Contrasting roles of cytochrome P450s in amitraz and chlorfenapyr resistance in the crop pest Tetranychus urticae.

The molecular mechanisms of amitraz and chlorfenapyr resistance remain only poorly understood for major agricultural pests and vectors of human diseases. This study focusses on a multi-resistant field strain of the crop pest Tetranychus urticae, which could be readily selected in the laboratory to high levels of amitraz and chlorfenapyr resistance. Toxicity experiments using tralopyril, the active toxophore of chlorfenapyr, suggested decreased activation as a likely mechanism underlying resistance. Starting from the same parental strain, transcriptome profiling revealed that a cluster of detoxifying genes was upregulated after amitraz selection, but unexpectedly downregulated after chlorfenapyr selection. Further functional validation associated the upregulation of CYP392A16 with amitraz metabolism and the downregulation of CYP392D8 with reduced activation of chlorfenapyr to tralopyril. Genetic mapping (QTL analysis by BSA) was conducted in an attempt to unravel the genetic mechanisms of expression variation and resistance. This revealed that chlorfenapyr resistance was associated with a single QTL, while 3 QTLs were uncovered for amitraz resistance. Together with the observed contrasting gene expression patterns, we argue that transcriptional regulators most likely underly the distinct expression profiles associated with resistance, but these await further functional validation.

Insights into unique features of Drosophila CYP4G enzymes.

The cytochrome P450 enzymes of the CYP4G subfamily are some of the most intriguing insect P450s in terms of structure and function. In Drosophila, CYP4G1 is highly expressed in the oenocytes and is the last enzyme in the biosynthesis of cuticular hydrocarbons, while CYP4G15 is expressed in the brain and is of unknown function. Both proteins have a CYP4G-specific and characteristic amino acid sequence insertion corresponding to a loop between the G and H helices whose function is unclear. Here we address these enigmatic structural and functional features of Drosophila CYP4Gs. First, we used reverse genetics to generate D. melanogaster strains in which all or part of the CYP4G-specific loop was removed from CYP4G1. We showed that the full loop was not needed for proper folding of the P450, but it is essential for function, and that just a short stretch of six amino acids is required for the enzyme's ability to make hydrocarbons. Second, we confirmed by immunocytochemistry that CYP4G15 is expressed in the brain and showed that it is specifically associated with the cortex glia cell subtype. We then expressed CYP4G15 ectopically in oenocytes, revealing that it can produce of a blend of hydrocarbons, albeit to quantitatively lower levels resulting in only a partial rescue of CYP4G1 knockdown flies. The CYP4G1 structural variants studied here should facilitate the biochemical characterization of CYP4G enzymes. Our results also raise the question of the putative role of hydrocarbons and their synthesis by cortex glial cells.

Chlorfenapyr metabolism by mosquito P450s associated with pyrethroid resistance identifies potential activation markers.

Chlorfenapyr is a pro-insecticide increasingly used in combination with pyrethroids such as a-cypermethrin or deltamethrin in insecticide treated bednets (ITNs) to control malaria transmitted by pyrethroid-resistant mosquito populations. Chlorfenapyr requires P450 activation to produce tralopyril and other bioactive metabolites. Pyrethroid resistance is often associated with elevated levels of chemoprotective P450s with broad substrate specificity, which could influence chlorfenapyr activity. Here, we have investigated chlorfenapyr metabolism by a panel of eight P450s commonly associated with pyrethroid resistance in An. gambiae and Ae. aegypti, the major vectors of malaria and arboviruses. Chlorfenapyr was activated to tralopyril by An. gambiae CYP6P3, CYP9J5, CYP9K1 and Ae. aegypti, CYP9J32. The Kcat/KM value of 0.66 µM-1 min-1 for CYP9K1 was, 6.7 fold higher than CYP6P3 and CYP9J32 (both 0.1 µM-1 min-1) and 22-fold higher than CYP9J5 (0.03 µM-1 min-1). Further investigation of the effect of -cypermethrin equivalent to the ratios used with chlorfenapyr in bed nets (~ 1:2 molar ratio) resulted in a reduction in chlorfenapyr metabolism by CYP6P3 and CYP6K1 of 76.8% and 56.8% respectively. This research provides valuable insights into the metabolism of chlorfenapyr by mosquito P450s and highlights the need for continued investigation into effective vector control strategies.

Increased metabolism in combination with the novel cytochrome b target-site mutation L258F confers cross-resistance between the Qo inhibitors acequinocyl and bifenazate in Tetranychus urticae.

Acequinocyl and bifenazate are potent acaricides acting at the Qo site of complex III of the electron transport chain, but frequent applications of these acaricides have led to the development of resistance in spider mites. Target-site resistance caused by mutations in the conserved cd1- and ef-helices of the Qo pocket of cytochrome b has been elucidated as the main resistance mechanism. We therefore monitored Qo pocket mutations in European field populations of Tetranychus urticae and uncovered a new mutation, L258F. The role of this mutation was validated by revealing patterns of maternal inheritance and by the independently replicated introgression in an unrelated susceptible genetic background. However, the parental strain exhibited higher resistance levels than conferred by the mutation alone in isogenic lines, especially for acequinocyl, implying the involvement of strong additional resistance mechanisms. This was confirmed by revealing a polygenic inheritance pattern with classical genetic crosses and via synergism experiments. Therefore, a genome-wide expression analysis was conducted that identified a number of highly overexpressed detoxification genes, including many P450s. Functional expression revealed that the P450 CYP392A11 can metabolize bifenazate by hydroxylation of the ring structure. In conclusion, the novel cytochrome b target-site mutation L258F was uncovered in a recently collected field strain and its role in acequinocyl and bifenazate resistance was validated. However, the high level of resistance in this strain is most likely caused by a combination of target-site resistance and P450-based increased detoxification, potentially acting in synergism.

Functional characterization of cytochrome P450s associated with pyrethroid resistance in the olive fruit fly Bactrocera oleae.

Resistance to pyrethroid insecticides has evolved in Bactrocera oleae populations in Greece, threatening the efficacy of control interventions based on this insecticide class. Here we report the collection of populations from Crete, with resistance levels reaching up to 132-folds, compared to susceptible laboratory strains and show that pyrethroid resistance is substantially suppressed by the PBO synergist, suggesting the involvement of detoxification enzymes. To identify specific candidate genes implicated in resistance, we performed comparative transcriptomic analysis, between the pyrethroid resistant populations from Crete and the susceptible laboratory strains, using both whole bodies and Malpighian tubules. Several genes were found differentially transcribed between resistant and susceptible flies in each comparison, with P450s being among the most highly over-expressed detoxification genes in pyrethroid resistant populations. Four of the over-expressed P450s (Cyp6A61, Cyp6G6, Cyp4P6 and Cyp6G28) were recombinantly expressed in Escherichia coli and in vitro metabolism assays revealed that CYP6A61 is capable of metabolizing alpha-cypermethrin, while CYP6G6, CYP4P6 and CYP6G28 are capable of metabolizing deltamethrin. No metabolism of neonicotinoid insecticides was recorded. We further silenced CYP6G6 in vivo, via RNAi, which led to a small, but significant increase in deltamethrin toxicity. The study provides valuable information towards the development of molecular diagnostics and evidence-based insecticide resistance management strategies.

High-resolution genetic mapping reveals cis-regulatory and copy number variation in loci associated with cytochrome P450-mediated detoxification in a generalist arthropod pest.

Chemical control strategies are driving the evolution of pesticide resistance in pest populations. Understanding the genetic mechanisms of these evolutionary processes is of crucial importance to develop sustainable resistance management strategies. The acaricide pyflubumide is one of the most recently developed mitochondrial complex II inhibitors with a new mode of action that specifically targets spider mite pests. In this study, we characterize the molecular basis of pyflubumide resistance in a highly resistant population of the spider mite Tetranychus urticae. Classical genetic crosses indicated that pyflubumide resistance was incompletely recessive and controlled by more than one gene. To identify resistance loci, we crossed the resistant population to a highly susceptible T. urticae inbred strain and propagated resulting populations with and without pyflubumide exposure for multiple generations in an experimental evolution set-up. High-resolution genetic mapping by a bulked segregant analysis approach led to the identification of three quantitative trait loci (QTL) linked to pyflubumide resistance. Two QTLs were found on the first chromosome and centered on the cytochrome P450 CYP392A16 and a cluster of CYP392E6-8 genes. Comparative transcriptomics revealed a consistent overexpression of CYP392A16 and CYP392E8 in the experimental populations that were selected for pyflubumide resistance. We further corroborated the involvement of CYP392A16 in resistance by in vitro functional expression and metabolism studies. Collectively, these experiments uncovered that CYP392A16 N-demethylates the toxic carboxamide form of pyflubumide to a non-toxic compound. A third QTL coincided with cytochrome P450 reductase (CPR), a vital component of cytochrome P450 metabolism. We show here that the resistant population harbors three gene copies of CPR and that this copy number variation is associated with higher mRNA abundance. Together, we provide evidence for detoxification of pyflubumide by cytochrome P450s that is likely synergized by gene amplification of CPR.

Functional characterization of CYP6A51, a cytochrome P450 associated with pyrethroid resistance in the Mediterranean fruit fly Ceratitis capitata.

Overexpression of the cytochrome P450 monooxygenase CYP6A51 has been previously associated with pyrethroid resistance in the Mediterranean fruit fly (medfly) Ceratitis capitata, an important pest species worldwide; however, this association has not been functionally validated. We expressed CYP6A51 gene in Escherichia coli and produced a functional enzyme with preference for the chemiluminescent substrate Luciferin-ME EGE. In vitro metabolism assays revealed that CYP6A51 is capable of metabolizing two insecticides that share the same mode of action, λ-cyhalothrin and deltamethrin, whereas no metabolism or substrate depletion was observed in the presence of spinosad or malathion. We further expressed CYP6A51 in vivo via a GAL4/UAS system in Drosophila melanogaster flies, driving expression with detoxification tissue-specific drivers. Toxicity bioassays indicated that CYP6A51 confers knock-down resistance to both λ-cyhalothrin and deltamethrin. Detection of CYP6A51 - associated pyrethroid resistance in field populations may be important for efficient Insecticide Resistance Management (IRM) strategies.

Rapid selection of a pyrethroid metabolic enzyme CYP9K1 by operational malaria control activities.

Since 2004, indoor residual spraying (IRS) and long-lasting insecticide-impregnated bednets (LLINs) have reduced the malaria parasite prevalence in children on Bioko Island, Equatorial Guinea, from 45% to 12%. After target site-based (knockdown resistance; kdr) pyrethroid resistance was detected in 2004 in Anopheles coluzzii (formerly known as the M form of the Anopheles gambiae complex), the carbamate bendiocarb was introduced. Subsequent analysis showed that kdr alone was not operationally significant, so pyrethroid-based IRS was successfully reintroduced in 2012. In 2007 and 2014-2015, mass distribution of new pyrethroid LLINs was undertaken to increase the net coverage levels. The combined selection pressure of IRS and LLINs resulted in an increase in the frequency of pyrethroid resistance in 2015. In addition to a significant increase in kdr frequency, an additional metabolic pyrethroid resistance mechanism had been selected. Increased metabolism of the pyrethroid deltamethrin was linked with up-regulation of the cytochrome P450 CYP9K1. The increase in resistance prompted a reversion to bendiocarb IRS in 2016 to avoid a resurgence of malaria, in line with the national Malaria Control Program plan.

Insecticide resistance in Trialeurodes vaporariorum populations and novel diagnostics for kdr mutations.

BACKGROUND: Neonicotinoids, pyrethroids and ketoenols are currently used for the control of Trialeurodes vaporariorum (Hemiptera: Aleyrodidae). In this study, insecticide resistance status and mechanisms were investigated using classical bioassays and molecular techniques. RESULTS: Dose-response bioassays were performed on 19 Greek populations, among the 35 different whitefly populations used for the whole analysis. Resistance factors scaled up to 207-, 4657- and 59-fold for imidacloprid, bifenthrin and spiromesifen, respectively. Molecular assays were used to investigate the frequency of known resistance mutations. A simple polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay was developed for detecting the pyrethroid-resistant alleles r1 (mutation L925I) and r2 (mutation T929I) of the para-type voltage-gated sodium channel gene (VGSC). Both alleles were present at high frequencies (on average 65% and 33%, respectively) in 14 populations from Greece. The M918 L pyrethroid resistance mutation was not detected in any of the Greek populations. Sequencing and a Taqman allelic discrimination were used to monitor the frequency of the mutation E645K of the acetyl-coenzyme A carboxylase gene (ACC) recently linked to spiromesifen resistance. This mutation was detected in 20 of the 24 populations examined in ∼38% frequency among the 433 individuals tested. However, its association with the spiromesifen resistance phenotype was not confirmed in the Greek populations. Finally, two homologues of the CYP6CM1 Bemisia tabaci P450, the known neonicotinoid metabolizer, were found upregulated in two T. vaporariorum neonicotinoid-resistant populations; they were both functionally expressed in Escherichia coli, but the recombinant proteins encoded did not metabolize those neonicotinoid insecticides tested. CONCLUSION: The development of simple diagnostics and their use alongside classical and molecular techniques for the early detection of resistant populations are of great importance for pest management strategies. The practical implications of our results are discussed in light of whitefly control. © 2017 Society of Chemical Industry.

A glutathione-S-transferase (TuGSTd05) associated with acaricide resistance in Tetranychus urticae directly metabolizes the complex II inhibitor cyflumetofen.

Cyflumetofen is a recently introduced acaricide with a novel mode of action, acting as an inhibitor of complex II of mitochondrial electron transport chain. It is activated by hydrolysis and the resulting de-esterified metabolite is a much stronger inhibitor. Cyflumetofen represents a great addition for the control of mite species including Tetranychus urticae, a major agricultural pest, which has the ability to develop resistance to most classes of pesticides rapidly. A resistant strain (Tu008R) was recently described and synergism experiments pointed towards the involvement of GSTs. Here, we conducted genome-wide gene expression analysis, comparing Tu008R with its parental susceptible strain, and identified the delta GST TuGSTd05 as the prime resistance-conferring candidate. Docking analysis suggests that both cyflumetofen and its de-esterified metabolite are potential substrates for conjugation by TuGSTd05. Several amino acids were identified that might be involved in the interaction, with Y107 and N103 possibly having an important role. To further investigate interaction as well as the role of Y107 and N103 in vitro, we recombinantly expressed and kinetically characterized the wild type TuGSTd05, TuGSTd05 Y107F and TuGSTd05 N103L mutants. While cyflumetofen was not found to act as a strong inhibitor, the de-esterified metabolite showed strong affinity for TuGSTd05 (IC50 = 4 µM), which could serve as a mechanism of rapid detoxification. Y107 and N103 might contribute to this interaction. HPLC-MS analysis provided solid indications that TuGSTd05 catalyzes the conjugation of ionized glutathione (GS-) to cyflumetofen and/or its de-esterified metabolite and the resulting metabolite and possible site of attack were identified.

Development of a lateral flow test to detect metabolic resistance in Bemisia tabaci mediated by CYP6CM1, a cytochrome P450 with broad spectrum catalytic efficiency.

Cotton whitefly, Bemisia tabaci (Genn.) (Homoptera: Aleyrodidae) is a major sucking pest in many agricultural and horticultural cropping systems globally. The frequent use of insecticides of different mode of action classes resulted in populations resisting treatments used to keep numbers under economic damage thresholds. Recently it was shown that resistance to neonicotinoids such as imidacloprid is linked to the over-expression of CYP6CM1, a cytochrome P450 monooxygenase detoxifying imidacloprid and other neonicotinoid insecticides when recombinantly expressed in insect cells. However over-expression of CYP6CM1 is also known to confer cross-resistance to pymetrozine, an insecticide not belonging to the chemical class of neonicotinoids. In addition we were able to demonstrate by LC-MS/MS analysis the metabolisation of pyriproxyfen by recombinantly expressed CYP6CM1. Based on our results CYP6CM1 is one of the most versatile detoxification enzymes yet identified in a pest of agricultural importance, as it detoxifies a diverse range of chemical classes used to control whiteflies. Therefore we developed a field-diagnostic antibody-based lateral flow assay which detects CYP6CM1 protein at levels providing resistance to neonicotinoids and other insecticides. The ELISA based test kit can be used as a diagnostic tool to support resistance management strategies based on the alternation of different modes of action of insecticides.

Molecular analysis of resistance to acaricidal spirocyclic tetronic acids in Tetranychus urticae: CYP392E10 metabolizes spirodiclofen, but not its corresponding enol.

Spirodiclofen is one of the most recently developed acaricides and belongs to the new family of spirocyclic tetronic acids (ketoenols). This new acaricidal family is an important chemical tool in resistance management strategies providing sustainable control of spider mites such as Tetranychus urticae. Spirodiclofen targets lipid biosynthesis mediated by direct inhibition of acetyl coenzyme A carboxylase (ACCase). In this study, we investigated two genetically distant spider mite strains with high resistance to spirodiclofen. Despite the strong resistance levels to spirodiclofen (up to 680-fold), only limited cross-resistance with other members of this group such as spiromesifen and spirotetramat could be detected. Amplification and sequencing of the ACCase gene from resistant and susceptible strains did not reveal common non-synonymous mutations, and expression levels of ACCase were similar in both resistant and susceptible strains, indicating the absence of target-site resistance. Furthermore, we collected genome-wide expression data of susceptible and resistant T. urticae strains using microarray technology. Analysis of differentially expressed genes revealed a broad response, but within the overlap of two resistant strains, several cytochrome P450s were prominent. Quantitative PCR confirmed the constitutive over-expression of CYP392E7 and CYP392E10 in resistant strains, and CYP392E10 expression was highly induced by spirodiclofen. Furthermore, stage specific expression profiling revealed that expression levels were not significantly different between developing stages, but very low in eggs, matching the age-dependent resistance pattern previously observed. Functional expression of CYP392E7 and CYP392E10 confirmed that CYP392E10 (but not CYP392E7) metabolizes spirodiclofen by hydroxylation as identified by LC-MS/MS, and revealed cooperative substrate binding and a Km of 43 µM spirodiclofen. CYP392E10 also metabolizes spiromesifen, but not spirotetramat. Surprisingly, no metabolism of the hydrolyzed spirodiclofen-enol metabolite could be detected. These findings are discussed in the light of a likely resistance mechanism.