Induction of cytochrome P450 CYP6B7 and cytochrome b5 mRNAs from Helicoverpa armigera (Hubner) by pyrethroid insecticides in organ culture

Trichoplusia ni Transcriptomic Responses to the Phytosaponin Aglycone Hederagenin: Sex-Related Differences

Many plant species, particularly legumes, protect themselves with saponins. Previously, a correlation was observed between levels of oleanolic acid-derived saponins, such as hederagenin-derived compounds, in the legume Medicago truncatula and caterpillar deterrence. Using concentrations that reflect the foliar levels of hederagenin-type saponins, the sapogenin hederagenin was not toxic to 4th instar caterpillars of the cabbage looper Trichoplusia ni nor did it act as a feeding deterrent. Female caterpillars consumed more diet than males, presumably to obtain the additional nutrients required for oogenesis, and are, thus, exposed to higher hederagenin levels. When fed the hederagenin diet, male caterpillars expressed genes encoding trypsin-like proteins (LOC113500509, LOC113501951, LOC113501953, LOC113501966, LOC113501965, LOC113499659, LOC113501950, LOC113501948, LOC113501957, LOC113501962, LOC113497819, LOC113501946, LOC113503910) as well as stress-responsive (LOC113503484, LOC113505107) proteins and cytochrome P450 6B2-like (LOC113493761) at higher levels than females. In comparison, female caterpillars expressed higher levels of cytochrome P450 6B7-like (LOC113492289). Bioinformatic tools predict that cytochrome P450s could catalyze the oxygenation of hederagenin which would increase the hydrophilicity of the compound. Expression of a Major Facilitator Subfamily (MFS) transporter (LOC113492899) showed a hederagenin dose-dependent increase in gene expression suggesting that this transporter may be involved in sapogenin efflux. These sex-related differences in feeding and detoxification should be taken into consideration in insecticide evaluations to minimize pesticide resistance.

Multiple recombination events between two cytochrome P450 loci contribute to global pyrethroid resistance in Helicoverpa armigera

The cotton bollworm, Helicoverpa armigera (Hübner) is one of the most serious insect pest species to evolve resistance against many insecticides from different chemical classes. This species has evolved resistance to the pyrethroid insecticides across its native range and is becoming a truly global pest after establishing in South America and having been recently recorded in North America. A chimeric cytochrome P450 gene, CYP337B3, has been identified as a resistance mechanism for resistance to fenvalerate and cypermethrin. Here we show that this resistance mechanism is common around the world with at least eight different alleles. It is present in South America and has probably introgressed into its closely related native sibling species, Helicoverpa zea. The different alleles of CYP337B3 are likely to have arisen independently in different geographic locations from selection on existing diversity. The alleles found in Brazil are those most commonly found in Asia, suggesting a potential origin for the incursion of H. armigera into the Americas.

Identification of SNPs involved in regulating a novel alternative transcript of P450 CYP6ER1 in the brown planthopper

Cytochrome P450-mediated metabolic resistance is one of the major mechanisms involved in insecticide resistance. Although the up-regulation of cytochrome P450 plays a vital role in insecticide metabolism, the molecular basis for the transcriptional regulation of cytochrome P450 remains largely unknown. The P450 gene CYP6ER1, has been reported to confer imidacloprid resistance to the brown planthopper, Nilaparvata lugens. Here, we identified a novel alternative transcript of CYP6ER1 (transcript A2) that had different expression patterns between resistant and susceptible populations, and was more stable after insecticide induction. The promoter of this transcript was sequenced and multiple single nucleotide polymorphisms (SNPs) were detected in individuals from susceptible and resistant field-collected populations. Resistant alleles of four SNPs were found to significantly enhance the promoter activity of the CYP6ER1 transcript A2. Electrophoretic mobility shift assays (EMSAs) revealed that these SNPs might regulate the binding of transcription factors to the promoter. Our findings provide novel evidence regarding the transcriptional regulation of a metabolic resistance-related gene and may be useful to understand the resistance mechanism of N. lugens in the field.

Heterologous Co-expression of CYP6B7 and NADPH-Dependent Cytochrome P450 Reductase From Helicoverpa armigera (Lepidoptera: Noctuidae) in Pichia pastoris

As important metabolic enzymes, the function of cytochrome P450 monooxygenases (CYPs) has been demonstrated repeatedly through various means, including heterologous expression systems. Unfortunately, most model systems typically lack expression of a conspecific NADPH-dependent cytochrome P450 reductase (CPR), which is the electron transfer partner of CYPs. As a result, the activities of heterologously expressed insect CYPs may not accurately reflect detoxification activities in vivo. Previously, CYP6B7 from Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) (HaCYP6B7) has been expressed in the Pichia pastoris GS115 strain and shown to detoxify bifenthrin, fenvalerate and chlorpyrifos. However, it remains to be determined if co-expression of HaCYP6B7 with HaCPR will enhance the detoxification ability of the expression system. In the present study, HaCYP6B7 and HaCPR genes were co-expressed in P. pastoris using a reconstituted expression vector, pPICZA-HaCYP6B7-HaCPR. Protein expression was confirmed by Western blot, and the detoxification activities of microsomal fractions to p-nitroanisole O-demethylation (PNOD), 7-ethoxycoumarin O-deethylation (ECOD), fenvalerate and chlorpyrifos were measured. Co-expression of HaCYP6B7 with HaCPR resulted in PNOD and ECOD activities of 1.90 nmol/min/mg·protein and 12.39 pmol/min/mg·protein, which were 1.6- and 1.5-fold of that catalyzed by HaCYP6B7 expressed alone, respectively. Furthermore, microsomes of pPICZA-HaCYP6B7-HaCPR-GS115 had higher detoxification activity than that of pPICZA-HaCYP6B7-GS115 to fenvalerate, but not chlorpyrifos. The results indicated that co-expression of HaCYP6B7 with conspecific CPR could enhance the detoxification activities to some substrates comparing with expression of HaCYP6B7 alone.

Transcriptional activity of detoxification genes is altered by ultraviolet filters in Chironomus riparius

Ultraviolet (UV) filters are compounds used to prevent the damage produced by UV radiation in personal care products, plastics, etc. They have been associated with endocrine disruption, showing anti-estrogen activity in vertebrates and altering the ecdysone pathway in invertebrates. Although they have attracted the attention of multiple research teams there is a lack of data about how animals activate detoxification systems, especially in invertebrates. Here, analysis of the effects of two UV filters, benzophenone-3 (BP3) and 4-methylbenzylidene camphor (4MBC), on the transcriptional activity of nine genes covering the three steps of the detoxification process has been performed. Four cytochrome P450 genes belonging to different members of this family, five GST genes, and the multidrug resistance protein 1 (MRP1) gene were studied by RT-PCR to analyze their transcriptional activity in fourth instar larvae exposed to the UV filters for 8 and 24h. The obtained results show a differential response with downregulation of the different Cyp450s tested by 4MBC while BP3 seems not to modify their expression. On the other hand, some of the GST genes were affected by one or other of the filters, showing a less homogenous response. Finally, MRP1 was activated by both filters but at different times. These results demonstrate for first time that UV filters alter the expression of genes involved in the different steps of the detoxification process and that they can be processed by phase I enzymes other than Cyp450s. They also suggest that UV filters affect biotransformation processes, compromising the ability of the individual to respond to chemical stress, so further research is needed to know the extent of the damage that they can produce in the resistance of the cell to chemicals.

Heterologous expression of Helicoverpa armigera cytochrome P450 CYP6B7 in Pichia pastoris and interactions of CYP6B7 with insecticides

BACKGROUND

Previous studies indicated that constitutive over-expression of cytochrome P450 CYP6B7 was involved in fenvalerate resistance in Helicoverpa armigera. In this study, the CYP6B7 gene from H. armigera (namely HaCYP6B7), was heterologously expressed in Pichia pastoris GS115. A vector pPICZA-HaCYP6B7 was constructed and transformed into P. pastoris GS115, the transformant of pPICZA-HaCYP6B7-GS115 was then cultured and induced by 1% (v/v) methanol and the heterologous expression of HaCYP6B7 protein in P. pastoris was confirmed by SDS-PAGE and western blot.

RESULTS

Microsomes containing the expressed HaCYP6B7 showed activities against model substrate p-nitroanisole and 7-ethoxycoumarin, with p-nitroanisole O-demethylation (PNOD) and 7-ethoxycoumarin O-deethylation (ECOD) activities of 15.66- and 4.75-fold of the control, respectively. Moreover, it showed degradation activities against the insecticides bifenthrin, fenvalerate and chlorpyrifos, with clearance activities of 6.88-, 1.49- and 2.27-fold of the control, respectively. The interactions of HaCYP6B7 with insecticides were further confirmed by molecular docking in silico with binding scores of 5.450, 5.295 and 2.197 between putative HaCYP6B7 protein and bifenthrin, fenvalerate and chlorpyrifos, respectively.

CONCLUSION

The results of present study provided more direct and important evidence on the role of HaCYP6B7 conferring pyrethroid resistance in H. armigera. © 2017 Society of Chemical Industry.

Isolation and Expression Analysis of CYP9A11 and Cytochrome P450 Reductase Gene in the Beet Armyworm (Lepidoptera: Noctuidae)

Cytochrome P450 monooxygenases (CYPs), as an enzyme superfamily, is widely distributed in organisms and plays a vital function in the metabolism of exogenous and endogenous compounds by interacting with its obligatory redox partner, CYP reductase (CPR). A novel CYP gene (CYP9A11) and CPR gene from the agricultural pest insect Spodoptera exigua were cloned and characterized. The complete cDNA sequences of SeCYP9A11 and SeCPR are 1,931 and 3,919 bp in length, respectively, and contain open reading frames of 1,593 and 2,070 nucleotides, respectively. Analysis of the putative protein sequences indicated that SeCYP9A11 contains a heme-binding domain and the unique characteristic sequence (SRFALCE) of the CYP9 family, in addition to a signal peptide and transmembrane segment at the N-terminal. Alignment analysis revealed that SeCYP9A11 shares the highest sequence similarity with CYP9A13 from Mamestra brassicae, which is 66.54%. The putative protein sequence of SeCPR has all of the classical CPR features, such as an N-terminal membrane anchor; three conserved domain flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN), and nicotinamide adenine dinucleotide phosphate (NADPH) domain; and characteristic binding motifs. Phylogenetic analysis revealed that SeCPR shares the highest identity with HaCPR, which is 95.21%. The SeCYP9A11 and SeCPR genes were detected in the midgut, fat body, and cuticle tissues, and throughout all of the developmental stages of S. exigua. The mRNA levels of SeCYP9A11 and SeCPR decreased remarkably after exposure to plant secondary metabolites quercetin and tannin. The results regarding SeCYP9A11 and SeCPR genes in the current study provide foundation for the further study of S. exigua P450 system.

An independent occurrence of the chimeric P450 enzyme CYP337B3 of Helicoverpa armigera confers cypermethrin resistance in Pakistan

The increasing resistance level of insect pest species is a major concern to agriculture worldwide. The cotton bollworm, Helicoverpa armigera, is one of the most important pest species due to being highly polyphagous, geographically widespread, and resistant towards many chemical classes of insecticides. We previously described the mechanism of fenvalerate resistance in Australian populations conferred by the chimeric cytochrome P450 monooxygenase CYP337B3, which arose by unequal crossing-over between CYP337B1 and CYP337B2. Here, we show that this mechanism is also present in the cypermethrin-resistant FSD strain from Pakistan. The Pakistani and the Australian CYP337B3 alleles differ by 18 synonymous and three nonsynonymous SNPs and additionally in the length and sequence of the intron. Nevertheless, the activity of both CYP337B3 proteins is comparable. We demonstrate that CYP337B3 is capable of metabolizing cypermethrin (trans- and especially cis-isomers) to the main metabolite 4'-hydroxycypermethrin, which exhibits no intrinsic toxicity towards susceptible larvae. In a bioassay, CYP337B3 confers a 7-fold resistance towards cypermethrin in FSD larvae compared to susceptible larvae from the Australian TWB strain lacking CYP337B3. Linkage analysis shows that presence of CYP337B3 accounts for most of the cypermethrin resistance in the FSD strain; up-regulation of other P450s in FSD plays no detectable role in resistance. The presence or absence of CYP337B3 can be easily detected by a simple PCR screen, providing a powerful tool to rapidly distinguish resistant from susceptible individuals in the field and to determine the geographical distribution of this resistance gene. Our results suggest that CYP337B3 evolved twice independently by unequal crossing-over between CYP337B2 and two different CYP337B1 alleles.

Aflatoxin B1: toxicity, bioactivation and detoxification in the polyphagous caterpillar Trichoplusia ni

Trichoplusia ni caterpillars are polyphagous foliage-feeders and rarely likely to encounter aflatoxin B1 (AFB1), a mycotoxin produced by Aspergillus flavus and A. parasiticus, in their host plants. To determine how T. ni copes with AFB1, we evaluated the toxicity of AFB1 to T. ni caterpillars at different developmental stages and found that AFB1 tolerance significantly increases with larval development. Diet incorporation of AFB1 at 1 μg/g completely inhibited larval growth and pupation of newly hatched larvae, but 3 μg/g AFB1 did not have apparent toxic effects on larval growth and pupation of caterpillars that first consume this compound 10 days after hatching. Piperonyl butoxide, a general inhibitor of cytochrome P450 monooxygenases (P450s), reduced the toxicity of AFB1, suggesting that AFB1 is bioactivated in T. ni and this bioactivation is mediated by P450s. Some plant allelochemicals, including flavonoids such as flavones, furanocoumarins such as xanthotoxin and imperatorin, and furanochromones such as visnagin, that induce P450s in other lepidopteran larvae ameliorated AFB1 toxicity, suggesting that P450s are also involved in AFB1 detoxification in T. ni.

Knockdown of several components of cytochrome P450 enzyme systems by RNA interference enhances the susceptibility of Helicoverpa armigera to fenvalerate

BACKGROUND

The function of cytochrome P450 proteins (P450s) in the metabolism of a variety of compounds by oxidation and reduction is well elucidated, but its interactions with other electron transfer components in the pyrethroid resistance of insect pests have been a mystery for a long time. In previous studies the authors cloned and characterised CYP6B7 and cytochrome b(5) (Cyt-b(5)) in the fenvalerate-resistant HDFR strain of cotton bollworm (Helicoverpa armigera Hübner) and showed that CYP6B7 mRNA was overexpressed and important for resistance to fenvalerate. In the present study, the functional interactions of CYP6B7, NADPH-dependent cytochrome P450 reductase (CPR) and Cyt-b(5) were assessed using RNA interference (RNAi) strategies and monitoring for fenvalerate resistance levels.

RESULTS

RT-qPCR analyses indicated that the expression levels of CYP6B7, CPR and Cyt-b(5) mRNA were decreased drastically in the midgut of fourth-instar larvae of the H. armigera HDFR strain after corresponding double-stranded RNA (dsRNA) injection, compared with that of the control. The knockdown of CYP6B7, CPR and Cyt-b(5) transcripts was time course dependent during a 12-48 h period after dsRNA injection. At the earlier time points analysed, significant suppression of CYP6B7 mRNA levels was observed in larvae injected with dsCYP6B7-313 as compared with controls, and further suppression was observed in larvae injected with dsCYP6B7-313, dsCPR-403 and dsCyt-b(5) . The injection of dsCYP6B7-313 together with dsCPR-403 and dsCyt-b(5) increased larval susceptibility of the HDFR strain to fenvalerate.

CONCLUSION

The results demonstrated that silencing of CYP6B7 alone or CYP6B7 together with CPR and/or Cyt-b(5) increased the susceptibility of H. armigera to fenvalerate, suggesting that CYP6B7, CPR and Cyt-b(5) collaboratively participated in enhanced metabolism of fenvalerate and played an important role in the resistance of H. armigera to fenvalerate.

Resistance of Australian Helicoverpa armigera to fenvalerate is due to the chimeric P450 enzyme CYP337B3

Worldwide, increasing numbers of insects have evolved resistance to a wide range of pesticides, which hampers their control in the field and, therefore, threatens agriculture. Members of the carboxylesterase and cytochrome P450 monooxygenase superfamilies are prominent candidates to confer metabolic resistance to pyrethroid insecticides. Both carboxylesterases and P450 enzymes have been shown to be involved in pyrethroid resistance in Australian Helicoverpa armigera, the noctuid species possessing by far the most reported resistance cases worldwide. However, specific enzymes responsible for pyrethroid resistance in field populations of this species have not yet been identified. Here, we show that the resistance toward fenvalerate in an Australian strain of H. armigera is due to a unique P450 enzyme, CYP337B3, which arose from unequal crossing-over between two parental P450 genes, resulting in a chimeric enzyme. CYP337B3 is capable of metabolizing fenvalerate into 4'-hydroxyfenvalerate, which exhibits no toxic effect on susceptible larvae; enzymes from the parental P450 genes showed no detectable fenvalerate metabolism. Furthermore, a polymorphic H. armigera strain could be bred into a susceptible line possessing the parental genes CYP337B1 and CYP337B2 and a resistant line possessing only CYP337B3. The exclusive presence of CYP337B3 in resistant insects of this strain confers a 42-fold resistance to fenvalerate. Thus, in addition to previously documented genetic mechanisms of resistance, recombination can also generate selectively advantageous variants, such as this chimeric P450 enzyme with an altered substrate specificity leading to a potent resistance mechanism.

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.

Ecologically appropriate xenobiotics induce cytochrome P450s in Apis mellifera

BACKGROUND

Honey bees are exposed to phytochemicals through the nectar, pollen and propolis consumed to sustain the colony. They may also encounter mycotoxins produced by Aspergillus fungi infesting pollen in beebread. Moreover, bees are exposed to agricultural pesticides, particularly in-hive acaricides used against the parasite Varroa destructor. They cope with these and other xenobiotics primarily through enzymatic detoxificative processes, but the regulation of detoxificative enzymes in honey bees remains largely unexplored.

METHODOLOGY/PRINCIPAL FINDINGS

We used several approaches to ascertain effects of dietary toxins on bee susceptibility to synthetic and natural xenobiotics, including the acaricide tau-fluvalinate, the agricultural pesticide imidacloprid, and the naturally occurring mycotoxin aflatoxin. We administered potential inducers of cytochrome P450 enzymes, the principal biochemical system for Phase 1 detoxification in insects, to investigate how detoxification is regulated. The drug phenobarbital induces P450s in many insects, yet feeding bees with phenobarbital had no effect on the toxicity of tau-fluvalinate, a pesticide known to be detoxified by bee P450s. Similarly, no P450 induction, as measured by tau-fluvalinate tolerance, occurred in bees fed xanthotoxin, salicylic acid, or indole-3-carbinol, all of which induce P450s in other insects. Only quercetin, a common pollen and honey constituent, reduced tau-fluvalinate toxicity. In microarray comparisons no change in detoxificative gene expression was detected in phenobarbital-treated bees. However, northern blot analyses of guts of bees fed extracts of honey, pollen and propolis showed elevated expression of three CYP6AS P450 genes. Diet did not influence tau-fluvalinate or imidacloprid toxicity in bioassays; however, aflatoxin toxicity was higher in bees consuming sucrose or high-fructose corn syrup than in bees consuming honey.

CONCLUSIONS/SIGNIFICANCE

These results suggest that regulation of honey bee P450s is tuned to chemicals occurring naturally in the hive environment and that, in terms of toxicological capacity, a diet of sugar is not equivalent to a diet of honey.

Co-up-regulation of three P450 genes in response to permethrin exposure in permethrin resistant house flies, Musca domestica

Background

Insects may use various biochemical pathways to enable them to tolerate the lethal action of insecticides. For example, increased cytochrome P450 detoxification is known to play an important role in many insect species. Both constitutively increased expression (overexpression) and induction of P450s are thought to be responsible for increased levels of detoxification of insecticides. However, unlike constitutively overexpressed P450 genes, whose expression association with insecticide resistance has been extensively studied, the induction of P450s is less well characterized in insecticide resistance. The current study focuses on the characterization of individual P450 genes that are induced in response to permethrin treatment in permethrin resistant house flies.

Results

The expression of 3 P450 genes, CYP4D4v2, CYP4G2, and CYP6A38, was co-up-regulated by permethrin treatment in permethrin resistant ALHF house flies in a time and dose-dependent manner. Comparison of the deduced protein sequences of these three P450s from resistant ALHF and susceptible aabys and CS house flies revealed identical protein sequences. Genetic linkage analysis located CYP4D4v2 and CYP6A38 on autosome 5, corresponding to the linkage of P450-mediated resistance in ALHF, whereas CYP4G2 was located on autosome 3, where the major insecticide resistance factor(s) for ALHF had been mapped but no P450 genes reported prior to this study.

Conclusion

Our study provides the first direct evidence that multiple P450 genes are co-up-regulated in permethrin resistant house flies through the induction mechanism, which increases overall expression levels of P450 genes in resistant house flies. Taken together with the significant induction of CYP4D4v2, CYP4G2, and CYP6A38 expression by permethrin only in permethrin resistant house flies and the correlation of the linkage of the genes with resistance and/or P450-mediated resistance in resistant ALHF house flies, this study sheds new light on the functional importance of P450 genes in response to insecticide treatment, detoxification of insecticides, the adaptation of insects to their environment, and the evolution of insecticide resistance.

Identification of candidate genes for fenvalerate resistance in Helicoverpa armigera using cDNA-AFLP

A cDNA-amplified fragment length polymorphisms approach was undertaken to screen for candidate genes associated with fenvalerate resistance in the AN02 strain of Helicoverpa armigera. Larvae and adults of this strain manifest approximately 50-fold resistance, which is suppressible by piperonyl butoxide and controlled by the semidominant gene RFen1 previously mapped to AFLP Linkage Group 13. Two cytochrome P450s (CYP337B1 and CYP4S1), one carboxylesterase-like protein and one glutathione transferase were found to be constitutively upregulated in resistant insects. Mapping of these potential detoxification genes showed that one of them, the novel P450 CYP337B1, was tightly linked to the resistance locus. This suggests that the RFen1(R) allele has a cis-acting effect on CYP337B1 expression, and possible trans-acting effects on expression of other genes.

Evaluation of the role of CYP6B cytochrome P450s in pyrethroid resistant Australian Helicoverpa armigera

The AN02 strain of Helicoverpa armigera from eastern Australia exhibits 50-fold, PBO-suppressible resistance to the pyrethroid insecticide fenvalerate. The semidominant resistance gene RFen1 was previously mapped to AFLP Linkage Group 13. In evaluating the cytochrome P450 genes CYP6B7, CYP6B6, and CYP6B2 as candidates for RFen1, we found that they occur in a tandem array in the genome, next to the gene encoding the para-type sodium channel; the target of pyrethroid insecticides. We mapped these genes to AFLP Linkage Group 14, thus rejecting mutations within the P450 cluster or para as candidates for RFen1. RFen1 genotypes produced slightly different mRNA levels of the three P450s, but the differences were too small to convincingly account for resistance. We conclude that even if one or more of these P450s metabolize fenvalerate, they are unlikely to be responsible for the resistance in AN02.

Do herbicide treatments reduce the sensitivity of mosquito larvae to insecticides?

Invasive mosquitoes are economic and sanitary concerns especially in Europe and America. Most work has emphasized the role of resistance [Berrada, S., Fournier, D., Cuany, A., Nguyen, T.X., 1994. Identification of resistance mechanisms in a selected laboratory strain of Cacopsylla pyri (Homoptera: Psyllidae): altered acetylcholinesterases and detoxifying oxidases. Pesticide Biochemistry and Physiology 48, 41-47; Hemingway, J., Hawkes, N.J., McCarroll, L., Ranson, H., 2004. The molecular basis of insecticide resistance in mosquitoes. Insect Biochemistry and Molecular Biology 34, 653-665] to insecticides. Compounds acting on larval sensitivity to insecticides are not well studied and their action remains poorly understood. Among several residual chemicals in ecosystems, particularly in wetlands, we identified a possible interaction of an herbicide on larval resistance to an insecticide. Our work contributes to the global control of mosquito populations by identifying possible pathways of resistance to insecticides of these vectors. Resistance or tolerance to insecticide treatments might contribute to successful invasion by mosquitoes. Here we report an ecotoxicological approach to test the hypothesis of an indirect effect of atrazine on mortality of an invasive vector. A brief contact (48h) between Aedes aegypti mosquito larvae and atrazine led to a modification of larval sensitivity to an insecticide: using atrazine as an inducer led to a decrease in the mortality of larvae treated with Bacillus thuringiensis var. israelensis (Bti).

Diversification of furanocoumarin-metabolizing cytochrome P450 monooxygenases in two papilionids: Specificity and substrate encounter rate

Diversification of cytochrome P450 monooxygenases (P450s) is thought to result from antagonistic interactions between plants and their herbivorous enemies. However, little direct evidence demonstrates the relationship between selection by plant toxins and adaptive changes in herbivore P450s. Here we show that the furanocoumarin-metabolic activity of CYP6B proteins in two species of swallowtail caterpillars is associated with the probability of encountering host plant furanocoumarins. Catalytic activity was compared in two closely related CYP6B4 and CYP6B17 groups in the polyphagous congeners Papilio glaucus and Papilio canadensis. Generally, P450s from P. glaucus, which feeds occasionally on furanocoumarin-containing host plants, display higher activities against furanocoumarins than those from P. canadensis, which normally does not encounter furanocoumarins. These P450s in turn catalyze a larger range of furanocoumarins at lower efficiency than CYP6B1, a P450 from Papilio polyxenes, which feeds exclusively on furanocoumarin-containing host plants. Reconstruction of the ancestral CYP6B sequences using maximum likelihood predictions and comparisons of the sequence and geometry of their active sites to those of contemporary CYP6B proteins indicate that host plant diversity is directly related to P450 activity and inversely related to substrate specificity. These predictions suggest that, along the lineage leading to Papilio P450s, the ancestral, highly versatile CYP6B protein presumed to exist in a polyphagous species evolved through time into a more efficient and specialized CYP6B1-like protein in Papilio species with continual exposure to furanocoumarins. Further diversification of Papilio CYP6Bs has likely involved interspersed events of positive selection in oligophagous species and relaxation of functional constraints in polyphagous species.

Cytochrome P450 CYP6X1 cDNAs and mRNA expression levels in three strains of the tarnished plant bug Lygus lineolaris (Heteroptera: Miridae) having different susceptibilities to pyrethroid insecticide

Three cDNAs, cloned from both pyrethroid-susceptible and -resistant strains of Lygus lineolaris, contained a 1548 nucleotide open reading frame encoding a 516 amino acid residue protein. Predicted cytochrome P450s from cDNAs were classified as the first three new members of subfamily CYP6X, CYP6X1v1 for a susceptible strain and CYP6X1v2 and CYP6X1v3 for two resistant strains. Putative cytochrome P450 CYP6X1s from L. lineolaris were highly similar (up to 42% amino acid sequence identity) to several insect CYP6s that are responsible for reduced sensitivity to pyrethroid insecticides. A total of twenty-six nucleotide substitutions were revealed between cDNAs of susceptible and resistant strains. Two nucleotide substitutions resulted in amino acid changes, Asp373 to Ala373 and Ser487 to Ala487, between susceptible and resistant strains. The resistant laboratory strain contained 2.1-fold higher cytochrome P450 mRNA per microgram total RNA than the susceptible laboratory strain. Topical treatment with 10 ng permethrin elevated cytochrome P450 mRNA levels by approximately 2-fold. The results of this study indicated that cytochrome P450 gene mutation, coupled with up-regulation, was present only in the pyrethroid resistant strains, and was possibly related to resistance development in the tarnished plant bug.