Disruption of nicotinic acetylcholine receptor α6 mediated by CRISPR/Cas9 confers resistance to spinosyns in Plutella xylostella

Key role of the ryanodine receptor I4790K mutation in mediating diamide resistance in Plutella xylostella

The diamondback moth Plutella xylostella, a global insect pest of cruciferous vegetables, has evolved resistance to many classes of insecticides including diamides. Three point mutations (I4790M, I4790K, and G4946E) in the ryanodine receptor of P. xylostella (PxRyR) have been identified to associate with varying levels of resistance. In this study, we generated a knockin strain (I4790K-KI) of P. xylostella, using CRISPR/Cas9 to introduce the I4790K mutation into PxRyR of the susceptible IPP-S strain. Compared to IPP-S, the edited I4790K-KI strain exhibited high levels of resistance to both anthranilic diamides (chlorantraniliprole 1857-fold, cyantraniliprole 1433-fold) and the phthalic acid diamide flubendiamide (>2272-fold). Resistance to chlorantraniliprole in the I4790K-KI strain was inherited in an autosomal and recessive mode, and genetically linked with the I4790K knockin mutation. Computational modeling suggests the I4790K mutation reduces the binding of diamides to PxRyR by disrupting key hydrogen bonding interactions within the binding cavity. The approximate frequencies of the 4790M, 4790K, and 4946E alleles were assessed in ten geographical field populations of P. xylostella collected in China in 2021. The levels of chlorantraniliprole resistance (2.3- to 1444-fold) in these populations were significantly correlated with the frequencies (0.017-0.917) of the 4790K allele, but not with either 4790M (0-0.183) or 4946E (0.017-0.450) alleles. This demonstrates that the PxRyR I4790K mutation is currently the major contributing factor to chlorantraniliprole resistance in P. xylostella field populations within China. Our findings provide in vivo functional evidence for the causality of the I4790K mutation in PxRyR with high levels of diamide resistance in P. xylostella, and suggest that tracking the frequency of the I4790K allele is crucial for optimizing the monitoring and management of diamide resistance in this crop pest.

The knockout of the nicotinic acetylcholine receptor subunit gene α1 (nAChR α1) through CRISPR/Cas9 technology exposes its involvement in the resistance of Spodoptera exigua to insecticides

Insect nicotinic acetylcholine receptors (nAChRs) are the directed targets of many insecticides. However, there have been no reports on the molecular characterization of the nAChR gene family or the causal association between nAChR α1 and resistance to insecticides in S. exigua, which is a significant agricultural pest. In this study, we identified a total of 9 candidate nAChR subunits in S. exigua, namely nAChR α1-α7 and nAChR β1-β2. For functional validation roles of Seα1 in insecticide resistance of S. exigua, we introduced a ∼ 1041-bp deletion of the Seα1 gene in a homozygous mutant strain (Seα1-KO) by CRISPR/Cas9 genome editing system, resulting in a premature truncation of the Seα1 protein and the subsequent loss of functional transmembrane (TM) 3 and TM4 elements. Compared with WH-S strain (wild-type strain), the Seα1-KO strain exhibited 2.62-folds resistant to trifluoropyrimidine, 8.3-folds resistant to dimehypo, and 5.28-folds resistant to dinotefuran, but no significant change in susceptibility to emamectin benzoate, spinetoram, lambda-cyhalothrin, permethrin and chlorpyrifos. Thus, this study has laid a solid foundation for investigating the role of nAChRs in S. exigua, and provides evidence for the crucial involvement of the α1 subunit in the mechanism of trifluoropyrimidine, dimehypo, and dinotefuran in S. exigua. Moreover, it provides a reference for the value of Seα1 subunit and its homologues in other species as insecticide targets.

Tissue-specific transcriptome analyses in Drosophila provide novel insights into the mode of action of the insecticide spinosad and the function of its target, nAChRα6

BACKGROUND

The insecticides spinosad and imidacloprid are neurotoxins with distinct modes of action. Both target nicotinic acetylcholine receptors (nAChRs), albeit different subunits. Spinosad is an allosteric modulator, that upon binding initiates endocytosis of its target, nAChRα6. Imidacloprid binding triggers excessive neuronal ion influx. Despite these differences, low-dose effects converge downstream in the precipitation of oxidative stress and neurodegeneration.

RESULTS

Using RNA-sequencing, we compared the transcriptional signatures of spinosad and imidacloprid, at low-dose exposures. Both insecticides cause up-regulation of glutathione S-transferase and cytochrome P450 genes in the brain and down-regulation in the fat body, whereas reduced expression of immune-related genes is observed in both tissues. Spinosad shows unique impacts on genes involved in lysosomal function, protein folding, and reproduction. Co-expression analyses revealed little to no correlation between genes affected by spinosad and nAChRα6 expressing neurons, but a positive correlation with glial cell markers. We also detected and experimentally confirmed nAChRα6 expression in fat body cells and male germline cells. This led us to uncover lysosomal dysfunction in the fat body following spinosad exposure, and a fitness cost in spinosad-resistant (nAChRα6 null) males - oxidative stress in testes, and reduced fertility.

CONCLUSION

Spinosad and imidacloprid share transcriptional perturbations in immunity-, energy homeostasis-, and oxidative stress-related genes. Low doses of other neurotoxic insecticides should be investigated for similar impacts. While target-site spinosad resistance mutation has evolved in the field, this may have a fitness cost. Our findings demonstrate the power of tissue-specific transcriptomics approach and the use of single-cell transcriptome data. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Equivalent intensity but differential dominance of sodium channel blocker insecticide resistance conferred by F1845Y and V1848I mutations of the voltage-gated sodium channel in Plutella xylostella

Two point mutations (F1845Y and V1848I) in the voltage-gated sodium channel gene of Plutella xylostella are involved in the target-site resistance to sodium channel blocker insecticides (SCBIs). The contribution of the individual mutations to the SCBI resistance and the associated inheritance modes is as yet unclear. Through 2 rounds of single-pair crossing and marker-assisted selection, 2 P. xylostella strains (1845Y and 1848I) bearing homozygous F1845Y or V1848I mutant alleles were successfully established from a field-collected population, and the contribution of each mutation to SCBI resistance, as well as associated inheritance patterns, was determined. When compared with the susceptible SZPS strain, each of the mutations individually conferred equally high-level resistance to indoxacarb (378 and 313 fold) and metaflumizone (734 and 674 fold), respectively. However, dominance levels of resistance to SCBIs were significantly different between the 2 resistant strains. Resistance of the 1845Y strain to indoxacarb and metaflumizone was inherited as an autosomal and incompletely dominant trait (D values ranged from 0.43 to 0.76). In contrast, that of the 1848I strain followed an autosomal but incompletely recessive to semidominant mode (D values: -0.24 to 0.09). Our findings enriched the current understanding of inheritance and mechanisms of SCBI resistance in P. xylostella, and will help develop resistance management programs for P. xylostella and other economic pests.

CRISPR-Cas Genome Editing for Insect Pest Stress Management in Crop Plants

Global crop yield and food security are being threatened by phytophagous insects. Innovative methods are required to increase agricultural output while reducing reliance on hazardous synthetic insecticides. Using the revolutionary CRISPR-Cas technology to develop insect-resistant plants appears to be highly efficient at lowering production costs and increasing farm profitability. The genomes of both a model insect, Drosophila melanogaster, and major phytophagous insect genera, viz. Spodoptera, Helicoverpa, Nilaparvata, Locusta, Tribolium, Agrotis, etc., were successfully edited by the CRISPR-Cas toolkits. This new method, however, has the ability to alter an insect’s DNA in order to either induce a gene drive or overcome an insect’s tolerance to certain insecticides. The rapid progress in the methodologies of CRISPR technology and their diverse applications show a high promise in the development of insect-resistant plant varieties or other strategies for the sustainable management of insect pests to ensure food security. This paper reviewed and critically discussed the use of CRISPR-Cas genome-editing technology in long-term insect pest management. The emphasis of this review was on the prospective uses of the CRISPR-Cas system for insect stress management in crop production through the creation of genome-edited crop plants or insects. The potential and the difficulties of using CRISPR-Cas technology to reduce pest stress in crop plants were critically examined and discussed.

CRISPR-mediated knockout of nicotinic acetylcholine receptor (nAChR) α6 subunit confers high levels of resistance to spinosyns in Spodoptera frugiperda

Spinosyn insecticides (spinosad and spinetoram) have been widely used to control a number of agricultural pests including the fall armyworm, Spodoptera frugiperda. Mutations of the nicotinic acetylcholine receptor α6 subunit (nAChRα6) have been reported to confer high levels of resistance to spinosyns in several insect pests. Here we used CRISPR-mediated gene knockout to determine the involvement of S. frugiperda nAChRα6 (Sfα6) in spinosyns susceptibility. A Sfα6 knockout strain of S. frugiperda (Sfα6-KO) was established using dual single guide RNA (sgRNA) directed large fragment deletion with the CRISPR/Cas9 system. Sfα6-KO showed high levels of resistance to spinosad (307-fold) and spinetoram (517-fold) compared with the progenitor strain YJ-19, while no resistance was observed to emamectin benzoate, indoxacarb, chlorfenapyr, chlorantraniliprole and broflanilide. Genetic analyses confirmed that spinosad resistance in Sfα6-KO was autosomal, incompletely recessive and tightly linked to the edited deletion mutation of Sfα6. Our results provided in vivo functional evidence for Sfα6 as the major target of spinosyns against S. frugiperda, and demonstrated that disruption of Sfα6 causes high level resistance to spinosyns. Although no mutations of Sfα6 have yet been reported in any field populations of S. frugiperda, it is critical to develop F₁ screens and/or DNA-based methods to detect and monitor the mutant allele frequencies of Sfα6 across global populations of S. frugiperda.

Effects of nicotinic acetylcholine receptor subunit deletion mutants on insecticide susceptibility and fitness in Drosophila melanogaster

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) are major excitatory neurotransmitter receptors in insects and also the target site for many insecticides. Unfortunately, the effectiveness of these insecticides is diminishing as a consequence of the evolution of insecticide resistance. Further exploration of insecticide targets is important to sustainable pest management.

RESULTS

In order to validate the role of nAChR subunits in insecticide susceptibility and test whether the subunit's absence imposes the fitness cost on insects, we determined the susceptibility of eight nAChR subunit deletion mutants of Drosophila melanogaster to nine insecticides. These findings highlighted the specific resistance of the Dα6 deletion mutant to spinosyns. Although triflumezopyrim, dinotefuran and imidacloprid are competitive modulators of nAChRs, differences in susceptibility of the insect with different deletion mutants suggested that the target sites of these three insecticides do not overlap completely. Mutants showed decreased susceptibility to insecticides, accompanied by a reduction in fitness. The number of eggs produced by Dα1^attP , Dα2^attP , Dβ2^attP and Dβ3^attP females was significantly lesser than that of the vas-Cas9 strain as the control. In addition, adults of Dα2^attP , Dα3^attP and Dα7^attP strains showed lower climbing performance. Meanwhile, males of Dα3^attP , Dα5^attP , Dβ2^attP and Dβ3^attP , and females of Dβ2^attP showed significantly shorter longevity than those of the vas-Cas9 strain.

CONCLUSION

This study provides new insights into the interactions of different insecticides with different nAChRs subunit in D. melanogaster as a research model, it could help better understand such interaction in agricultural pests whose genetic manipulations for toxicological research are often challenging. © 2022 Society of Chemical Industry.

Knockin of the G275E mutation of the nicotinic acetylcholine receptor (nAChR) α6 confers high levels of resistance to spinosyns in Spodoptera exigua

Spinosyns, including spinosad and spinetoram, act on the insect central nervous system, gradually paralyzing or destroying the target insect. Spinosad resistance is associated with loss-of-function mutations in the nicotinic acetylcholine receptor (nAChR) α6 subunit in a number of agricultural pests. Using gene editing, nAChR α6 has been verified as a target for spinosyns in five insect species. Recently, a point mutation (G275E) in exon 9 of nAChR α6 was identified in spinosad-resistant strains of Thrips palmi and Tuta absoluta. To date, no in vivo functional evidence has been obtained to support that this mutation is involved in spinosyn resistance in lepidopteran pests. In this study, the G275E mutation was introduced into the nAChR of Spodoptera exigua using clustered regularly interspaced short palindromic repeats (CRISPR) / CRISPR-associated protein 9 (Cas9) gene-editing technology. Reverse transcriptase-polymerase chain reaction and sequencing confirmed that this mutation was present in exon 9 of the nAChR transcripts in the edited 275E strain. The results of bioassays showed that the 275E strain was highly resistant to spinosad (230-fold) and spinetoram (792-fold) compared to the unedited background strain, directly confirming that the G275E mutation of the nAChR α6 subunit confers high levels of spinosyn resistance in S. exigua. Inheritance analysis showed that the resistance trait is autosomal and incompletely recessive. This study employs a reverse genetics approach to validate the functional role played by the G275E mutation in nAChR α6 of S. exigua in spinosyns resistance and provides another example of the use of CRISPR/Cas9 gene-editing technology to confirm the role played by candidate target site mutations in insecticide resistance.

Distinct roles of two RDL GABA receptors in fipronil action in the diamondback moth (Plutella xylostella)

The phenylpyrazole insecticide fipronil blocks resistance to dieldrin (RDL) γ-aminobutyric acid (GABA) receptors in insects, thereby impairing inhibitory neurotransmission. Some insect species, such as the diamondback moth (Plutella xylostella), possess more than one Rdl gene. The involvement of multiple Rdls in fipronil toxicity and resistance remains largely unknown. In this study, we investigated the roles of two Rdl genes, PxRdl1 and PxRdl2, in P. xylostella fipronil action. In Xenopus oocytes, PxRDL2 receptors were 40 times less sensitive to fipronil than PxRDL1. PxRDL2 receptors were also less sensitive to GABA compared with PxRDL1. Knockout of the fipronil-sensitive PxRdl1 reduced the fipronil potency 10-fold, whereas knockout of the fipronil-resistant PxRdl2 enhanced the fipronil potency 4.4-fold. Furthermore, in two fipronil-resistant diamondback moth field populations, PxRdl2 expression was elevated 3.7- and 4.1-fold compared with a susceptible strain, whereas PxRdl1 expression was comparable among the resistant and susceptible strains. Collectively, our results indicate antagonistic effects of PxRDL1 and PxRDL2 on fipronil action in vivo and suggest that enhanced expression of fipronil-resistant PxRdl2 is potentially a new mechanism of fipronil resistance in insects.

Varying contributions of three ryanodine receptor point mutations to diamide insecticide resistance in Plutella xylostella

BACKGROUND

Although decoding the molecular mechanisms underlying insecticide resistance has often proven difficult, recent progress has revealed that specific mutations in the ryanodine receptor (RyR) of the diamondback moth, Plutella xylostella, can confer resistance to diamide insecticides. The extent to which specific RyR mutations contribute to the diamide resistance phenotype, the associated genetic traits and fitness costs remain limited.

RESULTS

Three field-evolved PxRyR mutations (G4946E, I4790 M, and I4790 K) were respectively introgressed into a common susceptible background strain (IPP-S) of P. xylostella with marker-assisted backcrossing. The mutations alone can result in moderate to high levels of resistance to five commercial diamides (flubendiamide, chlorantraniliprole, cyantraniliprole, tetraniliprole, and cyclaniliprole), and the resistance intensity mediated by the three mutations was hierarchical in order of I4790 K (1199- to >2778-fold) > G4946E (39- to 739-fold) > I4790 M (16- to 57-fold). Flubendiamide resistance was autosomal and incompletely recessive, and was significantly linked with the introgressed mutations in the three constructed strains. In addition, the resistance levels to flubendiamide of hybrid progeny from any two resistant strains fell in between the status of their parents. Furthermore, by comparing the net replacement rate, the fitness of 4946E, 4790 M and 4790 K strains were 0.77, 0.93 and 0.92 relative to the IPP-S strain, respectively.

CONCLUSION

Three independent PxRyR mutations confer varying degrees of resistance to diamides in P. xylostella. Among the three mutations, I4790 K confers highest levels of resistance (> 1000-fold) to all five commercial diamides. The findings can guide resistance management practices for diamides in P. xylostella and other arthropods.

CRISPR/Cas9 in lepidopteran insects: Progress, application and prospects

Clustered regularly spaced short palindrome repeats (CRISPR) structure family forms the acquired immune system in bacteria and archaea. Recent advances in CRISPR/Cas genome editing as derived from prokaryotes, confirmed the characteristics of robustness, high target specificity and programmability, and also revolutionized the insect sciences field. The successful application of CRISPR in a wide variety of lepidopteran insects, with a high genetic diversity, provided opportunities to explore gene functions, insect modification and pest control. In this review, we present a detailed overview on the recent progress of CRISPR in lepidopteran insects, and described the basic principles of the system and its application. Major interest is on wing development, pigmentation, mating, reproduction, sex determination, metamorphosis, resistance and silkworm breeding innovation. Finally, we outlined the limitations of CRISPR/Cas system and discussed its application prospects in lepidopteran insects.

High-level field-evolved resistance to spinosad in Colorado potato beetle, Leptinotarsa decemlineata, in organically managed fields

BACKGROUND

Organic pest management eschews synthetic pesticides and insecticide resistance is rarely studied in organically managed systems. Spinosad is a biologically based insecticide used widely by both organic and conventional growers. Colorado potato beetle, Leptinotarsa decemlineata, is infamous for its ability to evolve resistance to insecticides. Spinosad resistance was surveyed in conventionally managed fields in eastern New York in 2006. In response to grower reports of spinosad failure on two organic farms in 2009, resistance to spinosad was assayed in both conventionally and organically managed fields the following year, and growers were surveyed for their prior spinosad use.

RESULTS

In 2006, spinosad resistance measured as median lethal dose (LD₅₀ ) varied 9.8-fold among the eight conventional fields sampled and a laboratory susceptible strain. In 2010, the resistance ratios of LD₅₀ values relative to a laboratory susceptible strain ranged from 17.5 to 40.6 in conventionally managed fields, and from 128.7 to 5750.3 in organically managed fields, a dramatic increase from 2006 with higher resistance ratios in organically managed fields. Organic growers reported much heavier use of spinosad in the years prior to 2010.

CONCLUSION

This is the first report of high-level resistance to spinosad in Coleopterans. Selection strength due to number of years used and number of applications per season appear to have been the primary factors driving the evolution of resistance to spinosad, highlighting the need for resistance management in organic production, where fewer alternative active ingredients for resistance management are available. © 2021 Society of Chemical Industry.

Identification and expression analysis of cuticular protein genes in the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)

Structural cuticular proteins (CPs) are major components of the insect cuticle, and they play critical roles in insect development and insecticide resistance. Here, a total of 196 CP genes were successfully annotated in the Plutella xylostella genome. On the basis of motif analysis, these CPs were classified into 10 different families, including 122 CPR, 12 CPAP1, 8 CPAP3, 9 CPLCP, 2 Tweedle, 1 CPF, 1 CPFL, 1 CPCFC, 17 CPG and 2 18 aa proteins, and the remaining 21 unclassified CPs were classed as cuticular proteins hypothetical (CPH). A phylogenetic analysis of CPs from different insects revealed species-specific clades of RR-1 and RR-2 genes, suggesting that CP gene duplication might occur independently among insect taxa, while we also found that some other CPs (such as CPAP1 and CPAP3) had a closer relationship based on their conserved domain architecture. Using available RNAseq libraries, the expression profiles of the CPs were analyzed over the four developmental stages of the insect (i.e., egg, larva, pupa, and adult), revealing stage-specific expression patterns for the CPs. In a chlorpyrifos resistant strain, 18 CP genes were found to be more than two-fold upregulated compared to the susceptible control strain, and qRT-PCR analysis showed that these CP genes were overexpressed after exposure to chlorpyrifos, suggesting a potential role in the molecular mechanism of insecticide resistance in P. xylostella. This study provides the tools and molecular basis to study the role of CPs in the post-embryonal development and the mechanisms of insecticide resistance of P. xylostella.

New insights on the effects of spinosad on the development of Helicoverpa armigera

Helicoverpa armigera (cotton bollworm) is one of the most destructive pests worldwide. Due to resistance to Bacillus thuringiensis and conventional insecticides, an effective management strategy to control this pest is urgently needed. Spinosad, a natural pesticide, is considered an alternative; however, the mechanism underlying the developmental effects of sublethal spinosad exposure remains elusive. In this study, the mechanism was examined using an insect model of H. armigera. Results confirmed that exposure to sublethal spinosad led to reduced larval wet weight, delayed larval developmental period, caused difficulty in molting, and deformed pupae. Further investigation demonstrated that exposure to sublethal spinosad caused a significant decrease in 20E titer and increase in JH titer, thereby leading to the discordance between 20E and JH titers, and consequently alteration in the expression levels of HR3 and Kr-h1. These results suggested that sublethal spinosad caused hormonal disorders in larvae, which directly affect insect development. Our study serves as a reference and basis for the toxicity evaluation of spinosad on molting and pupation in insect metamorphosis, which may contribute to identifying targets for effective control of cotton bollworm.

Inhibiting the proteasome reduces molecular and biological impacts of the natural product insecticide, spinosad

BACKGROUND

Insecticide targets are often identified by mutations that confer resistance, but the intricacies of insecticide binding and downstream processes leading to insect death often remain obscure. Mutations in α6-like nicotinic acetylcholine receptor subunit genes have been associated with high levels of resistance to spinosad in many insect species, including Drosophila melanogaster. Here, we aimed to expand our understanding of the effects of the natural product insecticide spinosad on its protein target, the α6 subunit, using genetic tools available in D. melanogaster.

RESULTS

Functional, fluorescently tagged Dα6 subunits (Dα6^YFP ) were developed to allow observation of the protein in vivo. Larvae expressing Dα6^YFP were exposed to a sub-lethal concentration of spinosyn A (0.025 ppm) for 6 days, leading to a 64% reduction in fluorescence relative to unexposed larvae. Direct application of high doses of spinosyn A to dissected larval brains resulted in a visible 38.25% decrease in Dα6^YFP within 20 min, indicating that degradation of the Dα6 protein occurred in response to spinosyn A exposure. Chemical inhibition of the proteasome system using the multiple myeloma treatment drug, PS-341 reduced loss of Dα6^YFP in response to spinosyn A at the 20-min time point to 6.35%. In addition, in vivo administration of PS-341 prior to spinosad exposure reduced the effect of spinosad on larval activity.

CONCLUSION

Based on these data, we propose that exposure to spinosad leads to degradation of the α6-like target protein, a potentially novel element in the mode of action of spinosyns that may contribute to their toxicity towards insects. © 2021 Society of Chemical Industry.

CRISPR/Cas9 modified An. gambiae carrying kdr mutation L1014F functionally validate its contribution in insecticide resistance and combined effect with metabolic enzymes

Insecticide resistance in Anopheles mosquitoes is a major obstacle in maintaining the momentum in reducing the malaria burden; mitigating strategies require improved understanding of the underlying mechanisms. Mutations in the target site of insecticides (the voltage gated sodium channel for the most widely used pyrethroid class) and over-expression of detoxification enzymes are commonly reported, but their relative contribution to phenotypic resistance remain poorly understood. Here we present a genome editing pipeline to introduce single nucleotide polymorphisms in An. gambiae which we have used to study the effect of the classical kdr mutation L1014F (L995F based on An. gambiae numbering), one of the most widely distributed resistance alleles. Introduction of 1014F in an otherwise fully susceptible genetic background increased levels of resistance to all tested pyrethroids and DDT ranging from 9.9-fold for permethrin to >24-fold for DDT. The introduction of the 1014F allele was sufficient to reduce mortality of mosquitoes after exposure to deltamethrin treated bednets, even as the only resistance mechanism present. When 1014F was combined with over-expression of glutathione transferase Gste2, resistance to permethrin increased further demonstrating the critical combined effect between target site resistance and detoxification enzymes in vivo. We also show that mosquitoes carrying the 1014F allele in homozygosity showed fitness disadvantages including increased mortality at the larval stage and a reduction in fecundity and adult longevity, which can have consequences for the strength of selection that will apply to this allele in the field.

Escalation of pyrethroid resistance in Anopheles mosquitoes threatens to reduce the effectiveness of our most important tools in malaria control. Studying the mechanisms underlying insecticide resistance is critical to design mitigation strategies. Here, using genome modified mosquitoes, we functionally characterize the most prevalent mutation in resistant mosquitoes, showing that it confers substantial levels of resistance to all tested pyrethroids and undermines the performance of pyrethroid-treated nets. Furthermore, we show that combining this mutation with elevated levels of a detoxification enzyme further increases resistance. The pipeline we have developed provides a robust approach to quantifying the contribution of different combinations of resistance mechanisms to the overall phenotype, providing the missing link between resistance monitoring and predictions of resistance impact.

Identification of the Aedes aegypti nAChR gene family and molecular target of spinosad

BACKGROUND

Spinosad is an insecticide with unique mode of action (MOA) of disrupting nicotinic acetylcholine receptor and is efficacious against many insect species. Mutations in the nicotinic acetylcholine receptor (nAChR) α6 subunit have been identified that are associated with levels of spinosad resistance, but the molecular characterization of the nAChR gene family and a causative association between nAChR α6 and resistance to spinosad in Aedes aegypti, a primary vector of many arboviruses, have not yet been reported.

RESULTS

In this study, we identified 10 candidate nAChR subunits in Ae. Aegypti, nAChRα1-α9 and nAChRβ1, showing similarly orthologous relationships with Anopheles gambiae. With the application of the CRISPR/Cas9 genome editing system, we introduced a 32-bp deletion at the 5' end of the Aaeα6 (Ae. aegypti nAChR α6) gene in a homozygous mutant strain (Aaeα6-KO). The mutation produced two successive pre-mature stop codons, resulting in loss of function in the target receptor. The Aaeα6-KO mutant strain exhibited a 320-fold level of resistance to spinosad compared with wildtype. A recessive mode of inheritance for spinosad resistance was found in the Aaeα6-KO strain.

CONCLUSION

CRISPR/Cas9 introduced truncated Aaeα6 receptor in Ae. aegypti resulted in an increased level of resistance to spinosad, suggesting that the conserved nAChR α6 subunit is the target for spinosad insecticide. © 2020 Society of Chemical Industry.

Role of nicotinic acetylcholine receptor subunits in the mode of action of neonicotinoid, sulfoximine and spinosyn insecticides in Drosophila melanogaster

Insecticides remain valuable tools for the control of insect pests that significantly impact human health and agriculture. A deeper understanding of insecticide targets is important in maintaining this control over pests. Our study systematically investigates the nicotinic acetylcholine receptor (nAChR) gene family, in order to identify the receptor subunits critical to the insect response to insecticides from three distinct chemical classes (neonicotinoids, spinosyns and sulfoximines). Applying the CRISPR/Cas9 gene editing technology in D. melanogaster, we were able to generate and maintain homozygous mutants for eight nAChR subunit genes. A ninth gene (Dβ1) was investigated using somatic CRISPR in neural cells to overcome the low viability of the homozygous germline knockout mutant. These findings highlight the specificity of the spinosyn class insecticide, spinosad, to receptors containing the Dα6 subunit. By way of contrast, neonicotinoids are likely to target multiple receptor subtypes, beyond those receptor subunit combinations previously identified. Significant differences in the impacts of specific nAChR subunit deletions on the resistance level of flies to neonicotinoids imidacloprid and nitenpyram indicate that the receptor subtypes they target do not completely overlap. While an R81T mutation in β1 subunits has revealed residues co-ordinating binding of sulfoximines and neonicotinoids differ, the resistance profiles of a deletion of Dβ1 examined here provide new insights into the mode of action of sulfoxaflor (sulfoximine) and identify Dβ1 as a key component of nAChRs targeted by both these insecticide classes. A comparison of resistance phenotypes found in this study to resistance reported in insect pests reveals a strong conservation of subunit targets across many different insect species and that mutations have been identified in most of the receptor subunits that our findings would predict to have the potential to confer resistance.

Variable resistance to spinetoram in populations of Thrips palmi across a small area unconnected to genetic similarity

The melon thrips, Thrips palmi, is an increasingly important pest of vegetables in northern China. Some populations have developed resistance in the field to the insecticide spinetoram. Understanding the origin and dispersal of insecticide-resistant populations can shed light on resistance management strategies. In this study, we tested susceptibility of seven greenhouse populations of T. palmi to spinetoram collected from a small area of about 300 km2 in Shandong Province and examined population genetic structure across the area based on a segment of mitochondrial cox1 gene and 22 microsatellite loci to infer the possible origin and dispersal of insecticide resistance. Levels of resistance to spinetoram differed among seven populations, which included one population with high resistance (LC50 = 759.34 mg/L), three populations with medium resistance (LC50 ranged from 28.69 to 34.79 mg/L), and three populations with low resistance (LC50 ranged from 7.61 to 8.97 mg/L). The populations were genetically differentiated into two groups unrelated to both levels of resistance and geographic distance. The molecular data indicated high levels of gene flow between populations with different levels of resistance to spinetoram and low gene flow among populations with the same level of resistance, pointing to a likely separate history of resistance evolution. Resistance levels of two tested populations to spinetoram decreased 23 and 4.6 times after five generations without any exposure to the pesticide. We therefore suspect that resistance of T. palmi most likely evolved in response to local applications of the insecticide. Our study suggests that the development of resistance could be avoided or resistance even reversed by reducing usage of spinetoram.

CRISPR/Cas9 mediated ryanodine receptor I4790M knockin confers unequal resistance to diamides in Plutella xylostella

The diamondback moth Plutella xylostella is a major destructive pest of Brassica worldwide. P. xylostella has evolved resistance to nearly all commercial insecticides used for its control, including the most recent chemical class, diamide insecticides. Several studies show that the G4946E and I4790M mutations of ryanodine receptor (RyR) are strongly associated with diamide resistance in insects. While the pivotal functional role of G4946E in conferring diamide resistance phenotype has confirmed by several studies in different species, no direct evidence has unambiguously confirmed the functional significance of the single I4790M mutation in diamide resistance. Here, we successfully constructed a knockin homozygous strain (I4790M-KI) of P. xylostella using CRISPR/Cas9 coupled with homology directed repair approach to introduce I4790M into RyR. When compared with the background susceptible IPP-S strain, the manipulated I4790M-KI strain exhibited moderate resistance to the phthalic acid diamide flubendiamide (40.5-fold) and low resistance to anthranilic diamides chlorantraniliprole (6.0-fold) and cyantraniliprole (7.7-fold), with no changes to the toxicities of indoxacarb and β-cypermethrin. Furthermore, the acquired flubendiamide resistance was inherited in an autosomally recessive mode and significantly linked with the I4790M mutation of RyR in this I4790M-KI strain. Our findings provide in vivo functional evidence for the causality of I4790M mutation of PxRyR with moderate levels of resistance to flubendiamide in P. xylostella, and support the hypothesis that the diamide classes have different interactions with RyRs.

CRISPR-mediated gene knockout reveals nicotinic acetylcholine receptor (nAChR) subunit α6 as a target of spinosyns in Helicoverpa armigera

BACKGROUND

The spinosyn insecticides (spinosad and spinetoram) have been intensively used to control a wide range of agricultural pests. However, resistance to spinosyns has evolved in several agricultural pests. Disruption of the nicotinic acetylcholine receptor subunit α6 (nAChRα6) has been associated with high levels of resistance to spinosyns in both field and laboratory-selected strains of several insect pests. Among the 12 nAChR subunits of Helicoverpa armigera, Haα6 has the closest sequence similarity (66.02%) to Haα7. Here we used CRISPR-mediated knockouts to evaluate the role of two nAChR subunits (Haα6 and Haα7) of H. armigera in toxicity of spinosyns.

RESULTS

Individual knockouts of Haα6 and Haα7 were created utilizing CRISPR/Cas9 system in H. armigera. The Haα6 knockout (Haα6-KO) strain exhibited high levels of resistance to spinosad (531-fold) and spinetoram (1105-fold) compared with the wild-type parent SCD strain, whereas the Haα7 knockout (Haα7-KO) strain showed no significant susceptibility changes to both spinosyns. Genetic analyses demonstrated that resistance to spinosad conferred by knockout of Haα6 was autosomal, incompletely recessive and tightly linked to the disruption mutation of Haα6. Both Haα6-KO and Haα7-KO strains had no significant effects on susceptibility to other four insecticides including emamectin benzoate, beta-cypermethrin, chlorantraniliprole and indoxacarb.

CONCLUSION

Our results provide in vivo functional evidence for Haα6 as a target of spinosyns in H. armigera, and little or no role of Haα7 in mediating toxicity of spinosyns. The results are valuable to the development of resistance monitoring and management methods for spinosyn resistance in H. armigera. © 2020 Society of Chemical Industry.

Functional validation of nicotinic acetylcholine receptor (nAChR) α6 as a target of spinosyns in Spodoptera exigua utilizing the CRISPR/Cas9 system

BACKGROUND

The beet armyworm, Spodoptera exigua, is a serious agricultural pest that is primarily controlled using chemical insecticides. Recently, resistance to the insecticide spinosad has been described in S. exigua field populations. To date, there has been no functional evidence proving the involvement of the nicotinic acetylcholine receptor (nAChR) α6 mutation in spinosad resistance in S. exigua.

RESULTS

In this study, using the CRISPR/Cas9 genome-editing system, a homozygous strain (Seα6-KO) with approximately 1760-bp deletion within Seα6 in S. exigua causing a premature truncation of Seα6 was successfully constructed. Insecticide bioassays showed that Seα6-KO exhibited 373-fold higher resistance to spinosad and 850-fold higher resistance to spinetoram compared to WH-S strain with the same genetic background but showed no significant change in susceptibility to emamectin benzoate and chlorantraniliprole. Genetic analysis revealed that Seα6-KO is inherited as an incompletely recessive trait.

CONCLUSION

The results clearly demonstrated the functional role of Seα6 in resistance to spinosyn insecticides and provide an example of using genome editing to verify a target premature truncation associated with resistance.

Using CRISPR/Cas9 genome modification to understand the genetic basis of insecticide resistance: Drosophila and beyond

Chemical insecticides are a major tool for the control of many of the world's most damaging arthropod pests. However, their intensive application is often associated with the emergence of resistance, sometimes with serious implications for sustainable pest control. To mitigate failure of insecticide-based control tools, the mechanisms by which insects have evolved resistance must be elucidated. This includes both identification and functional characterization of putative resistance genes and/or mutations. Research on this topic has been greatly facilitated by using powerful genetic model insects like Drosophila melanogaster, and more recently by advances in genome modification technology, notably CRISPR/Cas9. Here, we present the advances that have been made through the application of genome modification technology in insecticide resistance research. The majority of the work conducted in the field to date has made use of genetic tools and resources available in D. melanogaster. This has greatly enhanced our understanding of resistance mechanisms, especially those mediated by insensitivity of the pesticide target-site. We discuss this progress for a series of different insecticide targets, but also report a number of unsuccessful or inconclusive attempts that highlight some inherent limitations of using Drosophila to characterize resistance mechanisms identified in arthropod pests. We also discuss an experimental framework that may circumvent current limitations while retaining the genetic versatility and robustness that Drosophila has to offer. Finally, we describe examples of direct CRISPR/Cas9 use in non-model pest species, an approach that will likely find much wider application in the near future.

Significance and interpretation of molecular diagnostics for insecticide resistance management of agricultural pests

Insecticide resistant pests become increasingly difficult to control in current day agriculture. Because of environmental and health concerns, the insecticide portfolio to combat agricultural pests is gradually decreasing. It is therefore crucial to make rational decisions on insecticide use to assure effective resistance management. However, resistance monitoring programs that inform on pest susceptibility and resistance are not yet common practice in agriculture. Molecular markers of resistance that are turned into convenient diagnostic tools are urgently needed and will only increase in importance. This review investigates which factors determine the strength, diagnostic value, and success of a diagnostic marker, and in which cases recent technical advances might provide new opportunities for decision making in an operational meaningful way.

CRISPR-Mediated Knockout of the ABCC2 Gene in Ostrinia furnacalis Confers High-Level Resistance to the Bacillus thuringiensis Cry1Fa Toxin

The adoption of transgenic crops expressing Bacillus thuringiensis (Bt) insecticidal crystalline (Cry) proteins has reduced insecticide application, increased yields, and contributed to food safety worldwide. However, the efficacy of transgenic Bt crops is put at risk by the adaptive resistance evolution of target pests. Previous studies indicate that resistance to Bacillus thuringiensis Cry1A and Cry1F toxins was genetically linked with mutations of ATP-binding cassette (ABC) transporter subfamily C gene ABCC2 in at least seven lepidopteran insects. Several strains selected in the laboratory of the Asian corn borer, Ostrinia furnacalis, a destructive pest of corn in Asian Western Pacific countries, developed high levels of resistance to Cry1A and Cry1F toxins. The causality between the O. furnacalisABCC2 (OfABCC2) gene and resistance to Cry1A and Cry1F toxins remains unknown. Here, we successfully generated a homozygous strain (OfC2-KO) of O. furnacalis with an 8-bp deletion mutation of ABCC2 by the CRISPR/Cas9 approach. The 8-bp deletion mutation results in a frame shift in the open reading frame of transcripts, which produced a predicted protein truncated in the TM4-TM5 loop region. The knockout strain OfC2-KO showed much more than a 300-fold resistance to Cry1Fa, and low levels of resistance to Cry1Ab and Cry1Ac (<10-fold), but no significant effects on the toxicities of Cry1Aa and two chemical insecticides (abamectin and chlorantraniliprole), compared to the background NJ-S strain. Furthermore, we found that the Cry1Fa resistance was autosomal, recessive, and significantly linked with the 8-bp deletion mutation of OfABCC2 in the OfC2-KO strain. In conclusion, in vivo functional investigation demonstrates the causality of the OfABCC2 truncating mutation with high-level resistance to the Cry1Fa toxin in O. furnacalis. Our results suggest that the OfABCC2 protein might be a functional receptor for Cry1Fa and reinforces the association of this gene to the mode of action of the Cry1Fa toxin.