Identification and Characterization of the Gene CYP340W1 from Plutella xylostella and Its Possible Involvement in Resistance to Abamectin

Characterization of emamectin benzoate resistance in the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)

BACKGROUND

Plutella xylostella (L.) is a destructive pest of cruciferous crops worldwide that has evolved resistance to many insecticides. Here we examined the mode of inheritance, cross-resistance profile, and potential mechanisms of emamectin benzoate resistance in a field-derived strain of P. xylostella from Japan.

RESULTS

A field-collected population of P. xylostella, was found to exhibit strong (> 150-fold) resistance to emamectin benzoate in insecticide bioassays when compared with a laboratory susceptible strain. Genetic analysis showed that resistance is inherited as an autosomal, recessive trait, and is conferred by a single or a few closely linked loci. The emamectin benzoate resistant strain also exhibited resistance to abamectin, lepimectin, chlorantraniliprole, lufenuron, spinetoram, indoxacarb, fipronil, dieldrin, endosulfan and lambda-cyhalothrin, demonstrating a remarkable multi-resistance profile. Insecticide bioassays employing inhibitors of detoxification enzymes revealed that piperonyl butoxide (PBO) increased the toxicity of emamectin benzoate in the resistant strain by ten-fold indicating the potential involvement of cytochrome P450 monooxygenases in avermectin resistance. Furthermore, cloning and sequencing of the primary receptor of avermectins, the GluCl channel, revealed the absence of target-site mutations in the resistant strain.

CONCLUSIONS

Our data on the mode of inheritance and mechanisms of resistance to emamectin benzoate in a P. xylostella strain from Japan provide a foundation for the development of regional resistance management strategies. However, the high levels of phenotypic resistance in this strain to a diverse range of other insecticide classes available for control illustrate the challenges associated with the sustainable control of this important pest. © 2023 Society of Chemical Industry.

Ivermectin as a novel malaria control tool: Getting ahead of the resistance curse

Reduction in malaria clinical cases is strongly dependent on the ability to prevent Anopheles infectious bites. Vector control strategies using long-lasting insecticidal nets and indoor residual spraying with insecticides have contributed to significantly reduce the incidence of malaria in many endemic countries, especially in the Sub-Saharan region. However, global progress in reducing malaria cases has plateaued since 2015 mostly due to the increased insecticide resistance and behavioral changes in Anopheles vectors. Additional control strategies are thus required to further reduce the burden of malaria and contain the spread of resistant and invasive Anopheles vectors. The use of endectocides such as ivermectin as an additional malaria control tool is now receiving increased attention, driven by its different mode of action compared to insecticides used so far and its excellent safety record for humans. In this opinion article, we discuss the advantages and disadvantages of using ivermectin for malaria control with a focus on the risk of selecting ivermectin resistance in malaria vectors. We also highlight the importance of understanding how ivermectin resistance could develop in mosquitoes and what its underlying mechanisms and associated molecular markers are, and propose a research agenda to manage this phenomenon.

CRISPR/Cas9-mediated D472N substitution in the Rdl1 of Plutella xylostella confers low resistance to abamectin

BACKGROUND

Abamectin is one of the main insecticides used for the control of Plutella xylostella, a destructive pest of cruciferous crops. Target-site mutation plays an important role in insecticide resistance. A point mutation (D472N) has been reported in the Rdl1 γ-aminobutyric acid receptor (GABAR) in P. xylostella, but its roles in insecticide resistance remain unknown.

RESULTS

In this study, the D472N mutation of the Rdl1 GABAR was detected in several field populations of P. xylostella and showed a positive correlation with abamectin resistance. A knock-in homozygous mutation strain (D472N-KI) of P. xylostella was successfully constructed using CRISPR/Cas9 coupled with homology-directed repair, and the bioassay results demonstrated that compared with the susceptible strain, the D472N-KI strain had 11.1- and 3.7-fold increased resistance to abamectin and endosulfan, respectively. There was no difference in resistance to fipronil, broflanilide or isocycloseram, which also target the GABAR. In addition, the total fecundity of the D472N-KI strain was significantly reduced by 50.0%.

CONCLUSION

Our results suggest that the homozygous D472N mutation in Rdl1 confers a low level of resistance to abamectin in P. xylostella but causes significant fecundity disadvantages, which may delay the development of resistance to some extent. These results lay a foundation for further understanding the mechanisms of abamectin resistance in insect pests. © 2022 Society of Chemical Industry.

Functional analysis of a cytochrome P450 gene CYP9Z6 responding to terpinen-4-ol in the red flour beetle, Tribolium castaneum

Tribolium castaneum is an agricultural and stored pest found throughout the world. The cytochrome P450 genes of T. castaneum can encode various detoxification enzymes and catabolize heterologous substances, conferring tolerance to insecticides. Herein, we describe the identification of a P450 gene (CYP9Z6) from T. castaneum and investigated its expression profile and potential role in the detoxification of terpinen-4-ol. TcCYP9Z6 expression was significantly induced after exposure to terpinen-4-ol, and RNA-mediated silencing of TcCYP9Z6 increased terpinen-4-ol-induced larval mortality from 47.75% to 63.92%, showing that TcCYP9Z6 is closely related to the detoxification of terpinen-4-ol. The developmental expression profile revealed that TcCYP9Z6 was mainly expressed in late adults and late larvae. Tissue expression profiling revealed that the highest TcCYP9Z6 expression occurred in the head, in both the adult and the larval tissues, followed by the gut in larvae and the antennae in adults. These developmental stages and tissues with high TcCYP9Z6 expression are closely related to the detoxification of heterologous substances. These results indicated that TcCYP9Z6 may play a pivotal role in the detoxification of terpinen-4-ol, which provides support for using TcCYP9Z6 a potential gene for the RNAi-mediated prevention and control of T. castaneum.

Functionally characterized arthropod pest and pollinator cytochrome P450s associated with xenobiotic metabolism

The cytochrome P450 family (P450s) of arthropods includes diverse enzymes involved in endogenous essential physiological functions and in the oxidative metabolism of xenobiotics, insecticides and plant allelochemicals. P450s can also establish insecticide selectivity in bees and pollinators. Several arthropod P450s, distributed in different phylogenetic groups, have been associated with xenobiotic metabolism, and some of them have been functionally characterized, using different in vitro and in vivo systems. The purpose of this review is to summarize scientific publications on arthropod P450s from major insect and mite agricultural pests, pollinators and Papilio sp, which have been functionally characterized and shown to metabolize xenobiotics and/or their role (direct or indirect) in pesticide toxicity or resistance has been functionally validated. The phylogenetic relationships among these P450s, the functional systems employed for their characterization and their xenobiotic catalytic properties are presented, in a systematic approach, including critical aspects and limitations. The potential of the primary P450-based metabolic pathway of target and non-target organisms for the development of highly selective insecticides and resistance-breaking formulations may help to improve the efficiency and sustainability of pest control.

Characterization of field-evolved resistance to pyridalyl in a near-isogenic line of diamondback moth, Plutella xylostella

BACKGROUND

Plutella xylostella has developed resistance to a variety of pesticides in the field. Selection, inheritance, a near-isogenic line, cross-resistance and biochemical mechanisms of pyridalyl resistance were characterized in a field-collected resistant population of P. xylostella from China.

RESULTS

Compared with a susceptible IVF-S strain, the field-collected FZ population showed ~ 350-fold resistance to pyridalyl. The FZ-PY strain, selected from the FZ population using pyridalyl, developed ~ 640-fold resistance to pyridalyl. Inheritance tests indicated that pyridalyl resistance in the FZ-PY strain was autosomal and incompletely recessive. Through successive backcrossing to IVF-S, a near-isogenic strain (NIL-PY) was established that exhibited 191.21-fold resistance to pyridalyl and no cross-resistance to other tested popular insecticides. No significant effects of synergists and higher activities of metabolic enzymes were observed in NIL-PY compared with IVF-S. Furthermore, the survival rate of NIL-PY larvae, and female oviposition, fecundity and egg viability were markedly reduced in NIL-PY compared with IVF-S. The fitness of NIL-PY was found to be 0.56 compared with IVF-S.

CONCLUSION

Considering that no relevant effects of synergists or oxidative metabolism were observed in NIL-PY, and that pyridalyl resistance results in significant fitness costs compared with IVF-S, further research will be conducted on the mechanism of target-site resistance. © 2020 Society of Chemical Industry.

Differential regulation of cytochrome P450 genes associated with biosynthesis and detoxification in bifenthrin-resistant populations of navel orangewom (Amyelois transitella)

Pyrethroid resistance was first reported in 2013 for the navel orangeworm, Amyelois transitella, but the genetic underpinnings of pyrethroid resistance are unknown. We investigated the role of cytochrome P450 monooxygenases (P450s) belonging to the CYP3 and CYP4 clans using colonies derived from individuals collected in 2016 from almond orchards in two counties. One colony (ALM) originated from an almond orchard in Madera County with no reported pyrethroid resistance and the second colony (R347) originated from the same Kern County orchard where pyrethroid resistance was first reported. We used high-throughput quantitative real-time PCR (qRT-PCR) analyses of 65 P450s in the CYP3 and CYP4 clans of A. transitella to identify P450s induced by bifenthrin and associated with pyrethroid resistance. Nine P450s were constitutively overexpressed in R347 compared to ALM, including CYP6AE54 (11.7-fold), belonging to a subfamily associated with metabolic pesticide detoxification in Lepidoptera and CYP4G89 (33-fold) belonging to a subfamily associated with cuticular hydrocarbon (CHC) synthesis and resistance via reduced pesticide penetrance. Cuticular hydrocarbons analysis revealed that R347 produced twice as many total CHCs in the egg and adult stages as ALM. Topical toxicity bioassays for R347 determined that egg mortality was reduced at low bifenthrin concentrations and larval mortality was reduced at high concentrations of bifenthrin compared to ALM. Our discovery of both changes in metabolism and production of CHCs for R347 have implications for the possible decreased efficacy of other classes of insecticide used to control this insect. The threat of widespread pyrethroid resistance combined with the potential for cross-resistance to develop through the mechanism of reduced penetrance warrants developing management strategies that facilitate insecticide passage across the cuticle.

A Roadmap for the Development of Ivermectin as a Complementary Malaria Vector Control Tool

In the context of stalling progress against malaria, resistance of mosquitoes to insecticides, and residual transmission, mass drug administration (MDA) of ivermectin, an endectocide used for neglected tropical diseases (NTDs), has emerged as a promising complementary vector control method. Ivermectin reduces the life span of Anopheles mosquitoes that feed on treated humans and/or livestock, potentially decreasing malaria parasite transmission when administered at the community level. Following the publication by WHO of the preferred product characteristics for endectocides as vector control tools, this roadmap provides a comprehensive view of processes needed to make ivermectin available as a vector control tool by 2024 with a completely novel mechanism of action. The roadmap covers various aspects, which include 1) the definition of optimal dosage/regimens for ivermectin MDA in both humans and livestock, 2) the risk of resistance to the drug and environmental impact, 3) ethical issues, 4) political and community engagement, 5) translation of evidence into policy, and 6) operational aspects of large-scale deployment of the drug, all in the context of a drug given as a prevention tool acting at the community level. The roadmap reflects the insights of a multidisciplinary group of global health experts who worked together to elucidate the path to inclusion of ivermectin in the toolbox against malaria, to address residual transmission, counteract insecticide resistance, and contribute to the end of this deadly disease.

Cross-resistance, biochemical mechanism and fitness costs of laboratory-selected resistance to pyridalyl in diamondback moth, Plutella xylostella

Pyridalyl belongs to one novel type of insecticides with uncertain mode of action, and it showed significant efficacy against Plutella xylostella, which has been considered as one notorious insect pest in the world. To characterize pyridalyl resistance in P. xylostella, one susceptible strain XY-PS and one laboratory-selected pyridalyl-resistant strain XY-PR (34.4-fold) were used to establish cross-resistance patterns, and no cross-resistance to a series of popular insecticides in the XY-PR was observed. Activities of metabolic enzymes were measured and results showed that there was an approximate 5.2-fold significant increase in cytochrome P450 monooxygenase (P450) and no significant differences in glutathione S-transferase (GST) and esterase between XY-PR and XY-PS, indicating that the enhanced activity of P450 could be dominant mechanism of detoxification. Furthermore, expression profiles of three previously published resistance-associated P450 genes were established but no one was significantly different expression. Besides, fitness costs associated with pyridalyl resistance was observed in XY-PR, and it had been found that survival rate of larvae and hatchability were reduced in XY-PR. Then, by calculating the net replacement rate (R₀) of XY-PS, the fitness of XY-PR was established as 0.64. In conclusion, above results provided helpful data and information for studying further on mechanism of pyridalyl resistance, and will be conductive to design rational strategies of resistance management in P. xylostella.

The expression of Spodoptera exigua P450 and UGT genes: tissue specificity and response to insecticides

Cytochrome P450 and UDP-glucosyltransferase (UGT) as phase I and phase II metabolism enzymes, respectively, play vital roles in the breakdown of endobiotics and xenobiotics. Insects can increase the expression of detoxification enzymes to cope with the stress from xenobiotics including insecticides. However, the molecular mechanisms for insecticide detoxification in Spodoptera exigua remain elusive, and the genes conferring insecticide metabolisms in this species are less well reported. In this study, 68 P450 and 32 UGT genes were identified. Phylogenetic analysis showed gene expansions in CYP3 and CYP4 clans of P450 genes and UGT33 family of this pest. P450 and UGT genes exhibited specific tissue expression patterns. Insecticide treatments in fat body cells of S. exigua revealed that the expression levels of P450 and UGT genes were significantly influenced by challenges of abamectin, lambda-cyhalothrin, chlorantraniliprole, metaflumizone and indoxacarb. Multiple genes for detoxification were affected in expression levels after insecticide exposures. The results demonstrated that lambda-cyhalothrin, chlorantraniliprole, metaflumizone and indoxacarb induced similar responses in the expression of P450 and UGT genes in fat body cells; eight P450 genes and four UGT genes were co-up-regulated significantly, and no or only a few CYP/UGT genes were down-regulated significantly by these four insecticides. However, abamectin triggered a distinct response for P450 and UGT gene expression; more P450 and UGT genes were down-regulated by abamectin than by the other four compounds. In conclusion, P450 and UGT genes from S. exigua were identified, and different responses to abamectin suggest a different mechanism for insecticide detoxification.

Molecular cloning and characterization of GABA receptor and GluCl subunits in the western flower thrips, Frankliniella occidentalis

To understand the role of target site insensitivity in abamectin resistance in the western flower thrips (WFT), Frankliniella occidentalis (Pergande), cDNAs encoding gamma-aminobutyric acid receptor subunit (FoRdl) and glutamate-gated chloride channel (FoGluCl) were cloned from WFT, and both single nucleotide polymorphisms (SNPs) and mRNA expression levels of FoRdl and FoGluCl were detected in a susceptible strain (ABA-S) and a laboratory selected strain (ABA-R) displaying 45.5-fold resistance to abamectin. Multiple cDNA sequence alignment revealed three alternative splicing variants of FoRdl and two alternative splicing variants of FoGluCl generated by alternative splicing of exon 3. While sequence comparison of FoRdl and FoGluCl in ABA-S and ABA-R strains identified no resistance-associated mutations, the expression level of FoGluCl in ABA-R strain was 2.63-fold higher than that in ABA-S strain. Thus, our preliminary results provide the evidence that the increased mRNA expression of FoGluCl could be an important factor in FoGluCl-mediated target site insensitivity in WFT.

Complexin in ivermectin resistance in body lice

Ivermectin has emerged as very promising pediculicide, particularly in cases of resistance to commonly used pediculicides. Recently, however, the first field-evolved ivermectin-resistance in lice was reported. To gain insight into the mechanisms underlying ivermectin-resistance, we both looked for mutations in the ivermectin-target site (GluCl) and searched the entire proteome for potential new loci involved in resistance from laboratory susceptible and ivermectin-selected resistant body lice. Polymorphism analysis of cDNA GluCl showed no non-silent mutations. Proteomic analysis identified 22 differentially regulated proteins, of which 13 were upregulated and 9 were downregulated in the resistant strain. We evaluated the correlation between mRNA and protein levels by qRT-PCR and found that the trend in transcriptional variation was consistent with the proteomic changes. Among differentially expressed proteins, a complexin i.e. a neuronal protein which plays a key role in regulating neurotransmitter release, was shown to be the most significantly down-expressed in the ivermectin-resistant lice. Moreover, DNA-mutation analysis revealed that some complexin transcripts from resistant lice gained a premature stop codon, suggesting that this down-expression might be due, in part, to secondary effects of a nonsense mutation inside the gene. We further confirmed the association between complexin and ivermectin-resistance by RNA-interfering and found that knocking down the complexin expression induces resistance to ivermectin in susceptible lice. Our results provide evidence that complexin plays a significant role in regulating ivermectin resistance in body lice and represents the first evidence that links complexin to insecticide resistance.

Selecting and validating reference genes for quantitative real-time PCR in Plutella xylostella (L.)

Gene expression analysis provides important clues regarding gene functions, and quantitative real-time PCR (qRT-PCR) is a widely used method in gene expression studies. Reference genes are essential for normalizing and accurately assessing gene expression. In the present study, 16 candidate reference genes (ACTB, CyPA, EF1-α, GAPDH, HSP90, NDPk, RPL13a, RPL18, RPL19, RPL32, RPL4, RPL8, RPS13, RPS4, α-TUB, and β-TUB) from Plutella xylostella were selected to evaluate gene expression stability across different experimental conditions using five statistical algorithms (geNorm, NormFinder, Delta Ct, BestKeeper, and RefFinder). The results suggest that different reference genes or combinations of reference genes are suitable for normalization in gene expression studies of P. xylostella according to the different developmental stages, strains, tissues, and insecticide treatments. Based on the given experimental sets, the most stable reference genes were RPS4 across different developmental stages, RPL8 across different strains and tissues, and EF1-α across different insecticide treatments. A comprehensive and systematic assessment of potential reference genes for gene expression normalization is essential for post-genomic functional research in P. xylostella, a notorious pest with worldwide distribution and a high capacity to adapt and develop resistance to insecticides.

Identification of the pheromone biosynthesis genes from the sex pheromone gland transcriptome of the diamondback moth, Plutella xylostella

The diamondback moth was estimated to increase costs to the global agricultural economy as the global area increase of Brassica vegetable crops and oilseed rape. Sex pheromones traps are outstanding tools available in Integrated Pest Management for many years and provides an effective approach for DBM population monitoring and control. The ratio of two major sex pheromone compounds shows geographical variations. However, the limitation of our information in the DBM pheromone biosynthesis dampens our understanding of the ratio diversity of pheromone compounds. Here, we constructed a transcriptomic library from the DBM pheromone gland and identified genes putatively involved in the fatty acid biosynthesis, pheromones functional group transfer, and β-oxidation enzymes. In addition, odorant binding protein, chemosensory protein and pheromone binding protein genes encoded in the pheromone gland transcriptome, suggest that female DBM moths may receive odors or pheromone compounds via their pheromone gland and ovipositor system. Tissue expression profiles further revealed that two ALR, three DES and one FAR5 genes were pheromone gland tissue biased, while some chemoreception genes expressed extensively in PG, pupa, antenna and legs tissues. Finally, the candidate genes from large-scale transcriptome information may be useful for characterizing a presumed biosynthetic pathway of the DBM sex pheromone.

Cytochrome P450/ABC transporter inhibition simultaneously enhances ivermectin pharmacokinetics in the mammal host and pharmacodynamics in Anopheles gambiae

Mass administration of endectocides, drugs that kill blood-feeding arthropods, has been proposed as a complementary strategy to reduce malaria transmission. Ivermectin is one of the leading candidates given its excellent safety profile. Here we provide proof that the effect of ivermectin can be boosted at two different levels by drugs inhibiting the cytochrome or ABC transporter in the mammal host and the target mosquitoes. Using a mini-pig model, we show that drug-mediated cytochrome P450/ABC transporter inhibition results in a 3-fold increase in the time ivermectin remains above mosquito-killing concentrations. In contrast, P450/ABC transporter induction with rifampicin markedly impaired ivermectin absorption. The same ketoconazole-mediated cytochrome/ABC transporter inhibition also occurs outside the mammal host and enhances the mortality of Anopheles gambiae. This was proven by using the samples from the mini-pig experiments to conduct an ex-vivo synergistic bioassay by membrane-feeding Anopheles mosquitoes. Inhibiting the same cytochrome/xenobiotic pump complex in two different organisms to simultaneously boost the pharmacokinetic and pharmacodynamic activity of a drug is a novel concept that could be applied to other systems. Although the lack of a dose-response effect in the synergistic bioassay warrants further exploration, our study may have broad implications for the control of parasitic and vector-borne diseases.

Overexpression of a cytochrome P450 and a UDP-glycosyltransferase is associated with imidacloprid resistance in the Colorado potato beetle, Leptinotarsa decemlineata

Current control of insect pests relies on chemical insecticides, however, insecticide resistance development by pests is a growing concern in pest management. The main mechanisms for insecticide resistance typically involve elevated activity of detoxifying enzymes and xenobiotic transporters that break-down and excrete insecticide molecules. In this study, we investigated the molecular mechanisms of imidacloprid resistance in the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), an insect pest notorious for its capacity to develop insecticide resistance rapidly. We compared the transcriptome profiles of imidacloprid-resistant and sensitive beetle strains and identified 102 differentially expressed transcripts encoding detoxifying enzymes and xenobiotic transporters. Of these, 74 were up-regulated and 28 were down-regulated in the resistant strain. We then used RNA interference to knock down the transcript levels of seven up-regulated genes in the resistant beetles. Ingestion of double-stranded RNA successfully knocked down the expression of the genes for three cytochrome P450s (CYP6BQ15, CYP4Q3 and CYP4Q7), one ATP binding cassette (ABC) transporter (ABC-G), one esterase (EST1), and two UDP-glycosyltransferases (UGT1 and UGT2). Further, we demonstrated that silencing of CYP4Q3 and UGT2 significantly increased susceptibility of resistant beetles to imidacloprid, indicating that overexpression of these two genes contributes to imidacloprid resistance in this resistant strain.

RNA Interference in Moths: Mechanisms, Applications, and Progress

The vast majority of lepidopterans, about 90%, are moths. Some moths, particularly their caterpillars, are major agricultural and forestry pests in many parts of the world. However, some other members of moths, such as the silkworm Bombyx mori, are famous for their economic value. Fire et al. in 1998 initially found that exogenous double-stranded RNA (dsRNA) can silence the homolog endogenous mRNA in organisms, which is called RNA interference (RNAi). Soon after, the RNAi technique proved to be very promising not only in gene function determination but also in pest control. However, later studies demonstrate that performing RNAi in moths is not as straightforward as shown in other insect taxa. Nevertheless, since 2007, especially after 2010, an increasing number of reports have been published that describe successful RNAi experiments in different moth species either on gene function analysis or on pest management exploration. So far, more than 100 peer-reviewed papers have reported successful RNAi experiments in moths, covering 10 families and 25 species. By using classic and novel dsRNA delivery methods, these studies effectively silence the expression of various target genes and determine their function in larval development, reproduction, immunology, resistance against chemicals, and other biological processes. In addition, a number of laboratory and field trials have demonstrated that RNAi is also a potential strategy for moth pest management. In this review, therefore, we summarize and discuss the mechanisms and applications of the RNAi technique in moths by focusing on recent progresses.