adverse effects was observed, such as disease appearance (American foulbrood), higher mortality in the underbaskets (common to pesticide-stress site), behavioral alterations (queen changes, excess of honey storage) and biochemical anomalies (higher ALP activity at the end of the season). The overall results clearly indicate that the multi-stress conditions were able to induce biochemical, physiological and behavioral alterations which severely threatened bee colony survival.

Genome of the webworm Hyphantria cunea unveils genetic adaptations supporting its rapid invasion and spread

BACKGROUND

The fall webworm Hyphantria cunea is an invasive and polyphagous defoliator pest that feeds on nearly any type of deciduous tree worldwide. The silk web of H. cunea aids its aggregating behavior, provides thermal regulation and is regarded as one of causes for its rapid spread. In addition, both chemosensory and detoxification genes are vital for host adaptation in insects.

RESULTS

Here, a high-quality genome of H. cunea was obtained. Silk-web-related genes were identified from the genome, and successful silencing of the silk protein gene HcunFib-H resulted in a significant decrease in silk web shelter production. The CAFE analysis showed that some chemosensory and detoxification gene families, such as CSPs, CCEs, GSTs and UGTs, were expanded. A transcriptome analysis using the newly sequenced H. cunea genome showed that most chemosensory genes were specifically expressed in the antennae, while most detoxification genes were highly expressed during the feeding peak. Moreover, we found that many nutrient-related genes and one detoxification gene, HcunP450 (CYP306A1), were under significant positive selection, suggesting a crucial role of these genes in host adaptation in H. cunea. At the metagenomic level, several microbial communities in H. cunea gut and their metabolic pathways might be beneficial to H. cunea for nutrient metabolism and detoxification, and might also contribute to its host adaptation.

CONCLUSIONS

These findings explain the host and environmental adaptations of H. cunea at the genetic level and provide partial evidence for the cause of its rapid invasion and potential gene targets for innovative pest management strategies.

Adulticide Resistance Status of Aedes albopictus (Diptera: Culicidae) in Sabah, Malaysia: A Statewide Assessment

Resistance status of Aedes albopictus (Diptera: Culicidae) collected from Sabah, East Malaysia, was evaluated against four major classes of adulticides, namely pyrethroid, carbamate, organochlorine, and organophosphate. Adult bioassays conforming to WHO standard protocols were conducted to assess knockdown and mortality rates of Ae. albopictus. Among tested pyrethroid adulticides, only cyfluthrin, lambda-cyaholthrin, and deltamethrin were able to inflict total knockdown. The other adulticide classes mostly failed to cause any knockdown; the highest knockdown rate was only 18.33% for propoxur. With regards to mortality rate, Ae. albopictus was unanimously susceptible toward all pyrethroids, dieldrin, and malathion, but exhibited resistance toward bendiocarb, propoxur, dichlorodiphenyltrichloroethane, and fenitrothion. Additionally, correlation analysis demonstrated cross-resistance between bendiocarb and propoxur, and malathion and propoxur. In conclusion, this study has disclosed that pyrethroids are still generally effective for Aedes control in Sabah, Malaysia. The susceptibility status of Ae. albopictus against pyrethroids in descending order was cyfluthrin > lambda-cyhalothrin > deltamethrin > etofenprox > permethrin.

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.

Latrophilin mediates insecticides susceptibility and fecundity through two carboxylesterases, esterase4 and esterase6, in Tribolium castaneum

Latrophilin (LPH) is known as an adhesion G-protein-coupled receptor which involved in multiple physiological processes in organisms. Previous studies showed that lph not only involved the susceptibility to anticholinesterase insecticides but also affected fecundity in Tribolium castaneum. However, its regulatory mechanisms in these biological processes are still not clear. Here, we identified two potential downstream carboxylesterase (cce) genes of Tclph, esterase4 and esterase6, and further characterized their interactions with Tclph. After treatment of T. castaneum larvae with carbofuran or dichlorvos insecticides, the transcript levels of Tcest4 and Tcest6 were significantly induced from 12 to 72 h. RNAi against Tcest4 or Tcest6 led to the higher mortality compared with the controls after the insecticides treatment, suggesting that these two genes play a vital role in detoxification of insecticides in T. castaneum. Furthermore, with insecticides exposure to Tclph knockdown beetles, the expression of Tcest4 was upregulated but Tcest6 was downregulated, indicating that beetles existed a compensatory response against the insecticides. Additionally, RNAi of Tcest6 resulted in 43% reductions in female egg laying and completely inhibited egg hatching, which showed the similar phenotype as that of Tclph knockdown. These results indicated that Tclph affected fecundity by positively regulating Tcest6 expression. Our findings will provide a new insight into the molecular mechanisms of Tclph involved in physiological functions in T. castaneum.

Increased Biting Rate of Insecticide-Resistant Culex Mosquitoes and Community Adherence to IRS for Malaria Control in Urban Malabo, Bioko Island, Equatorial Guinea

Sustaining high levels of indoor residual spraying (IRS) coverage (≥85%) for community protection against malaria remains a challenge for IRS campaigns. We examined biting rates and insecticide resistance in Culex species and Anopheles gambiae s.l., and their potential effect on community adherence to IRS. The average IRS coverage in urban Malabo between 2015 and 2017 remained at 80%. Culex biting rate increased 6.0-fold (P < 0.001) between 2014 and 2017, reaching 8.08 bites per person per night, whereas that of An. gambiae s.l. remained steady at around 0.68. Although An. gambiae s.l. was susceptible to carbamates and organophosphates insecticides, Culex spp. were phenotypically resistant to all four main classes of WHO-recommended IRS insecticides. Similarly, the residual activity of the organophosphate insecticide used since 2017, ACTELLIC 300CS, was 8 mo for An. gambiae s.l., but was almost absent against Culex for 2 mo post-spray. A survey conducted in 2018 within urban Malabo indicated that 77.0% of respondents related IRS as means of protection against mosquito bites, but only 3.2% knew that only Anopheles mosquitoes transmit malaria. Therefore, the increasing biting rates of culicines in urban Malabo, and their resistance to all IRS insecticides, is raising concern that a growing number of people may refuse to participate in IRS as result of its perceived failure in controlling mosquitoes. Although this is not yet the case on Bioko Island, communication strategies need refining to sensitize communities about the effectiveness of IRS in controlling malaria vectors in the midst of insecticide resistance in nonmalaria vector mosquitoes.

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.

malERA: An updated research agenda for insecticide and drug resistance in malaria elimination and eradication

Resistance to first-line treatments for Plasmodium falciparum malaria and the insecticides used for Anopheles vector control are threatening malaria elimination efforts. Suboptimal responses to drugs and insecticides are both spreading geographically and emerging independently and are being seen at increasing intensities. Whilst resistance is unavoidable, its effects can be mitigated through resistance management practices, such as exposing the parasite or vector to more than one selective agent. Resistance contributed to the failure of the 20th century Global Malaria Eradication Programme, and yet the global response to this issue continues to be slow and poorly coordinated-too often, too little, too late. The Malaria Eradication Research Agenda (malERA) Refresh process convened a panel on resistance of both insecticides and antimalarial drugs. This paper outlines developments in the field over the past 5 years, highlights gaps in knowledge, and proposes a research agenda focused on managing resistance. A deeper understanding of the complex biological processes involved and how resistance is selected is needed, together with evidence of its public health impact. Resistance management will require improved use of entomological and parasitological data in decision making, and optimisation of the useful life of new and existing products through careful implementation, combination, and evaluation. A proactive, collaborative approach is needed from basic science and the development of new tools to programme and policy interventions that will ensure that the armamentarium of drugs and insecticides is sufficient to deal with the challenges of malaria control and its elimination.

Regulation of P450-mediated permethrin resistance in Culex quinquefasciatus by the GPCR/Gαs/AC/cAMP/PKA signaling cascade

This study explores the role of G-protein-coupled receptor-intracellular signaling in the development of P450-mediated insecticide resistance in mosquitoes, Culex quinquefasciatus, focusing on the essential function of the GPCRs and their downstream effectors of Gs alpha subunit protein (Gαs) and adenylyl cyclase (ACs) in P450-mediated insecticide resistance of Culex mosquitoes. Our RNAi-mediated functional study showed that knockdown of Gαs caused the decreased expression of the downstream effectors of ACs and PKAs in the GPCR signaling pathway and resistance P450 genes, whereas knockdown of ACs decreased the expression of PKAs and resistance P450 genes. Knockdown of either Gαs or ACs resulted in an increased susceptibility of mosquitoes to permethrin. These results add significantly to our understanding of the molecular basis of resistance P450 gene regulation through GPCR/Gαs/AC/cAMP-PKA signaling pathways in the insecticide resistance of mosquitoes. The temporal and spatial dynamic analyses of GPCRs, Gαs, ACs, PKAs, and P450s in two insecticide resistant mosquito strains revealed that all the GPCR signaling pathway components tested, namely GPCRs, Gαs, ACs and PKAs, were most highly expressed in the brain for both resistant strains, suggesting the role played by these genes in signaling transduction and regulation. The resistance P450 genes were mainly expressed in the brain, midgut and malpighian tubules (MTs), suggesting their critical function in the central nervous system and importance for detoxification. The temporal dynamics analysis for the gene expression showed a diverse expression profile during mosquito development, indicating their initially functional importance in response to exposure to insecticides during their life stages.

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GPCR, Gs alpha subunit protein (Gαs), adenylyl cyclase (ACs), and protein kinase A (PKAs) regulates resistance P450 gene expression and the development of insecticide resistance in mosquitoes, Culex quinquefasciatus.

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GPCR, Gαs, ACs, and PKAs, are highly expressed in the brain of mosquitoes, corresponding their role in signaling transduction, and regulation.

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GPCR, Gαs, ACs, PKAs, and P450s are expressed in the different life stages of mosquitoes, revealing their functional importance in response to exposure to insecticides during mosquito life stages.

Biology, Bionomics and Molecular Biology of Anopheles sinensis Wiedemann 1828 (Diptera: Culicidae), Main Malaria Vector in China

China has set a goal to eliminate all malaria in the country by 2020, but it is unclear if current understanding of malaria vectors and transmission is sufficient to achieve this objective. Anopheles sinensis is the most widespread malaria vector specie in China, which is also responsible for vivax malaria outbreak in central China. We reviewed literature from 1954 to 2016 on An. sinensis with emphasis on biology, bionomics, and molecular biology. A total of 538 references were relevant and included. An. sienesis occurs in 29 Chinese provinces. Temperature can affect most life-history parameters. Most An. sinensis are zoophilic, but sometimes they are facultatively anthropophilic. Sporozoite analysis demonstrated An. sinensis efficacy on Plasmodium vivax transmission. An. sinensis was not stringently refractory to P. falciparum under experimental conditions, however, sporozoite was not found in salivary glands of field collected An. sinensis. The literature on An. sienesis biology and bionomics was abundant, but molecular studies, such as gene functions and mechanisms, were limited. Only 12 molecules (genes, proteins or enzymes) have been studied. In addition, there were considerable untapped omics resources for potential vector control tools. Existing information on An. sienesis could serve as a baseline for advanced research on biology, bionomics and genetics relevant to vector control strategies.

Transcriptome Analysis of Green Peach Aphid (Myzus persicae): Insight into Developmental Regulation and Inter-Species Divergence

Green peach aphid (Myzus persicae) and pea aphid (Acyrthosiphon pisum) are two phylogenetically closely related agricultural pests. While pea aphid is restricted to Fabaceae, green peach aphid feeds on hundreds of plant species from more than 40 families. Transcriptome comparison could shed light on the genetic factors underlying the difference in host range between the two species. Furthermore, a large scale study contrasting gene expression between immature nymphs and fully developed adult aphids would fill a previous knowledge gap. Here, we obtained transcriptomic sequences of green peach aphid nymphs and adults, respectively, using Illumina sequencing technology. A total of 2244 genes were found to be differentially expressed between the two developmental stages, many of which were associated with detoxification, hormone production, cuticle formation, metabolism, food digestion, and absorption. When searched against publically available pea aphid mRNA sequences, 13,752 unigenes were found to have no homologous counterparts. Interestingly, many of these unigenes that could be annotated in other databases were involved in the "xenobiotics biodegradation and metabolism" pathway, suggesting the two aphids differ in their adaptation to secondary metabolites of host plants. Conversely, 3989 orthologous gene pairs between the two species were subjected to calculations of synonymous and nonsynonymous substitutions, and 148 of the genes potentially evolved in response to positive selection. Some of these genes were predicted to be associated with insect-plant interactions. Our study has revealed certain molecular events related to aphid development, and provided some insight into biological variations in two aphid species, possibly as a result of host plant adaptation.

Characterization and functional analysis of a novel glutathione S-transferase gene potentially associated with the abamectin resistance in Panonychus citri (McGregor)

The citrus red mite, Panonychus citri (McGregor), a major citrus pest distributed worldwide, has been found to be resistant to various insecticides and acaricides used in China. However, the molecular mechanisms associated with the abamectin resistance in this species have not yet been reported. In this study, results showed over-expression of a novel glutathione S-transferases (GSTs) gene (PcGSTm5) in abamectin-resistant P. citri. Quantitative real-time PCR analysis showed that the transcripts of PcGSTm5 were also significantly up-regulated after exposure to abamectin and the maximum mRNA expression level at nymphal stage. The recombinant protein of PcGSTm5-pET-28a produced by Escherichia coli showed a pronounced activity toward the conjugates of 1-chloro-2,4 dinitrobenzene (CDNB) and glutathione (GSH). The kinetics of CDNB and GSH and its optimal pH and thermal stability were also determined. Reverse genetic study through a new method of leaf-mediated dsRNA feeding further support a link between the expression of PcGSTm5 and abamectin resistance. However, no direct evidence was found in metabolism or inhibition assays to confirm the hypothesis that PcGSTm5 can metabolize abamectin. Finally, it is here speculated that PcGSTm5 may play a role in abamectin detoxification through other pathway such as the antioxidant protection.

Know your ABCs: Characterization and gene expression dynamics of ABC transporters in the polyphagous herbivore Helicoverpa armigera

Polyphagous insect herbivores are adapted to many different secondary metabolites of their host plants. However, little is known about the role of ATP-binding cassette (ABC) transporters, a multigene family involved in detoxification processes. To study the larval response of the generalist Helicoverpa armigera (Lepidoptera) and the putative role of ABC transporters, we performed developmental assays on artificial diet supplemented with secondary metabolites from host plants (atropine-scopolamine, nicotine and tomatine) and non-host plants (taxol) in combination with a replicated RNAseq experiment. A maximum likelihood phylogeny identified the subfamily affiliations of the ABC transporter sequences. Larval performance was equal on the atropine-scopolamine diet and the tomatine diet. For the latter we could identify a treatment-specific upregulation of five ABC transporters in the gut. No significant developmental difference was detected between larvae fed on nicotine or taxol. This was also mirrored in the upregulation of five ABC transporters when fed on either of the two diets. The highest number of differentially expressed genes was recorded in the gut samples in response to feeding on secondary metabolites. Our results are consistent with the expectation of a general detoxification response in a polyphagous herbivore. This is the first study to characterize the multigene family of ABC transporters and identify gene expression changes across different developmental stages and tissues, as well as the impact of secondary metabolites in the agricultural pest H. armigera.

Identification, Validation, and Application of Molecular Diagnostics for Insecticide Resistance in Malaria Vectors

Insecticide resistance is a major obstacle to control of Anopheles malaria mosquitoes in sub-Saharan Africa and requires an improved understanding of the underlying mechanisms. Efforts to discover resistance genes and DNA markers have been dominated by candidate gene and quantitative trait locus studies of laboratory strains, but with greater availability of genome sequences a shift toward field-based agnostic discovery is anticipated. Mechanisms evolve continually to produce elevated resistance yielding multiplicative diagnostic markers, co-screening of which can give high predictive value. With a shift toward prospective analyses, identification and screening of resistance marker panels will boost monitoring and programmatic decision making.

Selection for chlorpyrifos resistance in Liriomyza sativae Blanchard: Cross-resistance patterns, stability and biochemical mechanisms

The vegetable leafminer (VLM), Liriomyza sativae (Diptera: Agromyzidae) is a serious pest of vegetable crops and ornamentals worldwide. In cropping systems with inappropriate management strategies, development of resistance to insecticides in leafminers is probable. Chlorpyrifos is a commonly used pesticide for controlling leafminers in Iran, but resistance to this insecticide in leafminers has not been characterized. In order to develop strategies to minimize resistance in the field and greenhouse, a laboratory selected chlorpyrifos resistant strain of L. sativae was used to characterize resistance and determine the rate of development and stability of resistance. Selecting for resistance in the laboratory after 23 generations yielded a chlorpyrifos resistant selected strain (CRSS) with a resistance ratio of 40.34, determined on the larval stage. CRSS exhibited no cross-resistance to other tested insecticides except for diazinon. Synergism and biochemical assays indicated that esterases (EST) had a key role in metabolic resistance to chlorpyrifos, but glutathione S-transferase (GST) and mixed function oxidase (MFO) were not mediators in this resistance. In CRSS acetylcholinesterase (AChE) was more active than the susceptible strain, Sharif (SH). AChE in CRSS was also less sensitive to inhibition by propoxur. The kinetics parameters (Km and Vmax) of AChE indicated that affinities and hydrolyzing efficiencies of this enzyme in CRSS were higher than SH. Susceptibility to chlorpyrifos in L. sativae was re-gained in the absence of insecticide pressure. Synergism, biochemical and cross-resistance assays revealed that overactivity of metabolic enzymes and reduction in target site sensitivity are probably joint factors in chlorpyrifos resistance. An effective insecticide resistance management program is necessary to prevent fast resistance development in crop systems.

Aniline exposure associated with up-regulated transcriptional responses of three glutathione S-transferase Delta genes in Drosophila melanogaster

Complex transcriptional profile of glutathione S-transferase Delta cluster genes occurred in the developmental process of the fruit fly Drosophila melanogaster. The purpose of this project was to quantify the expression levels of Gst Delta class genes altered by aniline exposure and to understand the relationship between aniline dosages and the variation of Gst Delta genes expressed in D. melanogaster. Using RT-PCR expression assays, the expression patterns of the transcript mRNAs of the glutathione S-transferase Delta genes were revealed and their expression levels were measured at eggs, larvae, pupae and adults. The adult stage was selected for further dose-response assays. After analysis, the results indicated that three Gst Delta genes (Gst D2, Gst D5 and Gst D6) were found to show a peak of up-regulated transcriptional response at 6-8h of exposure of aniline. Furthermore, the dose-response relationship of their induction levels within the dose regiments (from 1.2 to 2.0 μl/tube) had been measured. The expression patterns and annotations of these genes were discussed in the context.

Molecular cloning and sequence analysis of novel cytochrome P450 cDNA fragments from Dastarcus helophoroides

The predatory beetle Dastarcus helophoroides (Fairmaire) (Coleoptera: Bothrideridae) is a natural enemy of many longhorned beetles and is mainly distributed in both China and Japan. To date, no research on D. helophoroides P450 enzymes has been reported. In our study, for the better understanding of P450 enzymes in D. helophoroides, 100 novel cDNA fragments encoding cytochrome P450 were amplified from the total RNA of adult D. helophoroides abdomens using five pairs of degenerate primers designed according to the conserved amino acid sequences of the CYP6 family genes in insects through RT-PCR. The obtained nucleotide sequences were 250 bp, 270 bp, and 420 bp in length depending on different primers. Ninety-six fragments were determined to represent CYP6 genes, mainly from CYP6BK, CYP6BQ, and CYP6BR subfamilies, and four fragments were determined to represent CYP9 genes. Twenty-two fragments, submitted to GenBank, were selected for further homologous analysis, which revealed that some fragments of different sizes might be parts of the same P450 gene.

Multiple glutathione S-transferase genes: identification and expression in oriental fruit fly, Bactrocera dorsalis

BACKGROUND

The oriental fruit fly, Bactrocera dorsalis (Hendel), is widely distributed in Asia-Pacific regions, where it is a serious pest of a wide range of tropical and subtropical fruit and vegetable crops. In this study, 17 cDNA encoding glutathione S-transferases (GSTs) in B. dorsalis were sequenced and characterised.

RESULTS

Phylogenetic analysis revealed that 16 GSTs belonged to five different cytosolic classes, including four in delta, eight in epsilon, two in omega, one in theta, and one in zeta. The remaining GST (BdGSTu1) was unclassified. RT-qPCR assay showed that the relative expression levels of five GST genes were significantly higher in larval stages than in adulthood. Tissue-specific expression analysis found that BdGSTe3, BdGSTe9 and BdGSTd5 were expressed highly in the midgut, BdGSTe4, BdGSTe6, BdGSTd6 and BdGSTz2 were higher in the fat body, and six GSTs were higher in Malpighian tubules. RT-qPCR confirmed that the expressions of nine GST genes were increased by malathion exposure at various times and doses, while BdGSTe4, BdGSTe9 and BdGSTt1 were increased by β-cypermethrin exposure.

CONCLUSION

The increases in GST gene expression levels after malathion and β-cypermethrin exposure in B. dorsalis might increase the ability of this species to detoxify other insecticides and xenobiotics.

Identification and characterization of seven glutathione S-transferase genes from citrus red mite, Panonychus citri (McGregor)

The citrus red mite, Panonychus citri (McGregor), is a global citrus pest, and has developed severe resistance to several types of acaricides. However, the molecular mechanisms of resistance in this mite remain unknown. In this study, seven full-length cDNAs encoding glutathione S-transferases (GSTs) genes were identified and characterized in P. citri. The effects of pyridaben and fenpropathrin exposure on the expression of these genes were also investigated. Phylogenetic analysis revealed that the seven GSTs genes in P. citri cloned in this study belong to three different cytosolic classes, including four in mu, two in delta and one in zeta. Among these seven GSTs genes, the relative expression level of PcGSTm1 was significantly higher in adult than in the other life stages (egg, larvae and nymph). Compared with the control, the mRNA levels of the seven GST genes did not change significantly following exposure to pyridaben at LC10. However, RT-qPCR results showed that, when exposed to LC10 of fenpropathrin, six GSTs gene (PcGSTm1, PcGSTm3, PcGSTm4, PcGSTd1, PcGSTd2 and PcGSTz1) transcripts increased in a time-dependent manner. This is the first insight into the molecular characteristics of GSTs gene cDNAs in P. citri. The elevated GSTs gene transcripts following exposure to fenpropathrin might be one of the mechanisms involved in detoxification of this acaricide.

A whole transcriptomal linkage analysis of gene co-regulation in insecticide resistant house flies, Musca domestica

Background

Studies suggest that not only is insecticide resistance conferred via multiple gene up-regulation, but it is mediated through the interaction of regulatory factors. However, no regulatory factors in insecticide resistance have yet been identified, and there has been no examination of the regulatory interaction of resistance genes. Our current study generated the first reference transcriptome from the adult house fly and conducted a whole transcriptome analysis for the multiple insecticide resistant strain ALHF (wild-type) and two insecticide susceptible strains: aabys (with morphological recessive markers) and CS (wild type) to gain valuable insights into the gene interaction and complex regulation in insecticide resistance of house flies, Musca domestica.

Results

Over 56 million reads were used to assemble the adult female M. domestica transcriptome reference and 14488 contigs were generated from the de novo transcriptome assembly. A total of 6159 (43%) of the contigs contained coding regions, among which 1316 genes were identified as being co-up-regulated in ALHF in comparison to both aabys and CS. The majority of these up-regulated genes fell within the SCOP categories of metabolism, general, intra-cellular processes, and regulation, and covered three key detailed function categories: redox detailed function category in metabolism, signal transduction and kinases/phosphatases in regulation, and proteases in intra-cellular processes. The redox group contained detoxification gene superfamilies, including cytochrome P450s, glutathione S-transferases, and esterases. The signal transduction and kinases/phosphatases groups contained gene families of rhodopsin-like GPCRs, adenylate and guanylate cyclases, protein kinases and phosphatases. The proteases group contained genes with digestive, catalytic, and proteinase activities. Genetic linkage analysis with house fly lines comparing different autosomal combinations from ALHF revealed that the up-regulation of gene expression in the three key SCOP detailed function categories occurred mainly through the co-regulation of factors among multiple autosomes, especially between autosomes 2 and 5, suggesting that signaling transduction cascades controlled by GPCRs, protein kinase/phosphates and proteases may be involved in the regulation of resistance P450 gene regulation.

Conclusion

Taken together, our findings suggested that not only is insecticide resistance conferred via multi-resistance mechanisms or up-regulated genes, but it is mediated through the trans and/or cis co-regulations of resistance genes.

Molecular characterization of two carboxylesterase genes of the citrus red mite, Panonychus citri (Acari: Tetranychidae)

The citrus red mite, Panonychus citri, is known for its ability to rapidly evolve resistance to insecticides/acaricides and to adapt to hosts that produce toxins. To get better insight into the detoxification mechanism of P. citri, two carboxylesterase (CarE) genes, PCE1 and PCE2, were isolated and characterized. PCE1 and PCE2 contained open reading frames of 1,653 and 1,392 nucleotides, encoding proteins of 550 and 463 amino acid residues, respectively. Phylogenetic analyses showed that PCE1 and PCE2 were most closely related to the CarE genes from other phytophagous mites. The transcriptional profiles of two CarE genes among developmental stages (egg, larva, nymph, adult female, and adult male), after exposing to four acaricides (avermectin, azocyclotin, pyridaben, and spirodiclofen) and acid rain were investigated using real-time quantitative PCR (qPCR). The results showed that during development, PCE1 was highly expressed at the egg stage, whereas PCE2 was abundantly expressed at the adult stage of males. The expression levels of PCE1 were highly induced upon exposure to acaricides and acid rain. On the other hand, the expression levels of PCE2 were increased after treatment with avermectin and pyridaben. These results suggest that PCE1 and PCE2 may have distinct roles in different developmental stages and participate in the detoxification of acaricides.

The transcriptome profile of the mosquito Culex quinquefasciatus following permethrin selection

To gain valuable insights into the gene interaction and the complex regulation system involved in the development of insecticide resistance in mosquitoes Culex quinquefasciatus, we conducted a whole transcriptome analysis of Culex mosquitoes following permethrin selection. Gene expression profiles for the lower resistant parental mosquito strain HAmCq^G0 and their permethrin-selected high resistant offspring HAmCq^G8 were compared and a total of 367 and 3982 genes were found to be up- and down-regulated, respectively, in HAmCq^G8, indicating that multiple genes are involved in response to permethrin selection. However, a similar overall cumulative gene expression abundance was identified between up- and down-regulated genes in HAmCq^G8 mosquitoes following permethrin selection, suggesting a homeostatic response to insecticides through a balancing of the up- and down-regulation of the genes. While structural and/or cuticular structural functions were the only two enriched GO terms for down-regulated genes, the enriched GO terms obtained for the up-regulated genes occurred primarily among the catalytic and metabolic functions where they represented three functional categories: electron carrier activity, binding, and catalytic activity. Interestingly, the functional GO terms in these three functional categories were overwhelmingly overrepresented in P450s and proteases/serine proteases. The important role played by P450s in the development of insecticide resistance has been extensively studied but the function of proteases/serine proteases in resistance is less well understood. Hence, the characterization of the functions of these proteins, including their digestive, catalytic and proteinase activities; regulation of signaling transduction and protein trafficking, immunity and storage; and their precise function in the development of insecticide resistance in mosquitoes will provide new insights into how genes are interconnected and regulated in resistance.

Novel cytochrome P450 genes, CYP6EB1 and CYP6EC1, are over-expressed in acrinathrin-resistant Frankliniella occidentalis (Thysanoptera: Thripidae)

Control of Frankliniella occidentalis (Pergande) is a serious problem for agriculture all over the world because of the limited range of insecticides that are available. Insecticide resistance in F. occidentalis has been reported for all major insecticide groups. Our previous studies showed that cytochrome P450-mediated detoxification is a major mechanism responsible for insecticide resistance in this pest. Degenerate polymerase chain reaction was used to identify P450 genes that might be involved in acrinathrin resistance, in a laboratory population of F. occidentalis. Associated sequences were classified as belonging to the CYP4 and CYP6 families. Real-time quantitative polymerase chain reaction analyses revealed that two genes, CYP6EB1 and CYP6EC1, were over-expressed in adults and L2 larvae of the resistant population, when compared with the susceptible population, suggesting their possible involvement in resistance to acrinathrin.

Transcription profiling associated with life cycle of Anopheles gambiae

Complex biological events occur during the developmental process of the mosquito Anopheles gambiae (Giles). Using cDNA expression microarrays, the expression patterns of 13,440 clones representing 8,664 unique transcripts were revealed from six different developmental stages: early larvae (late third instar/early fourth instar), late larvae (late fourth instar), early pupae (< 30 min after pupation), late pupae (after tanning), and adult female and male mosquitoes (24 h postemergence). After microarray analysis, 560 unique transcripts were identified to show at least a fourfold up- or down-regulation in at least one developmental stage. Based on the expression patterns, these gene products were clustered into 13 groups. In total, eight genes were analyzed by quantitative real-time polymerase chain reaction to validate microarray results. Among 560 unique transcripts, 446 contigs were assigned to respective genes from the An. gambiae genome. The expression patterns and annotations of the genes in the 13 groups are discussed in the context of development including metabolism, transport, protein synthesis and degradation, cellular processes, cellular communication, intra- or extra-cellular architecture maintenance, response to stress or immune-related defense, and spermatogenesis.

Identification and transcription profiling of trypsin in Aedes taeniorhynchus (Diptera: Culicidae): developmental regulation, blood feeding, and permethrin exposure

The cDNA of a trypsin gene from Aedes (Ochlerotatus) taeniorhynchus (Weidemann) was cloned and sequenced. The full-length mRNA sequence (890 bp) for trypsin from Ae. taeniorhynchus (AetTryp1) was obtained, which encodes an open reading frame of 765 bp (i.e., 255 amino acids). To detect whether AetTryp is developmentally regulated, a quantitative real-time polymerase chain reaction was used to examine AetTrypl mRNA expression levels in different developmental stages of Ae. taeniorhynchus. AetTryp1 was expressed at low levels in egg, larval, and pupal stages, but was differentially expressed in adult Ae. taeniorhynchus, with highest levels found in 5-d-old female adults when compared with teneral adults. In addition, AetTryp1 mRNA expression differed between sexes, with expression levels much lower in males. However, in both males and females, there was a significant increase in AetTryp1 transcription levels as age increased and peaked in 5-d-old adults. AetTrypl expressed in 5-d-old female Ae. taeniorhynchus significantly increased after 30 min postblood feeding compared with the control. The AetTryp1 mRNA expression in 5-d-old female Ae. taeniorhynchus was affected by different concentrations of permethrin.

Molecular characterization of two novel deltamethrin-inducible P450 genes from Liposcelis bostrychophila Badonnel (Psocoptera: Liposcelididae)

Two novel P450 genes, CYP6CE1 and CYP6CE2 (GenBank accession number: EF421245 and EF421246), were cloned and characterized from psocid, Liposcelis bostrychophila. CYP6CE1 and CYP6CE2 contain open reading frames of 1,581 and 1,563 nucleotides that encode 527 and 521 amino acid residues, respectively. The putative proteins of CYP6CE1 and CYP6CE2 show predicted molecular weights of 60.76 and 59.83 kDa with a theoretical pI of 8.58 and 8.78, respectively. CYP6CE1 and CYP6CE2 share 74% identity with each other, and the deduced proteins are typical microsomal P450s sharing signature sequences with other insect CYP6 P450s. Both CYP6CE1 and CYP6CE2 share the closest identities with Hodotermopsis sjoestedti CYP6AM1 at 48% among the published sequences. Phylogenetic analysis showed a closer relationship of CYP6CE1 and CYP6CE2 with CYP6 members of other insects than with those from other families. Quantitative real-time RT-PCR showed that both CYP6CE1 and CYP6CE2 are expressed at all developmental stages tested. Interestingly, CYP6CE2 transcripts decreased from the highest in 1st nymph to the lowest in adults, which seemed to suggest developmental regulation. However, neither CYP6CE1 nor CYP6CE2 were stage specific. The CYP6CE1 and CYP6CE2 transcripts in adults increased significantly after deltamethrin exposure. Recombinant protein expression studies are needed to determine the real functions of these proteins.

Transcription profiling of eleven cytochrome P450s potentially involved in xenobiotic metabolism in the mosquito Aedes aegypti

Transcription profiles of 11 Aedes aegypti P450 genes from CYP6 and CYP9 subfamilies potentially involved in xenobiotic metabolism were investigated. Many genes were preferentially transcribed in tissues classically involved in xenobiotic metabolism including midgut and Malpighian tubules. Life-stage transcription profiling revealed important variations amongst larvae, pupae, and adult males and females. Exposure of mosquito larvae to sub-lethal doses of three xenobiotics induced the transcription of several genes with an induction peak after 48 to 72 h exposure. Several CYP genes were also induced by oxidative stress and one gene strongly responded to 20-hydroxyecdysone. Overall, this study revealed that these P450s show different transcription profiles according to xenobiotic exposures, life stages or sex. Their putative chemoprotective functions are discussed.

Mitochondrial gene cytochrome b developmental and environmental expression in Aedes aegypti (Diptera: Culicidae)

Cytochrome b, coded by mitochondrial DNA, is one of the cytochromes involved in electron transport in the respiratory chain of mitochondria. Cytochrome b is a critical intermediate in a mitochondrial death pathway. To reveal whether cytochrome b of the mosquito Aedes aegypti (L.) (Diptera: Culicidae) (AeaCytB) is developmentally regulated, we used real-time quantitative polymerase chain reaction (qPCR) to examine AeaCytB gene expression levels in different developmental stages of Ae. aegypti. The qPCR showed that AeaCytB was expressed in each developmental stage, with peaks at first and second instars and was highly expressed in teneral male and female Ae. aegypti adults. Because mitochondrial genes exist as multiple copies, AeaCytB has much higher expression levels in all developmental stages in Ae. aegypti compared with nuclear genes. We also investigated the effect of abiotic environmental factors (e.g., high temperatures, ultraviolet radiation, and pesticide) on AeaCytB gene expression. Taken together, these results suggest that AeaCytB gene plays an important role in the development of Ae. aegypti and its response to environmental stress.

High, clustered, nucleotide diversity in the genome of Anopheles gambiae revealed through pooled-template sequencing: implications for high-throughput genotyping protocols

Background

Association mapping approaches are dependent upon discovery and validation of single nucleotide polymorphisms (SNPs). To further association studies in Anopheles gambiae we conducted a major resequencing programme, primarily targeting regions within or close to candidate genes for insecticide resistance.

Results

Using two pools of mosquito template DNA we sequenced over 300 kbp across 660 distinct amplicons of the An. gambiae genome. Comparison of SNPs identified from pooled templates with those from individual sequences revealed a very low false positive rate. False negative rates were much higher and mostly resulted from SNPs with a low minor allele frequency. Pooled-template sequencing also provided good estimates of SNP allele frequencies. Allele frequency estimation success, along with false positive and negative call rates, improved significantly when using a qualitative measure of SNP call quality. We identified a total of 7062 polymorphic features comprising 6995 SNPs and 67 indels, with, on average, a SNP every 34 bp; a high rate of polymorphism that is comparable to other studies of mosquitoes. SNPs were significantly more frequent in members of the cytochrome p450 mono-oxygenases and carboxy/cholinesterase gene-families than in glutathione-S-transferases, other detoxification genes, and control genomic regions. Polymorphic sites showed a significantly clustered distribution, but the degree of SNP clustering (independent of SNP frequency) did not vary among gene families, suggesting that clustering of polymorphisms is a general property of the An. gambiae genome.

Conclusion

The high frequency and clustering of SNPs has important ramifications for the design of high-throughput genotyping assays based on allele specific primer extension or probe hybridisation. We illustrate these issues in the context of the design of Illumina GoldenGate assays.

The salivary transcriptome of Anopheles gambiae (Diptera: Culicidae) larvae: A microarray-based analysis

In spite of the many recent developments in the field of vector sialomics, the salivary glands of larval mosquitoes have been largely unexplored. We used whole-transcriptome microarray analysis to create a gene-expression profile of the salivary gland tissue of fourth-instar Anopheles gambiae larvae, and compare it to the gene-expression profile of a matching group of whole larvae. We identified a total of 221 probes with expression values that were (a) significantly enriched in the salivary glands, and (b) sufficiently annotated as to allow the prediction of the presence/absence of signal peptides in their corresponding gene products. Based on available annotation of the protein sequences associated with these probes, we propose that the main roles of larval salivary secretions include: (a) immune response, (b) mouthpart lubrication, (c) nutrient metabolism, and (d) xenobiotic detoxification. Other highlights of the study include the cloning of a transcript encoding a previously unknown salivary defensin (AgDef5), the confirmation of mucus secretion by the larval salivary glands, and the first report of salivary lipocalins in the Culicidae.

Expression of the cytochrome P450s, CYP6P3 and CYP6M2 are significantly elevated in multiple pyrethroid resistant populations of Anopheles gambiae s.s. from Southern Benin and Nigeria

Background

Insecticide resistance in Anopheles mosquitoes is threatening the success of malaria control programmes. This is particularly true in Benin where pyrethroid resistance has been linked to the failure of insecticide treated bed nets. The role of mutations in the insecticide target sites in conferring resistance has been clearly established. In this study, the contribution of other potential resistance mechanisms was investigated in Anopheles gambiae s.s. from a number of localities in Southern Benin and Nigeria. The mosquitoes were sampled from a variety of breeding sites in a preliminary attempt to investigate the role of contamination of mosquito breeding sites in selecting for resistance in adult mosquitoes.

Results

All mosquitoes sampled belonged to the M form of An. gambiae s.s. There were high levels of permethrin resistance in an agricultural area (Akron) and an urban area (Gbedjromede), low levels of resistance in mosquito samples from an oil contaminated site (Ojoo) and complete susceptibility in the rural Orogun location. The target site mutation kdrW was detected at high levels in two of the populations (Akron f = 0.86 and Gbedjromede f = 0.84) but was not detected in Ojoo or Orogun. Microarray analysis using the Anopheles gambiae detox chip identified two P450s, CYP6P3 and CYP6M2 up regulated in all three populations, the former was expressed at particularly high levels in the Akron (12.4-fold) and Ojoo (7.4-fold) populations compared to the susceptible population. Additional detoxification and redox genes were also over expressed in one or more populations including two cuticular pre-cursor genes which were elevated in two of the three resistant populations.

Conclusion

Multiple resistance mechanisms incurred in the different breeding sites contribute to resistance to permethrin in Benin. The cytochrome P450 genes, CYP6P3 and CYP6M2 are upregulated in all three resistant populations analysed. Several additional potential resistance mechanisms were also identified that warrant further investigation. Metabolic genes were over expressed irrespective of the presence of kdr, the latter resistance mechanism being absent in one resistant population. The discovery that mosquitoes collected from different types of breeding sites display differing profiles of metabolic genes at the adult stage may reflect the influence of a range of xenobiotics on selecting for resistance in mosquitoes.

Functional Genomics Approaches to Studies of the Cytochrome P450 Superfamily

Functional genomics approaches are widely implemented in current research and have found application in many areas of biology. This review will present research fields, novel findings and new tools developed in the cytochrome P450 field using the functional genomics techniques. The most widely used method is microarray technology, which has already greatly contributed to the understanding of the cytochromes P450 function and expression. Several focused CYP microarrays have been developed for genotyping, toxicogenomics and studies of CYP function of many different organisms. Our contribution to the CYP field by development of Steroltalk microarrays to study the cross-talk of cholesterol homeostasis and drug metabolism is also presented.

Evidence of multiple pyrethroid resistance mechanisms in the malaria vector Anopheles gambiae sensu stricto from Nigeria

Pyrethroid insecticide resistance in Anopheles gambiae sensu stricto is a major concern to malaria vector control programmes. Resistance is mainly due to target-site insensitivity arising from a single point mutation, often referred to as knockdown resistance (kdr). Metabolic-based resistance mechanisms have also been implicated in pyrethroid resistance in East Africa and are currently being investigated in West Africa. Here we report the co-occurrence of both resistance mechanisms in a population of An. gambiae s.s. from Nigeria. Bioassay, synergist and biochemical analysis carried out on resistant and susceptible strains of An. gambiae s.s. from the same geographical area revealed >50% of the West African kdr mutation in the resistant mosquitoes but <3% in the susceptible mosquitoes. Resistant mosquitoes synergized using pyperonyl butoxide before permethrin exposure showed a significant increase in mortality compared with the non-synergized. Biochemical assays showed an increased level of monooxygenase but not glutathione-S-transferase or esterase activities in the resistant mosquitoes. Microarray analysis using the An. gambiae detox-chip for expression of detoxifying genes showed five over-expressed genes in the resistant strain when compared with the susceptible one. Two of these, CPLC8 and CPLC#, are cuticular genes not implicated in pyrethroid metabolism in An. gambiae s.s, and could constitute a novel set of candidate genes that warrant further investigation.

Juvenile hormone induction of glutathione S-transferase activity in the larval fat body of the common cutworm, Spodoptera litura (Lepidoptera: Noctuidae)

The effect of pyriproxyfen, a juvenile hormone analog (JHA), on the pupation of S. litura was examined. A topical application of 100 mug JHA/larva on the newly ecdysed (0-day) sixth instar larvae resulted in more than 80% pupation, while most of the 1- or 2-day-old larvae similarly treated developed into supernumerary larvae. Glutathione S-transferse (GST) activity in the fat body of 0-day-old sixth instar larvae was significantly induced within 12 h of JHA (100 microg/larva) treatment. In contrast, no such induction was found when 1- and 2-day-old sixth instar larvae were similarly treated. This induction phenomenon was also observed when 0-day-old sixth instar larvae were treated with the natural JH III. The suppressive effects of alpha-amanitin and cycloheximide suggest that JHA induction of GST activity in these cutworm larvae presumably occurred at the gene transcription level.

Comparative molecular modeling of Anopheles gambiae CYP6Z1, a mosquito P450 capable of metabolizing DDT

One of the challenges faced in malarial control is the acquisition of insecticide resistance that has developed in mosquitoes that are vectors for this disease. Anopheles gambiae, which has been the major mosquito vector of the malaria parasite Plasmodium falciparum in Africa, has over the years developed resistance to insecticides including dieldrin, 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), and pyrethroids. Previous microarray studies using fragments of 230 An. gambiae genes identified five P450 loci, including CYP4C27, CYP4H15, CYP6Z1, CYP6Z2, and CYP12F1, that showed significantly higher expression in the DDT-resistant ZAN/U strain compared with the DDT-susceptible Kisumu strain. To predict whether either of the CYP6Z1 and CYP6Z2 proteins might potentially metabolize DDT, we generated and compared molecular models of these two proteins with and without DDT docked in their catalytic sites. This comparison indicated that, although these two CYP6Z proteins share high sequence identity, their metabolic profiles were likely to differ dramatically from the larger catalytic site of CYP6Z1, potentially involved in DDT metabolism, and the more constrained catalytic site of CYP6Z2, not likely to metabolize DDT. Heterologous expressions of these proteins have corroborated these predictions: only CYP6Z1 is capable of metabolizing DDT. Overlays of these models indicate that slight differences in the backbone of SRS1 and variations of side chains in SRS2 and SRS4 account for the significant differences in their catalytic site volumes and DDT-metabolic capacities. These data identify CYP6Z1 as one important target for inhibitor design aimed at inactivating insecticide-metabolizing P450s in natural populations of this malarial mosquito.

Cytochrome c gene and protein expression: developmental regulation, environmental response, and pesticide sensitivity in Aedes aegypti

Cytochrome c is a highly conserved protein that is found in many multicellular and unicellular organisms. Cytochrome c is a critical intermediate in apoptosis: a controlled form of cell death that kills cells as part of their natural process of development and in response to environmental condition. To detect whether cytochrome c of the mosquito Aedes aegypti (L.) (AeaCytC) is developmentally regulated, we used quantitative real-time polymerase chain reaction (PCR) to examine AeaCytC gene expression levels in different developmental stages ofAe. aegypti. Quantitative real-time PCR showed that AeaCytC was expressed in each developmental stage, at different points in time, and it was highly expressed in teneral female Ae. aegypti. Ae. aegypti cytochrome c protein (AeaCYTC) was detected only in adult mosquitoes, not in early developmental stages of Ae. aegypti. We also investigated the effect of certain environmental factors (e.g., temperature, UV-light, and permethrin insecticide) on AeaCytC gene and AeaCYTC protein expression in adult mosquitoes, and we found that response varied with age. These results suggest that AeaCytC gene and AeaCYTC protein play functional roles in the development of Ae. aegypti and the differential expression of cytochrome c has potential as a biomarker for environmental and chemical stress.

Monooxygenase levels and knockdown resistance (kdr) allele frequencies in Anopheles gambiae and Anopheles arabiensis in Kenya

Pyrethroid-treated bed-nets and indoor spray are important components of malaria control strategies in Kenya. Information on resistance to pyrethroid insecticides in Anopheles gambiae and An. arabiensis populations is essential to the selection of appropriate insecticides and the management of insecticide resistance. Monooxygenase activity and knockdown resistance (kdr) allele frequency are biochemical and molecular indicators of mosquito resistance to pyrethroids. This study determined baseline information on monooxygenase activity and kdr allele frequency in anopheline mosquitoes in the western region, the Great Rift Valley-central region, and the coastal region of Kenya. In total, 1,990 field-collected individuals, representing 12 An. gambiae and 22 An. arabiensis populations were analyzed. We found significant among-population variation in monooxygenase activity in An. gambiae and An. arabiensis and substantial variability among individuals within populations. Nine of 12 An. gambiae populations exhibited significantly higher average monooxygenase activity than the susceptible Kisumu reference strain. The kdr alleles (L1014S) were detected in three An. gambiae populations, and one An. arabiensis population in western Kenya, but not in the Rift Valley-central region and the coastal Kenya region. All genotypes with the kdr alleles were heterozygous, and the conservative estimation of kdr allele frequency was below 1% in these four populations. Information on monooxygenase activity and kdr allele frequency reported in this study provided baseline data for monitoring insecticide resistance changes in Kenya during the era when large-scale insecticide-treated bed-net and indoor residual spray campaigns were being implemented.

A microarray-based analysis of transcriptional compartmentalization in the alimentary canal of Anopheles gambiae (Diptera: Culicidae) larvae

The alimentary canal of the larval mosquito displays a considerable degree of physiological compartmentalization among its different anatomical sub-divisions (gastric caeca, anterior midgut, posterior midgut and hindgut). We performed a comparative microarray analysis in order to identify transcripts which are particularly enriched in each gut section. Based on the available annotation of the selected transcripts, we suggest that the metabolism and absorption of proteins and carbohydrates takes place mainly in the gastric caeca and posterior midgut, whereas the anterior midgut specializes in the metabolism and absorption of lipids. Transcripts encoding antimicrobial peptides were found to be enriched in the gastric caeca, and a high enrichment of transcripts associated with enzymes involved in xenobiotic detoxification was found in the anterior midgut. Furthermore, our data support the notion that the region encompassing the hindgut and Malpighian tubes plays important roles in avoiding the excretion of nutrients, as well as in xenobiotic detoxification.

Continuous exposure to Plasmodium results in decreased susceptibility and transcriptomic divergence of the Anopheles gambiae immune system

Background

Plasmodium infection has been shown to compromise the fitness of the mosquito vector, reducing its fecundity and longevity. However, from an evolutionary perspective, the impact of Plasmodium infection as a selective pressure on the mosquito is largely unknown.

Results

In the present study we have addressed the effect of a continuous Plasmodium berghei infection on the resistance to infection and global gene expression in Anopheles gambiae.

Exposure of A. gambiae to P. berghei-infected blood and infection for 16 generations resulted in a decreased susceptibility to infection, altered constitutive expression levels for approximately 2.4% of the mosquito's total transcriptome and a lower basal level of immune genes expression, including several anti-Plasmodium factors. The infection-responsiveness for several defense genes was elevated in the P. berghei exposed mosquito colonies.

Conclusion

Our study establishes the existence of a selective pressure exerted by the parasite P. berghei on the malaria vector A. gambiae that results in a decreased permissiveness to infection and changes in the mosquito transcriptome regulation that suggest a decreased constitutive immune gene activity but a more potent immune response upon Plasmodium challenge.

Gene expression in pyriproxyfen-resistant Bemisia tabaci Q biotype

Pyriproxyfen is a biorational insecticide that acts as a juvenile hormone (JH) analogue and disrupts insect development with an unknown molecular mode of action. Pyriproxyfen is one of the major insecticides used to control the whitefly Bemisia tabaci (Gennadius) and comply with integrated pest management (IPM) programmes, resulting in minimal effects on the environment, humans and beneficial organisms. During the last few years, resistance to pyriproxyfen has been observed in several locations in Israel, sometimes reaching a thousandfold or more. No information exists about the molecular basis underlying this resistance that may lead to understanding the mode of action of pyriproxyfen and developing molecular markers for rapid monitoring of resistance outbreaks. In this communication, a cDNA microarray from B. tabaci was used to monitor changes in gene expression in a resistant B. tabaci population. Based on statistical analysis, 111 expressed sequence tags (ESTs) were identified that were differentially upregulated in the resistant strain after pyriproxyfen treatment. Many of the upregulated ESTs observed in the present study belong to families usually associated with resistance and xenobiotic detoxification such as mitochondrial genes, P450s and oxidative stress, genes associated with protein, lipid and carbohydrate metabolism and others related to JH-associated processes in insects such as oocyte and egg development.

Transcription profiling of a recently colonised pyrethroid resistant Anopheles gambiae strain from Ghana

BACKGROUND

Mosquito resistance to the pyrethroid insecticides used to treat bednets threatens the sustainability of malaria control in sub-Saharan Africa. While the impact of target site insensitivity alleles is being widely discussed the implications of insecticide detoxification--though equally important--remains elusive. The successful development of new tools for malaria intervention and management requires a comprehensive understanding of insecticide resistance, including metabolic resistance mechanisms. Although three enzyme families (cytochrome P450s, glutathione S-transferases and carboxylesterases) have been widely associated with insecticide detoxification the role of individual enzymes is largely unknown.

RESULTS

Here, constitutive expression patterns of genes putatively involved in conferring pyrethroid resistance was investigated in a recently colonised pyrethroid resistant Anopheles gambiae strain from Odumasy, Southern Ghana. RNA from the resistant strain and a standard laboratory susceptible strain, of both sexes was extracted, reverse transcribed and labelled with either Cy3- or Cy5-dye. Labelled cDNA was co-hybridised to the detox chip, a custom-made microarray containing over 230 A. gambiae gene fragments predominantly from enzyme families associated with insecticide resistance. After hybridisation, Cy3- and Cy5-signal intensities were measured and compared gene by gene. In both females and males of the resistant strain the cytochrome P450s CYP6Z2 and CYP6M2 are highly over-expressed along with a member of the superoxide dismutase (SOD) gene family.

CONCLUSION

These genes differ from those found up-regulated in East African strains of pyrethroid resistant A. gambiae and constitute a novel set of candidate genes implicated in insecticide detoxification. These data suggest that metabolic resistance may have multiple origins in A. gambiae, which has strong implications for the management of resistance.