Knockout of a P-glycoprotein gene increases susceptibility to abamectin and emamectin benzoate in Spodoptera exigua

Characterization of ATP-binding cassette transporters associated with emamectin benzoate tolerance: from the model insect Drosophila melanogaster to the agricultural pest Spodoptera frugiperda

BACKGROUND

Multiple families of detoxification genes, including the increasingly recognized family of ATP-binding cassette (ABC) transporters, work together to influence the toxicity of synthetic insecticides and thus their resistance. Effective management of insecticide resistance requires identification of all toxicity-affecting members from each family of toxicity-related genes.

RESULTS

Here, we used emamectin benzoate (EB), ABC transporters and Spodoptera frugiperda as a working case to test whether the strategy of 'from the model insect Drosophila melanogaster to agricultural pests' can identify all or most ABC transporter members related to EB tolerance in S. frugiperda. After confirming the involvement of ABC transporters in the toxicity of EB against fruit fly with the ABC inhibitor verapamil, four ABC transporter genes (DmCG3327, DmCG11147, DmCG4822, and DmCG7627) were found to be involved in EB tolerance using RNA interference-based family-wide functional screening. A combination of phylogenic analysis and a reciprocal TBLASTN search identified five S. frugiperda ABC transporter members as homologs (SfABCC4, SfABCG1, and SfABCG23) or one-way best hits (SfABCG4 and SfABCG20) of the four fly ABC genes. Real-time quantitative polymerase chain reaction (qPCR) analysis found that all five S. frugiperda ABC transporter genes were inducible by EB, and expressed in all the developmental stages and larval tissues, but with significant quantitative differences among stages and tissues. A cytotoxicity assay of ABC-overexpressing Sf9 cell lines showed that all the five S. frugiperda ABC transporter genes made Sf9 cells tolerant to EB.

CONCLUSIONS

This study not only identifies nine ABC transporter genes related to EB tolerance from D. melanogaster (four genes) and S. frugiperda (five genes), but also demonstrates the utility and effectiveness of the 'model to pests' strategy to identify most toxicity-affecting members from a given family of toxicity-related genes. © 2024 Society of Chemical Industry.

CRISPR/Cas9-Based Genome Editing of Fall Armyworm (Spodoptera frugiperda): Progress and Prospects

The fall armyworm (Spodoptera frugiperda) poses a substantial threat to many important crops worldwide, emphasizing the need to develop and implement advanced technologies for effective pest control. CRISPR/Cas9, derived from the bacterial adaptive immune system, is a prominent tool used for genome editing in living organisms. Due to its high specificity and adaptability, the CRISPR/Cas9 system has been used in various functional gene studies through gene knockout and applied in research to engineer phenotypes that may cause economical losses. The practical application of CRISPR/Cas9 in diverse insect orders has also provided opportunities for developing strategies for genetic pest control, such as gene drive and the precision-guided sterile insect technique (pgSIT). In this review, a comprehensive overview of the recent progress in the application of the CRISPR/Cas9 system for functional gene studies in S. frugiperda is presented. We outline the fundamental principles of applying CRISPR/Cas9 in S. frugiperda through embryonic microinjection and highlight the application of CRISPR/Cas9 in the study of genes associated with diverse biological aspects, including body color, insecticide resistance, olfactory behavior, sex determination, development, and RNAi. The ability of CRISPR/Cas9 technology to induce sterility, disrupt developmental stages, and influence mating behaviors illustrates its comprehensive roles in pest management strategies. Furthermore, this review addresses the limitations of the CRISPR/Cas9 system in studying gene function in S. frugiperda and explores its future potential as a promising tool for controlling this insect pest.

CRISPR/Cas9-mediated genome editing technique to control fall armyworm (Spodoptera frugiperda) in crop plants with special reference to maize

Fall Armyworm imposes a major risk to agricultural losses. Insecticides have historically been used to manage its infestations, but it eventually becomes resistant to them. To combat the pest, a more recent strategy based on the use of transgenic maize that expresses Bt proteins such as Cry1F from the bacteria has been used. Nonetheless, there have been numerous reports of Cry1F maize resistance in FAW populations. Nowadays, the more effective and less time-consuming genome editing method known as CRISPR/Cas9 technology has gradually supplanted these various breeding techniques. This method successfully edits the genomes of various insects, including Spodoptera frugiperda. On the other hand, this new technique can change an insect's DNA to overcome its tolerance to specific insecticides or to generate a gene drive. The production of plant cultivars resistant to fall armyworms holds great potential for the sustainable management of this pest, given the swift advancement of CRISPR/Cas9 technology and its varied uses. Thus, this review article discussed and critically assessed the use of CRISPR/Cas9 genome-editing technology in long-term fall armyworm pest management. However, this review study focuses primarily on the mechanism of the CRISPR-Cas9 system in both crop plants and insects for FAW management.

Identification of differentially expressed miRNAs associated with diamide detoxification pathways in Spodoptera frugiperda

The fall armyworm (FAW) Spodoptera frugiperda is a severe economic pest of multiple crops globally. Control of this pest is often achieved using insecticides; however, over time, S. frugiperda has developed resistance to new mode of action compounds, including diamides. Previous studies have indicated diamide resistance is a complex developmental process involving multiple detoxification genes. Still, the mechanism underlying the possible involvement of microRNAs in post-transcriptional regulation of resistance has not yet been elucidated. In this study, a global screen of microRNAs (miRNAs) revealed 109 known and 63 novel miRNAs. Nine miRNAs (four known and five novel) were differentially expressed between insecticide-resistant and -susceptible strains. Gene Ontology analysis predicted putative target transcripts of the differentially expressed miRNAs encoding significant genes belonging to detoxification pathways. Additionally, miRNAs are involved in response to diamide exposure, indicating they are probably associated with the detoxification pathway. Thus, this study provides comprehensive evidence for the link between repressed miRNA expression and induced target transcripts that possibly mediate diamide resistance through post-transcriptional regulation. These findings highlight important clues for further research to unravel the roles and mechanisms of miRNAs in conferring diamide resistance.

Reduced expression of the P-glycoprotein gene HaABCB1 is linked to resistance to Bacillus thuringiensis Cry1Ac toxin but not Cry2Ab toxin in Helicoverpa armigera

Rapid evolution of pest resistance to Bt insecticidal proteins presents a serious threat to the sustainable use of Bt crops. The cotton bollworm has been extensively exposed to Bt cotton worldwide and has evolved resistance in laboratory and field. Previous studies have highlighted the significant roles played by the ABC transporter proteins in Bt resistance. In this study, the ORF of HaABCB1 was cloned and analyzed. The expression of HaABCB1 was detected in all developmental stages and tissues, with the highest expression in third instar larvae stage and hindgut tissue. Compared with susceptible strain, a remarkable decrease of HaABCB1 expression in Cry1Ac resistant strain while no significant change in Cry2Ab resistant strain were found. The HaABCB1 expression reduced after susceptible larvae induced by Cry1Ac, but no obvious expression changes after Cry2Ab exposure. RNAi-mediated down-regulation of HaABCB1 could lead to a significant reduction in larval susceptibility to Cry1Ac, but not to Cry2Ab, in susceptible strain. Genetic linkage analysis confirmed that decreased expression of the HaABCB1 mediates resistance to Cry1Ac, but not Cry2Ab resistance. This knowledge contributes to better understanding of the complex molecular mechanisms underlying Bt resistance and provide theoretical foundation for the development of new strategies for pest resistance management.

Advancements and prospects of CRISPR/Cas9 technologies for abiotic and biotic stresses in sugar beet

Sugar beet is a crop with high sucrose content, known for sugar production and recently being considered as an emerging raw material for bioethanol production. This crop is also utilized as cattle feed, mainly when animal green fodder is scarce. Bioethanol and hydrogen gas production from this crop is an essential source of clean energy. Environmental stresses (abiotic/biotic) severely affect the productivity of this crop. Over the past few decades, the molecular mechanisms of biotic and abiotic stress responses in sugar beet have been investigated using next-generation sequencing, gene editing/silencing, and over-expression approaches. This information can be efficiently utilized through CRISPR/Cas 9 technology to mitigate the effects of abiotic and biotic stresses in sugar beet cultivation. This review highlights the potential use of CRISPR/Cas 9 technology for abiotic and biotic stress management in sugar beet. Beet genes known to be involved in response to alkaline, cold, and heavy metal stresses can be precisely modified via CRISPR/Cas 9 technology for enhancing sugar beet's resilience to abiotic stresses with minimal off-target effects. Similarly, CRISPR/Cas 9 technology can help generate insect-resistant sugar beet varieties by targeting susceptibility-related genes, whereas incorporating Cry1Ab and Cry1C genes may provide defense against lepidopteron insects. Overall, CRISPR/Cas 9 technology may help enhance sugar beet's adaptability to challenging environments, ensuring sustainable, high-yield production.

Involvement of Glutamate Cysteine Ligase Genes in Tolerance to Emamectin Benzoate in Spodoptera frugiperda and Their Putative Regulatory Mechanisms

As the rate-limiting enzyme in de novo Glutathione (GSH) biosynthesis, the mammalian glutamate cysteine ligase (Gcl) catalytic (Gclc) and modifier (Gclm) subunits are regulated at multiple levels, whereas the function and regulatory mechanism of insect Gcl remain to be explored. In this study, we identified and characterized SfGclc and SfGclm in Spodoptera frugiperda. SfGclc and SfGclm were highly expressed in the hindgut and relatively less expressed in other tissues. The exposure of the third instar larvae to LC30 of emamectin benzoate (EMB) significantly reduced the GSH content with a concomitant upregulation of SfGclc and SfGclm. Further in vivo pretreatment with L-BSO, the Gcl inhibitor, increased the susceptibility of S. frugiperda to EMB. Consistently, overexpression of SfGclc and SfGclm increased the Sf9 cell viability under EMB treatment. Finally, both RNAi and the dual-luciferase reporter assay in Sf9 cells revealed that SfGclc is regulated by transcription factor CncC. These data provide insights into the function and regulatory mechanism of insect Gcl, and they imply that disruption of the redox homeostasis might be a practical strategy to enhance the insecticidal activity of EMB and other insecticides.

Assessment of the role of an ABCC transporter TuMRP1 in the toxicity of abamectin to Tetranychus urticae

The rapid evolution of pest resistance threatens the sustainable utilization of bioinsecticides such as abamectin, and so deciphering the molecular mechanisms affecting toxicity and resistance is essential for their long-term application. Historical studies of abamectin resistance in arthropods have mainly focused on mechanisms involving the glutamate-gated chloride channel (GluCl) targets, with the role of metabolic processes less clear. The two-spotted spider mite, Tetranychus urticae, is a generalist herbivore notorious for rapidly developing resistance to pesticides worldwide, and abamectin has been widely used for its control in the field. After reanalyzing previous transcriptome and RNA-seq data, we here identified an ABC transporter subfamily C gene in T. urticae named multidrug resistance-associated protein 1 (TuMRP1), whose expression differed between susceptible and resistant populations. Synergism bioassays with the inhibitor MK-571, the existence of a genetic association between TuMRP1 expression and susceptibility to abamectin, and the effect of RNA interference mediated silencing of TuMRP1 were all consistent with a direct role of this transporter protein in the toxicity of abamectin. Although ABC transporters are often involved in removing insecticidal compounds from cells, our data suggest either an alternative role for these proteins in the mechanism of action of abamectin or highlight an indirect association between their expression and abamectin toxicity.

Investigating the Regulatory Mechanism of the Sesquiterpenol Nerolidol from a Plant on Juvenile Hormone-Related Genes in the Insect Spodoptera exigua

Various plant species contain terpene secondary metabolites, which disrupt insect growth and development by affecting the activity of juvenile hormone-degrading enzymes, and the juvenile hormone (JH) titers maintained in insects. Nerolidol, a natural sesquiterpenol belonging to the terpenoid group, exhibits structural similarities to insect JHs. However, the impact of nerolidol on insect growth and development, as well as its underlying molecular mechanism, remains unclear. Here, the effects of nerolidol on Spodoptera exigua were investigated under treatment at various sub-lethal doses (4.0 mg/mL, 1.0 mg/mL, 0.25 mg/mL). We found that a higher dose (4.0 mg/mL) of nerolidol significantly impaired the normal growth, development, and population reproduction of S. exigua, although a relatively lower dose (0.25 mg/mL) of nerolidol had no significant effect on this growth and development. Combined transcriptome sequencing and gene family analysis further revealed that four juvenile hormone esterase (JHE)-family genes that are involved in juvenile hormone degradation were significantly altered in S. exigua larvae after nerolidol treatment (4.0 mg/mL). Interestingly, the juvenile hormone esterase-like (JHEL) gene Sexi006721, a critical element responsive to nerolidol stress, was closely linked with the significant augmentation of JHE activity and JH titer in S. exigua (R2 = 0.94, p < 0.01). Taken together, we speculate that nerolidol can function as an analog of JH by modulating the expression of the enzyme genes responsible for degrading JH, resulting in JH disorders and ultimately disrupting the development of insect larvae. This study ultimately provides a theoretical basis for the sustainable control of S. exigua in the field whilst proposing a new perspective for the development of novel biological pesticides.

Group V Chitin Deacetylases Influence the Structure and Composition of the Midgut of Beet Armyworm, Spodoptera exigua

Chitin deacetylase (CDA) can accelerate the conversion of chitin to chitosan, influencing the mechanical properties and permeability of the cuticle structures and the peritrophic membrane (PM) in insects. Putative Group V CDAs SeCDA6/7/8/9 (SeCDAs) were identified and characterized from beet armyworm Spodoptera exigua larvae. The cDNAs of SeCDAs contained open reading frames of 1164 bp, 1137 bp, 1158 bp and 1152 bp, respectively. The deduced protein sequences showed that SeCDAs are synthesized as preproteins of 387, 378, 385 and 383 amino acid residues, respectively. It was revealed via spatiotemporal expression analysis that SeCDAs were more abundant in the anterior region of the midgut. The SeCDAs were down-regulated after treatment with 20-hydroxyecdysone (20E). After treatment with a juvenile hormone analog (JHA), the expression of SeCDA6 and SeCDA8 was down-regulated; in contrast, the expression of SeCDA7 and SeCDA9 was up-regulated. After silencing SeCDAV (the conserved sequences of Group V CDAs) via RNA interference (RNAi), the layer of intestinal wall cells in the midgut became more compact and more evenly distributed. The vesicles in the midgut were small and more fragmented or disappeared after SeCDAs were silenced. Additionally, the PM structure was scarce, and the chitin microfilament structure was loose and chaotic. It was indicated in all of the above results that Group V CDAs are essential for the growth and structuring of the intestinal wall cell layer in the midgut of S. exigua. Additionally, the midgut tissue and the PM structure and composition were affected by Group V CDAs.

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.

Systematic identification and functional analysis of long noncoding RNAs involved in indoxacarb resistance in Spodoptera litura

Long noncoding RNAs (lncRNAs) are noncoding transcripts that are more than 200 nucleotides long. They play essential roles in regulating a variety of biological processes in many species, including insects, and some lncRNAs have been found to be associated with insecticide resistance. However, the characteristics and biological functions of lncRNAs involved in indoxacarb resistance are unknown in Spodoptera litura. We performed RNA sequencing in the SS, InRS, and FInRS of S. litura and identified 11 978 lncRNAs, including 3 136 intergenic lncRNAs, 7 393 intronic lncRNAs, and 1 449 anti-sense lncRNAs. Compared with the SS, 51 lncRNAs were upregulated and 134 lncRNAs were downregulated in the two resistant strains, and 908 differentially expressed mRNAs were predicted as the target genes of the 185 differentially expressed lncRNAs. Further analysis showed that 112 of differentially expressed lncRNAs may be associated with indoxacarb resistance by regulating the expression of 14 P450s, seven CCEs, one GST, six UGTs, five ABC transporters, and 24 cuticle protein genes, and 79 of differentially expressed lncRNAs may regulate the expression of 14 detoxification genes and 19 cuticle protein genes to participate in indoxacarb resistance by sponging 10 microRNAs. Interestingly, 47 of differentially expressed lncRNAs may mediate indoxacarb resistance through both lncRNA-mRNA and lncRNA-miRNA-mRNA regulatory pathways. Furthermore, quantitative PCR, RNA interference, and indoxacarb bioassay analyses indicated that overexpressed LNC_004867 and LNC_006576 were involved in indoxacarb resistance. This study provides comprehensive information for lncRNAs of S. litura, and presents evidence that lncRNAs have key roles in conferring insecticide resistance in S. litura.

Functional Inquiry into ATP-Binding Cassette Transporter Genes Contributing to Spirotetramat Resistance in Aphis gossypii Glover

ATP-binding cassette (ABC) transporters regulate the efflux of a broad spectrum of substrates to extracellular transporting, which play an important role in the detoxification process in arthropods. Here, we described a comprehensive approach to explore the involvement of ABC transporters in spirotetramat resistance in cotton aphids. In this study, synergism bioassays showed 17.05% and 35.42% increases in the toxicity to spirotetramat with the ABC inhibitor verapamil in adult and 3rd instar nymph aphids of the SR strain, respectively. In a competitive assay based on the microinjection of a fluorescent ABC transporter substrate, verapamil (a general ABC inhibitor) and spirotetramat significantly inhibited the elimination of Texas Red. Based on transcriptome data of midguts of spirotetramat-susceptible (SS) and -resistant (SR) strains, the expression levels of ABCB4, ABCB5, ABCF2, MRP11, and MRP12 were significantly upregulated in the SR strain midgut compared to that of the SS strain. Gene functional analysis based on ectopic expression and RNA interference (RNAi) proved that ABCB4, ABCB5, ABCF2, MRP11, and MRP12 were involved in the tolerance of cotton aphids to spirotetramat. Moreover, the upregulated ABCF2, ABCB4, and ABCB5 in the midgut of the SR strain contributed more to the resistance of spirotetramat in in vitro functional analysis. In summary, these results demonstrate that candidate ABC transporter genes in the midgut tissue were involved in spirotetramat resistance, which will help reveal the relationship between ABC transporters and the development of spirotetramat resistance in field populations.

miRNA novel_268 targeting NlABCG3 is involved in nitenpyram and clothianidin resistance in Nilaparvata lugens

The brown planthopper (BPH), Nilaparvata lugens (Stål), is one of the most destructive pests that seriously threatens the high-quality and safe production of rice. However, due to the unscientific use of chemical insecticides, N. lugens has developed varying levels of resistance to insecticides, including nitenpyram and clothianidin. The ATP-binding cassette (ABC) transporter plays a nonnegligible role in phase III of the detoxification process, which may play an important role in insecticide resistance. In the present study, NlABCG3 was significantly overexpressed in both the NR and CR populations compared with susceptible populations. Silencing NlABCG3 significantly increased the susceptibility of BPH to nitenpyram and clothianidin. In addition, RNAi-mediated knockdown of three key genes in the miRNA biogenesis pathway altered the level of NlABCG3. Subsequently, the luciferase reporter assays demonstrated that novel_268 binds to the NlABCG3 coding region and downregulates its expression. Furthermore, injection of miRNA inhibitors or mimics of novel_268 significantly altered the susceptibility of N. lugens to nitenpyram and clothianidin. These results suggest that miRNA novel_268 targeting NlABCG3 is involved in nitenpyram and clothianidin resistance in N. lugens. These findings may help to enhance our knowledge of the transcriptional regulation of the ABC transporter that mediate insecticide resistance in N. lugens.

Molecular Cloning and Expression Profiling of CncC in Bactrocera dorsalis Hendel

The cap 'n' collar isoform C (CncC) transcription factor is thought to be a regulator associated with antioxidant and detoxification genes that can enhance pest resistance by regulating the expression of detoxification enzyme genes. However, this transcription factor has not been well studied in the important agricultural pest Bactrocera dorsalis. In this study, the cDNA sequence of CncC in B. dorsalis was cloned, and the complete ORF sequence was obtained; it had a sequence length of 3378 bp, encoding a total of 1125 amino acids. Phylogenetic tree analysis showed that B. dorsalis CncC belonged to the CNC family and that its amino acid sequence showed the closest relationship with B. tryoni. The conserved structural region of BdCncC was analyzed and was found to include a conserved bZIP superfamily structural domain. Spatiotemporal expression analysis revealed that BdCncC was most highly expressed in the adult Malpighian tubules, followed by the antennae, foregut, and midgut, and then the brain, hemolymph, hindgut, and fat body. BdCncC was expressed at every developmental stage, and the highest expression was found in mature males. This study provides a theoretical basis for an in-depth investigation of the function of BdCncC in regulating pesticide resistance in B. dorsalis.

Characterization of a novel pesticide transporter and P-glycoprotein orthologues in Drosophila melanogaster

Pesticides remain one of the most effective ways of controlling agricultural and public health insects, but much is still unknown regarding how these compounds reach their targets. Specifically, the role of ABC transporters in pesticide absorption and excretion is poorly understood, especially compared to the detailed knowledge about mammalian systems. Here, we present a comprehensive characterization of pesticide transporters in the model insect Drosophila melanogaster. An RNAi screen was performed, which knocked down individual ABCs in specific epithelial tissues and examined the subsequent changes in sensitivity to the pesticides spinosad and fipronil. This implicated a novel ABC drug transporter, CG4562, in spinosad transport, but also highlighted the P-glycoprotein orthologue Mdr65 as the most impactful ABC in terms of chemoprotection. Further characterization of the P-glycoprotein family was performed via transgenic overexpression and immunolocalization, finding that Mdr49 and Mdr50 play enigmatic roles in pesticide toxicology perhaps determined by their different subcellular localizations within the midgut. Lastly, transgenic Drosophila lines expressing P-glycoprotein from the major malaria vector Anopheles gambiae were used to establish a system for in vivo characterization of this transporter in non-model insects. This study provides the basis for establishing Drosophila as a model for toxicology research on drug transporters.

Functional Diversity of the Lepidopteran ATP-Binding Cassette Transporters

The ATP-binding cassette (ABC) transporter gene family is ubiquitous in the living world. ABC proteins bind and hydrolyze ATP to transport a myriad of molecules across various lipid-containing membrane systems. They have been studied well in plants for transport of a variety of compounds and particularly, in vertebrates due to their direct involvement in resistance mechanisms against several toxic molecules/metabolites. ABC transporters in insects are found within large multigene families involved in the efflux of chemical insecticides and toxic/undesired metabolites originating from food and endogenous metabolism. This review deals with ABC transporter subfamilies of few agronomically important Lepidopteran pests. The transcriptional dynamics and regulation of ABC transporters during insect development emphasizes their functional diversity against insecticides, Cry toxins, and plant specialized metabolites. To generate insights about molecular function and physiological roles of ABCs, functional and structural characterization is necessary. Also, expansion and divergence of ABC transporter gene subfamilies in Lepidopteran insects needs more systematic investigation. We anticipate that newer methods of insect control in agriculture can benefit from an understanding of ABC transporter interactions with a vast range of natural specialized molecules and synthetic compounds.

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.

Knockout of the ABCB1 Gene Increases Susceptibility to Emamectin Benzoate, Beta-Cypermethrin and Chlorantraniliprole in Spodoptera frugiperda

ATP-binding cassette transporter B1 (ABCB1, or P-glycoprotein) is known to be an important participant in multidrug resistance in mammals, and it also has been proved as a transporter for some insecticides in several lepidopteran insects, yet the precise function of this transporter in Spodoptera frugiperda is unknown. Here, we generated a SfABCB1 knockout strain of the S. frugiperda using the CRISPR/Cas9 system to explore its potential roles in determining susceptibility to chemical insecticides or Bt toxins. Bioassay results showed that the susceptibility of SfABCB1 knockout strain to beta-cypermethrin, chlorantraniliprole and emamectin benzoate were significantly increased compared with the wild-type strain DH19, whereas there were no changes to Bt toxins for Cry1Ab, Cry1Fa and Vip3Aa. Our results revealed that SfABCB1 plays important roles in the susceptibility of S. frugiperda to beta-cypermethrin, chlorantraniliprole and emamectin benzoate, and imply that overexpression of ABCB1 may contribute to beta-cypermethrin, chlorantraniliprole and emamectin benzoate resistance in S. frugiperda.

Why are Drosophila larvae more sensitive to avermectin than adults?

The insects have different physiological and morphological characteristics in various developmental stages. The difference in the characteristics may be related to the different sensitivity of insects to insecticides. In avermectin resistant strain screening assay, we found that the Drosophila larvae displayed a higher sensitivity to the insecticidal effect of avermectin, compared with adults. In this study, we found that the Drosophila larvae have relatively thicker chitin layer, faster avermectin metabolism and lower P-glycoprotein (P-gp) level, when compared with the adults. Besides, the expression levels of the molecular targets of avermectin, glutamate-gated chloride channel and γ-aminobutyric acid (GABA)-gated chloride channel, are lower in the larval stage than the adult. These results suggested that lower P-gp level in the body especially in brain may be the major reason for the higher sensitivity of Drosophila larvae to the insecticide. In summary, these results shed new light on the concept that different developmental stages of insects display different sensitivity to the same insecticide, which also provided a physiological explanation of the relevant mechanism of the difference of sensitivity of insect at its larval and adult stages to insecticide.

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.

Comparative analysis of the detoxification gene inventory of four major Spodoptera pest species in response to xenobiotics

The genus Spodoptera (Lepidoptera: Noctuidae) comprises some of the most polyphagous and destructive agricultural pests worldwide. The success of many species of this genus is due to their striking abilities to adapt to a broad range of host plants. Superfamilies of detoxification genes play a crucial role in the adaption to overcome plant defense mechanisms mediated by numerous secondary metabolites and toxins. Over the past decade, a substantial amount of expression data in Spodoptera larvae was produced for those genes in response to xenobiotics such as plant secondary metabolites, but also insecticide exposure. However, this information is scattered throughout the literature and in most cases does not allow to clearly identify candidate genes involved in host-plant adaptation and insecticide resistance. In the present review, we analyzed and compiled information on close to 600 pairs of inducers (xenobiotics) and induced genes from four main Spodoptera species: S. exigua, S. frugiperda, S. littoralis and S. litura. The cytochrome P450 monooxygenases (P450s; encoded by CYP genes) were the most upregulated detoxification genes across the literature for all four species. Most of the data was provided from studies on S. litura, followed by S. exigua, S. frugiperda and S. littoralis. We examined whether these detoxification genes were reported for larval survival under xenobiotic challenge in forward and reverse genetic studies. We further analyzed whether biochemical assays were carried out showing the ability of corresponding enzymes and transporters to breakdown and excrete xenobiotics, respectively. This revealed a clear disparity between species and the lack of genetic and biochemical information in S. frugiperda. Finally, we discussed the biological importance of detoxification genes for this genus and propose a workflow to study the involvement of these enzymes in an ecological and agricultural context.

Inheritance patterns, cross-resistance and synergism in Spodoptera frugiperda (Lepidoptera: Noctuidae) resistant to emamectin benzoate

BACKGROUND

Fall armyworm, Spodoptera frugiperda (J. E. Smith), is a relevant global pest due to severe damage caused on agricultural crops and its capacity to evolve resistance to insecticides. Here, we selected a strain of S. frugiperda resistant to emamectin benzoate under laboratory conditions to understand the inheritance patterns, cross-resistance and synergism involved in the resistance.

RESULTS

The emamectin benzoate-resistant (Ben-R) strain was isolated by using F₂ screen in a field population collected in Lucas do Rio Verde, Mato Grasso state, Brazil. After ten generations of selection pressure with emamectin benzoate, the estimated LC₅₀ of the Ben-R strain was 678.38 μg a.i. mL^-1 whereas that of the susceptible (Sus) strain was 0.29 μg a.i.mL^-1 , resulting in a resistance ratio (RR) of ~ 2340-fold. The LC₅₀ values of the offspring from reciprocal crosses of Sus and Ben-R strains were 93.37 and 105.32 μg a.i. mL^-1 , suggesting that resistance is an autosomal incompletely dominant trait. The high survival of heterozygous and Ben-R strains (>92%) on non-Bt maize sprayed with the field rate of emamectin benzoate confirmed that resistance is functionally dominant. The minimum number of segregations influencing resistance was 3.55, suggesting a polygenic effect. Low cross-resistance was detected between emamectin benzoate and the insecticides methomyl, chlorpyrifos, lambda-cyhalothrin, spinetoram, indoxacarb and chlorantraniliprole (RR <5.75-fold). There was no effect of synergists piperonyl butoxide, diethyl maleate and S, S, S-tributyl phosphorotrithiotate on the Ben-R strain, suggesting a minor role of metabolic resistance.

CONCLUSIONS

Our results showed a high risk of resistance evolution of S. frugiperda to emamectin benzoate, based on incompletely dominant inheritance. Rotation of insecticides with different modes of action can be one of the resistance management strategies to be implemented to delay the evolution of resistance of S. frugiperda to emamectin benzoate in Brazil.

Transcription factor CncC potentially regulates the expression of multiple detoxification genes that mediate indoxacarb resistance in Spodoptera litura

The xenobiotic transcription factor cap 'n' collar isoform C (CncC) is considered the central regulator of antioxidant and detoxification genes. Previous research indicated that CncC regulates three-phase enzymes responsible for insecticide resistance. In this study, the SlituCncC gene from Spodoptera litura was obtained and characterized. Quantitative polymerase chain reaction (qPCR) analysis showed that SlituCncC was expressed in all developmental stages and tissues, but was highly expressed in 3rd- and 4th-instar larvae, and in the Malpighian tubule, fat body, and midgut. In addition, SlituCncC was up-regulated and more highly induced with indoxacarb treatment in the indoxacarb-resistant strains compared with the susceptible strain. RNA interference-mediated gene silencing of SlituCncC significantly increased mortality of S. litura when exposed to indoxacarb. Furthermore, comparative transcriptome analysis showed that 842 genes were down-regulated and 127 genes were up-regulated in SlituCncC knockdown S. litura. Further analysis indicated that 18 three-phase enzymes were identified in the down-regulated genes, of which seven were associated with indoxacarb resistance in S. litura. qPCR analysis confirmed that expression of six of these seven genes was consistent with RNA sequencing data. All six detoxification genes were induced by indoxacarb, and the expression patterns were similar to that of SlituCncC. Finally, the CncC-Maf binding site was predicted in all six gene promoters. This study indicates that the transcription factor SlituCncC may regulate multiple detoxification genes that mediate indoxacarb resistance in S. litura.

Transcriptome analysis and response of three important detoxifying enzymes to Serratia marcescens Bizio (SM1) in Hyphantria cunea (Drury) (Lepidoptera: Noctuidae)

Hyphantria cunea (Drury) (Lepidoptera: Noctuidae) is a main pest of forest trees. In this study, the effects of Serratia marcescens Bizio (SM1) infection on the transcriptome of H. cunea were studied. The expression of 1068 unigenes in the transcriptome of H. cunea infected by S. marcescens was markedly different from that in the control of H. cunea; 474 genes were upregulated, and 594 genes were downregulated in the former. Among them, 8 cytochrome P450s (CYPs), 5 uridine diphosphate-glycosyltransferases (UGTs) and 3 glutathione S-transferases (GSTs) were significantly differentially expressed. Pathway enrichment analysis indicated that these differentially expressed detoxification enzyme genes were mainly involved in the drug metabolism pathway, glutathione metabolism pathway and ABC transporter pathway. Interestingly, we found that five UGTs were related to oestradiol metabolism in the steroid hormone biosynthesis pathway. Furthermore, the real-time fluorescent quantitative PCR results showed that SM1 could induce the expression of CYPs and UGTs, but inhibit the expression of GSTs. This research will identify the response of important detoxification enzymes to S. marcescens, which will provide a theoretical foundation for the development of new immunosuppressants for H. cunea control. Furthermore, H. cunea was performed transcriptome sequencing to explore the key metabolic pathways, signalling pathways and genes affected by S. marcescens, which will clarify the mechanisms of S. marcescens infection of H. cunea. In addition, this study also explored the relationship between H. cunea and S. marcescens, which will provide a theoretical basis for the biological control of H. cunea by using S. marcescens.

Overexpression of ATP-binding cassette transporters associated with sulfoxaflor resistance in Aphis gossypii glover

BACKGROUND

Sulfoxaflor is a new insecticide for controlling against Aphis gossypii in the field. ATP-binding cassette (ABC) transporters belong to a large superfamily of proteins and play an important role in the detoxification process. However, the potential role of ABC transporters in sulfoxaflor resistance in A. gossypii is unknown.

RESULTS

In this study, an ABC transporter inhibitor, verapamil, dramatically increased the toxicity of sulfoxaflor in the resistant population with a synergistic ratio of 8.55. However, verapamil did not synergize sulfoxaflor toxicity in the susceptible population. The contents of ABC transporters were significantly increased in the Sul-R population. Based on RT-qPCR analysis, 10 of 23 ABC transcripts, ABCA1, ABCA2, ABCB1, ABCB5, ABCD1, ABCG7, ABCG16, ABCG26, ABCG27, and MRP7, were up-regulated in the Sul-R population compared to the Sus population. Meanwhile, inductive effects of ABCA1, ABCD1, ABCG7 and ABCG26 by sulfoxaflor were found in A. gossypii. Furthermore, knockdown of ABCA1 and ABCD1 using RNAi significantly increased the sulfoxaflor sensitivity in Sul-R aphids.

CONCLUSION

These results suggested that ABC transporters, especially the ABCA1 and ABCD1 genes, might be related with sulfoxaflor resistance in A. gossypii. This study will promote further work to validate the functional roles of these ABCs in sulfoxaflor resistance and might be helpful for the management of sulfoxaflor-resistant A. gossypii.

Comparative and functional genomics of the ABC transporter superfamily across arthropods

BACKGROUND

The ATP-binding cassette (ABC) transporter superfamily is comprised predominantly of proteins which directly utilize energy from ATP to move molecules across the plasma membrane. Although they have been the subject of frequent investigation across many taxa, arthropod ABCs have been less well studied. While the manual annotation of ABC transporters has been performed in many arthropods, there has so far been no systematic comparison of the superfamily within this order using the increasing number of sequenced genomes. Furthermore, functional work on these genes is limited.

RESULTS

Here, we developed a standardized pipeline to annotate ABCs from predicted proteomes and used it to perform comparative genomics on ABC families across arthropod lineages. Using Kruskal-Wallis tests and the Computational Analysis of gene Family Evolution (CAFE), we were able to observe significant expansions of the ABC-B full transporters (P-glycoproteins) in Lepidoptera and the ABC-H transporters in Hemiptera. RNA-sequencing of epithelia tissues in the Lepidoptera Helicoverpa armigera showed that the 7 P-glycoprotein paralogues differ substantially in their tissue distribution, suggesting a spatial division of labor. It also seems that functional redundancy is a feature of these transporters as RNAi knockdown showed that most transporters are dispensable with the exception of the highly conserved gene Snu, which is probably due to its role in cuticular formation.

CONCLUSIONS

We have performed an annotation of the ABC superfamily across > 150 arthropod species for which good quality protein annotations exist. Our findings highlight specific expansions of ABC transporter families which suggest evolutionary adaptation. Future work will be able to use this analysis as a resource to provide a better understanding of the ABC superfamily in arthropods.

A Comparative Perspective on Functionally-Related, Intracellular Calcium Channels: The Insect Ryanodine and Inositol 1,4,5-Trisphosphate Receptors

Calcium (Ca²⁺) homeostasis is vital for insect development and metabolism, and the endoplasmic reticulum (ER) is a major intracellular reservoir for Ca²⁺. The inositol 1,4,5- triphosphate receptor (IP₃R) and ryanodine receptor (RyR) are large homotetrameric channels associated with the ER and serve as two major actors in ER-derived Ca²⁺ supply. Most of the knowledge on these receptors derives from mammalian systems that possess three genes for each receptor. These studies have inspired work on synonymous receptors in insects, which encode a single IP₃R and RyR. In the current review, we focus on a fundamental, common question: “why do insect cells possess two Ca²⁺ channel receptors in the ER?”. Through a comparative approach, this review covers the discovery of RyRs and IP₃Rs, examines their structures/functions, the pathways that they interact with, and their potential as target sites in pest control. Although insects RyRs and IP₃Rs share structural similarities, they are phylogenetically distinct, have their own structural organization, regulatory mechanisms, and expression patterns, which explains their functional distinction. Nevertheless, both have great potential as target sites in pest control, with RyRs currently being targeted by commercial insecticide, the diamides.

Genome mapping coupled with CRISPR gene editing reveals a P450 gene confers avermectin resistance in the beet armyworm

The evolution of insecticide resistance represents a global constraint to agricultural production. Because of the extreme genetic diversity found in insects and the large numbers of genes involved in insecticide detoxification, better tools are needed to quickly identify and validate the involvement of putative resistance genes for improved monitoring, management, and countering of field-evolved insecticide resistance. The avermectins, emamectin benzoate (EB) and abamectin are relatively new pesticides with reduced environmental risk that target a wide number of insect pests, including the beet armyworm, Spodoptera exigua, an important global pest of many crops. Unfortunately, field resistance to avermectins recently evolved in the beet armyworm, threatening the sustainable use of this class of insecticides. Here, we report a high-quality chromosome-level assembly of the beet armyworm genome and use bulked segregant analysis (BSA) to identify the locus of avermectin resistance, which mapped on 15-16 Mbp of chromosome 17. Knockout of the CYP9A186 gene that maps within this region by CRISPR/Cas9 gene editing fully restored EB susceptibility, implicating this gene in avermectin resistance. Heterologous expression and in vitro functional assays further confirm that a natural substitution (F116V) found in the substrate recognition site 1 (SRS1) of the CYP9A186 protein results in enhanced metabolism of EB and abamectin. Hence, the combined approach of coupling gene editing with BSA allows for the rapid identification of metabolic resistance genes responsible for insecticide resistance, which is critical for effective monitoring and adaptive management of insecticide resistance.

Resistance in the Genus Spodoptera: Key Insect Detoxification Genes

The genus Spodoptera (Lepidoptera: Noctuidae) includes species that are among the most important crop pests in the world. These polyphagous species are able to feed on many plants, including corn, rice and cotton. In addition to their ability to adapt to toxic compounds produced by plants, they have developed resistance to the chemical insecticides used for their control. One of the main mechanisms developed by insects to become resistant involves detoxification enzymes. In this review, we illustrate some examples of the role of major families of detoxification enzymes such as cytochromes P450, carboxyl/cholinesterases, glutathione S-transferases (GST) and transporters such as ATP-binding cassette (ABC) transporters in insecticide resistance. We compare available data for four species, Spodoptera exigua, S. frugiperda, S. littoralis and S. litura. Molecular mechanisms underlying the involvement of these genes in resistance will be described, including the duplication of the CYP9A cluster, over-expression of GST epsilon or point mutations in acetylcholinesterase and ABCC2. This review is not intended to be exhaustive but to highlight the key roles of certain genes.

Pyrrole-2 carboxamides - A novel class of insect ryanodine receptor activators

The ryanodine receptor (RyR) is an intracellular calcium channel critical to the regulation of insect muscle contraction and the target site of diamide insecticides such as chlorantraniliprole, cyantraniliprole and flubendiamide. To-date, diamides are the only known class of synthetic molecules with high potency against insect RyRs. Target-based screening of an informer library led to discovery of a novel class of RyR activators, pyrrole-2-carboxamides. Efforts to optimize receptor activity resulted in analogs with potency comparable to that of commercial diamides when tested against RyR of the fruit fly, Drosophila melanogaster. Surprisingly, testing of pyrrole-2-carboxamides in whole-insect screens showed poor insecticidal activity, which is partially attributed to differential selectivity among insect receptors and rapid detoxification. Among various lepidopteran species field resistance to diamide insecticides has been well documented and in many cases has been attributed to a single point mutation, G4946E, of the RyR gene. As with diamide insecticides, the G4946E mutation confers greatly reduced sensitivity to pyrrole-2-carboxamides. This, coupled with findings from radioligand binding studies, indicates a shared binding domain between anthranilic diamides and pyrrole-2-carboxamides.

The Essential and Enigmatic Role of ABC Transporters in Bt Resistance of Noctuids and Other Insect Pests of Agriculture

Simple Summary

The insect family, Noctuidae, contains some of the most damaging pests of agriculture, including bollworms, budworms, and armyworms. Transgenic cotton and maize expressing Cry-type insecticidal proteins from Bacillus thuringiensis (Bt) are protected from such pests and greatly reduce the need for chemical insecticides. However, evolution of Bt resistance in the insects threatens the sustainability of this environmentally beneficial pest control strategy. Understanding the interaction between Bt toxins and their targets in the insect midgut is necessary to evaluate the risk of resistance evolution. ABC transporters, which in eukaryotes typically expel small molecules from cells, have recently been proposed as a target for the pore-forming Cry toxins. Here we review the literature surrounding this hypothesis in noctuids and other insects. Appreciation of the critical role of ABC transporters will be useful in discovering counterstrategies to resistance, which is already evolving in some field populations of noctuids and other insects.

Abstract

In the last ten years, ABC transporters have emerged as unexpected yet significant contributors to pest resistance to insecticidal pore-forming proteins from Bacillus thuringiensis (Bt). Evidence includes the presence of mutations in resistant insects, heterologous expression to probe interactions with the three-domain Cry toxins, and CRISPR/Cas9 knockouts. Yet the mechanisms by which ABC transporters facilitate pore formation remain obscure. The three major classes of Cry toxins used in agriculture have been found to target the three major classes of ABC transporters, which requires a mechanistic explanation. Many other families of bacterial pore-forming toxins exhibit conformational changes in their mode of action, which are not yet described for the Cry toxins. Three-dimensional structures of the relevant ABC transporters, the multimeric pore in the membrane, and other proteins that assist in the process are required to test the hypothesis that the ATP-switch mechanism provides a motive force that drives Cry toxins into the membrane. Knowledge of the mechanism of pore insertion will be required to combat the resistance that is now evolving in field populations of insects, including noctuids.

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.

Application of CRISPR/Cas9 System to Reverse ABC-Mediated Multidrug Resistance

Multidrug resistance (MDR) is the main obstacle in cancer chemotherapy. ATP-binding cassette (ABC) transporters can transport a wide range of antitumor drugs out of cells, which is the most common reason in the development of resistance to drugs. Currently, various therapeutic strategies are used to reverse MDR, among which CRISPR/Cas9 gene editing technique is expected to be an effective way. Here, we reviewed the research progress of reversing ABC-mediated drug resistance by CRISPR/Cas9 system.

Plant-Derived Compounds as a Tool for the Control of Gastrointestinal Nematodes: Modulation of Abamectin Pharmacological Action by Carvone

The combination of synthetic anthelmintics and bioactive phytochemicals may be a pharmacological tool for improving nematode control in livestock. Carvone (R-CNE) has shown in vitro activity against gastrointestinal nematodes; however, the anthelmintic effect of bioactive phytochemicals either alone or combined with synthetic drugs has been little explored in vivo. Here, the pharmacological interaction of abamectin (ABM) and R-CNE was assessed in vitro and in vivo. The efficacy of this combination was evaluated in lambs naturally infected with resistant gastrointestinal nematodes. Additionally, the ligand and molecular docking of both molecules to P-glycoprotein (P-gp) was studied in silico. The presence of R-CNE produced a significant (p < 0.05) increase of Rho123 and ABM accumulation in the intestinal explants. After 60 min of incubation, Rho123 incubated with R-CNE had a 67 ± 21% higher concentration (p < 0.01) than when it was incubated alone. In the case of ABM, a significant increase in the intestinal concentrations was observed at 15 and 30 min after incubation with R-CNE. In the in vivo assay, no undesirable effects were observed after the oral administration of R-CNE. The coadministration of the natural compound prolonged ABM absorption in lambs. ABM T_½ absorption was 1.57-fold longer (p < 0.05) in the coadministered group. Concentrations of R-CNE between 420 and 2,593 ng/mL were detected in the bloodstream between 1 and 48 h posttreatment. The in vivo efficacy of ABM against gastrointestinal nematodes increased from 94.9 to 99.8% in the presence of R-CNE, with the lower confidence interval limit being >90%. In vitro/in vivo pharmacoparasitological studies are relevant for the knowledge of the interactions and the efficacy of bioactive natural products combined with synthetic anthelmintics. While ADMET (absorption, distribution, metabolism, excretion, and toxicity) predictions and the molecular docking study showed a good interaction between ABM and P-gp, R-CNE does not appear to modulate this efflux protein. Therefore, the pharmacokinetic–pharmacodynamic effect of R-CNE on ABM should be attributed to its effect on membrane permeability. The development of pharmacology-based information is critical for the design of successful strategies for the parasite control.

Identification and transcriptional response of ATP-binding cassette transporters to chlorantraniliprole in the rice striped stem borer, Chilo suppressalis

BACKGROUND

As the largest transporter gene family in metazoans, ATP-binding cassette (ABC) transporters regulate the efflux of a broad spectrum of substrates from the cytoplasm to the outside of the cell. In arthropods, ABC transporters are involved in phase III of the detoxification process, and play important roles in the metabolism and transport of insecticides.

RESULTS

We identified 54 ABC transporters from the genome and transcriptome of Chilo suppressalis, one of the most damaging pests of rice in China. The identified ABC transporters were classified into eight subfamilies (ABCA to ABCH) based on NCBI BLAST and phylogenetic analysis. Synergism studies showed that treatment with verapamil, a potent inhibitor of ABC transporters, resulted in significantly increased toxicity of chlorantraniliprole against C. suppressalis larvae. Among the 21 tested ABC genes, three ABC transporter genes including CsABCC8, CsABCG1C and CsABCH1 were significantly upregulated after chlorantraniliprole treatment.

CONCLUSION

ABC transporters play important roles in the detoxification and transport of chlorantraniliprole in C. suppressalis. The results from our study provide valuable information on C. suppressalis ABC transporters, and are helpful in understanding the roles of ABC transporters in chlorantraniliprole resistance mechanisms in C. suppressalis and other insect pests. © 2020 Society of Chemical Industry.

Transcriptome analysis of Spodoptera litura reveals the molecular mechanism to pyrethroids resistance

Spodoptera litura is a destructive agricultural pest and has evolved resistance to multiple insecticides, especially pyrethroids. At present, the resistance mechanism to pyrethroids remains unclear. Four field-collected populations, namely CZ, LF, NJ and JD, were identified to have high resistance to pyrethroids comparing to pyrethroid-susceptible population (GX), with resistant ratio ranging from 11.5- to 9123.5-fold. To characterize pyrethroid resistance mechanism, the transcriptomes between two pyrethroid-resistant (LF and NJ) and a pyrethroid-susceptible (GX) populations were compared by RNA-sequencing. Results showed that multiple differentially expressed genes were enriched in metabolism-related GO terms and KEGG pathways. 35 up-regulated metabolism-related unigenes were selected to verify by qRT-PCR and 15 unigenes, including 4 cytochrome P450s (P450s), 5 glutathione S-transferase (GSTs), 1 UDP-glycosyltransferase (UGT), 4 carboxylesterases (COEs) and 1 and ATP-binding cassette transporters (ABC), were all up-regulated in the four pyrethroid-resistant populations. The expression levels of CYP3 and GST3, which were annotated as CYP6A13 and GSTE1, respectively, showed positive correlation with their pyrethroid resistance levels among the four pyrethroid-resistant populations. While the expression levels of CYP5, CYP12, COE4 and ABC5 showed good correlation with their pyrethroid resistance levels in at least three populations. UGT5 had the highest expression level among the tested UGT genes in the four pyrethroid-resistant populations. RNAi mediated silencing of CYP6 increased the cumulative mortality treated by beta cypermethrin and cyhalothrin significantly, while silencing of GST3 increased the cumulative mortality treated by fenvalerate significantly. CYP3, CYP5, CYP6, CYP12, GST3, COE4, UGT5 and ABC5 play important roles in pyrethroid resistance among the four pyrethroid-resistant populations. Our work provides a valuable clue for further study of pyrethroid resistance mechanisms in S. litura.

Down-regulation of lysosomal protein ABCB6 increases gossypol susceptibility in Helicoverpa armigera

Cotton bollworm (Helicoverpa armigera) is the major insect herbivore of cotton plants. As its larvae feed and grow on cotton, H. armigera can likely tolerate gossypol, the main defense metabolite produced by cotton plants, through detoxification and sequestration mechanisms. Recent reports have shown that various P450 monooxygenases and UDP-glycosyltransferases in H. armigera are involved in gossypol detoxification, while the roles of ABC transporters, another gene family widely associated with metabolite detoxification, remain to be elucidated. Here, we show that ingestion of gossypol-infused artificial diet and cotton leaves significantly induced the expression of HaABCB6 in H. armigera larvae. Knockdown and knockout of HaABCB6 increased sensitivity of H. armigera larvae to gossypol. Moreover, HaABCB6-GFP fusion protein was localized on lysosomes in Hi5 cells and its overexpression significantly enhanced gossypol tolerance in vitro. These experimental results strongly support that HaABCB6 plays an important role in gossypol detoxification by H. armigera.

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.

Evaluation of five candidate receptors for three Bt toxins in the beet armyworm using CRISPR-mediated gene knockouts

Insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) can provide safe and effective control of some major pests, but evolution of resistance by pests diminishes these benefits. Better understanding of the genetics and mechanisms of resistance is urgently needed to improve methods for monitoring, managing, and countering pest resistance to Bt toxins. Here we used CRISPR-mediated knockouts to evaluate the role of five genes encoding candidate Bt toxin receptors in Spodoptera exigua (beet armyworm), a devastating pest of vegetable, field and flower crops. We compared susceptibility to Bt toxins Cry1Ac, Cry1Fa, and Cry1Ca between the parent susceptible strain and each of five strains homozygous for the knockout of one of the candidate genes (SeAPN1, SeCad1, SeABCC1, SeABCC2 or SeABCC3). The results from the 15 pairwise comparisons reveal that SeABCC2 has a major role and SeCad1 a minor role in mediating toxicity of Cry1Ac and Cry1Fa. SeABCC2 also has a minor role in toxicity of Cry1Ca. In addition, the results imply little or no role for the other three candidate receptors in toxicity of Cry1Ac or Cry1Fa; or for the four candidate receptors other than SeABCC2 in toxicity of Cry1Ca.

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

BACKGROUND

The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system provides some advantages over other reverse genetic techniques to investigate the causal relationship between insecticide resistance phenotype and candidate gene. Several studies published to date point to the nicotinic acetylcholine receptor (nAChR) α6 subunit strongly associated with spinosyns resistance in insects, including Plutella xylostella. However, reverse genetic verification of the P. xylostella nAChRα6 has not yet been achieved via an in vivo approach.

RESULTS

Here, we successfully constructed a homozygous strain (Pxα6-KO) with a 2-nt deletion mutation of nAChRα6 by CRISPR/Cas9 coupled with non-homologous end joining approach in P. xylostella. The manipulated mutation results in a frame shift in the open reading frame of transcripts, which produces a predicted protein truncated in the TM3-TM4 loop region. When compared to the background strain IPP-S, the knockout strain Pxα6-KO exhibited 229- and 1462-fold resistance to spinosad and spinetoram, respectively, but no or limited (resistance ratios <3-fold) effects on the toxicities of imidacloprid, abamectin, β-cypermethrin, indoxacarb, metaflumizone and chlorantraniliprole. Furthermore, the mode of inheritance of the acquired spinetoram resistance was autosomal recessive and significantly linked with the 2-nt deletion mutation of nAChRα6 in the Pxα6-KO strain.

CONCLUSION

In vivo functional investigation demonstrates the causality of the Pxα6 truncating mutation with high levels of resistance to spinosyns in P. xylostella. Our results suggest the Pxα6-KO strain underlies an autosomal, recessive mode of inheritance for spinetoram resistance, and reinforces the association of this gene to the mode of action of spinosyns. © 2019 Society of Chemical Industry.

A Cross-Sectional Survey of Biosafety Professionals Regarding Genetically Modified Insects

BACKGROUND

Genetic technologies such as gene editing and gene drive create challenges for existing frameworks used to assess risk and make regulatory determinations by governments and institutions. Insect genetic technologies including transgenics, gene editing, and gene drive may be particularly challenging because of the large and increasing number of insect species being genetically modified and the degree of familiarity with these organisms and technologies by biosafety officials charged with making containment decisions.

METHODS

An anonymous online survey of biosafety professionals was distributed to the membership of ABSA International, a global society of biosafety professionals, to investigate their perspectives on their preparedness to meet these new challenges.

RESULTS

Existing guidance used to make containment decisions for nongenetically modified insects was widely seen as adequate, and most respondents thought the available guidance for making containment decisions for genetically modified insects with and without gene drives was inadequate. Most respondents reported having less confidence in their decisions concerning containment of genetically modified insects compared to decisions involving genetically modified microbes, (noninsect) animals, and plants.

CONCLUSIONS

These results reveal a need for additional support for biosafety professionals to improve the quality of and confidence in containment decisions regarding genetically modified insects with and without gene drive. These needs might be addressed by increasing training, updating existing guidance, creating new guidance, and creating a third-party accreditation entity to support institutions. Sixty percent of the respondents said they either would or might use a voluntary third-party accreditation service to support insect containment decisions.

The ABCB Multidrug Resistance Proteins Do Not Contribute to Ivermectin Detoxification in the Colorado Potato Beetle, Leptinotarsa decemlineata (Say)

The Colorado potato beetle, Leptinotarsa decemlineata (Say), is a significant agricultural pest that has developed resistance to many insecticides that are used to control it. Investigating the mechanisms of insecticide detoxification in this pest is important for ensuring its continued control, since they may be contributors to such resistance. Multidrug resistance (MDR) genes that code for the ABCB transmembrane efflux transporters are one potential source of insecticide detoxification activity that have not been thoroughly examined in L. decemlineata. In this study, we annotated the ABCB genes found in the L. decemlineata genome and then characterized the expression profiles across midgut, nerve, and Malpighian tubule tissues of the three full transporters identified. To investigate if these genes are involved in defense against the macrocyclic lactone insecticide ivermectin in this insect, each gene was silenced using RNA interference or MDR protein activity was inhibited using a chemical inhibitor, verapamil, before challenging the insects with a dose of ivermectin. Survival of the insects did not significantly change due to gene silencing or protein inhibition, suggesting that MDR transporters do not significantly contribute to defense against ivermectin in L. decemlineata.

Reduced expression of the P-glycoprotein gene PxABCB1 is linked to resistance to Bacillus thuringiensis Cry1Ac toxin in Plutella xylostella (L.)

BACKGROUND

Rapid evolution of pest resistance has seriously threatened the sustainable use of Bacillus thuringiensis (Bt). The diamondback moth, Plutella xylostella (L.), is the first pest to develop resistance to Bt biopesticides in the open field, which renders it an excellent model to explore the molecular basis of Bt resistance in insects. Our previous midgut transcriptome and RNA-Seq profiles showed that the P-glycoprotein gene PxABCB1 was down-regulated in two Cry1Ac-resistant P. xylostella strains, suggesting its potential involvement in Cry1Ac resistance in P. xylostella.

RESULTS

In this study, the bona fide full-length cDNA sequence of the PxABCB1 gene was cloned and analyzed, and the expression of the PxABCB1 gene was detected in all tissues and developmental stages, with the highest expression in midgut tissue and the female adult stage. Although no consistent non-synonymous mutations were identified between the susceptible and resistant strains, PxABCB1 gene expression was remarkably decreased in all resistant strains, and the association was further validated by Cry1Ac selection in the moderately resistant SZ-R strain. Moreover, knockdown of the PxABCB1 gene expression resulted in significantly reduced larval susceptibility to Cry1Ac toxin in the DBM1Ac-S strain, and decreased expression of the PxABCB1 gene was tightly linked to Cry1Ac resistance in P. xylostella.

CONCLUSION

Our results demonstrated that down-regulation of the PxABCB1 gene is associated with both laboratory-selected and field-evolved Cry1Ac resistance in P. xylostella. This knowledge will be conducive to further elucidating the complicated molecular basis of Bt resistance and developing new insect resistance management tactics. © 2019 Society of Chemical Industry.

Transcriptional response of ATP-binding cassette (ABC) transporters to insecticides in the cotton bollworm, Helicoverpa armigera

When any living organism is frequently exposed to any drugs or toxic substances, they evolve different detoxification mechanism to confront with toxicants during absorption and metabolism. Likewise, the insects have evolved detoxification mechanisms as they are frequently exposed to different toxic secondary plant metabolites and commercial insecticides. ABC transporter superfamily is one of the largest and ubiquitous group of proteins which play an important role in phase III of the detoxification process. However, knowledge about this gene family remains largely unknown. To help fill this gap, we have identified a total of 54 ABC transporters in the Helicoverpa armigera genome which are classified into eight subfamilies (A-H) by phylogenetic analysis. The temporal and spatial expression profiles of these 54 ABC transporters throughout H. armigera development stages and seven tissues and their responses to five different insecticides, were investigated using RNA-seq analysis. Furthermore, the mRNA expression of eight selected genes in different tissues and six genes responses to insecticides were confirmed by the quantitative real-time PCR (RT-qPCR). Moreover, H. armigera become more sensitive to abamectin and indoxacarb when P-gp was inhibited. These results provide a foundation for further studies of ABCs in H. armigera.

Changes in the expression of four ABC transporter genes in response to imidacloprid in Bemisia tabaci Q (Hemiptera: Aleyrodidae)

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), a globally invasive species complex that causes serious damage to field crops, has developed resistance to imidacloprid and many other pesticides. Insect detoxify to pesticides may partially depend on ABC transporters, which contribute to the detoxification of xenobiotics. To determine whether genes in the ABCG subfamily are involved in imidacloprid detoxification in B. tabaci Q, we cloned four ABCG subfamily genes based on the published MED/Q genome and on our previous study of the transcriptional response of ABC transporters in B. tabaci Q adults to imidacloprid. As indicated by the quantification of mRNA levels after a 6-h exposure, the expression level of ABCG3 was 3.3-fold higher in B. tabaci Q adults exposed to 100 μg/mL imidacloprid rather than to the buffer control. The expression level of ABCG3 was higher in females than in males but did not significantly differ among eggs or nymphal stages and did not significantly differ among head, thorax, and abdomen tissues of adults. Knockdown of ABCG3 via RNA interference significantly increased the mortality of imidacloprid-treated laboratory and field-collected adults of B. tabaci Q. These results indicate that the ABCG3 gene may be involved in imidacloprid detoxification by B. tabaci Q.

CRISPR-Cas9: A New Addition to the Drug Metabolism and Disposition Tool Box

Clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR associated protein 9 (Cas9), i.e., CRISPR-Cas9, has been extensively used as a gene-editing technology during recent years. Unlike earlier technologies for gene editing or gene knockdown, such as zinc finger nucleases and RNA interference, CRISPR-Cas9 is comparably easy to use, affordable, and versatile. Recently, CRISPR-Cas9 has been applied in studies of drug absorption, distribution, metabolism, and excretion (ADME) and for ADME model generation. To date, about 50 papers have been published describing in vitro or in vivo CRISPR-Cas9 gene editing of ADME and ADME-related genes. Twenty of these papers describe gene editing of clinically relevant genes, such as ATP-binding cassette drug transporters and cytochrome P450 drug-metabolizing enzymes. With CRISPR-Cas9, the ADME tool box has been substantially expanded. This new technology allows us to develop better and more predictive in vitro and in vivo ADME models and map previously underexplored ADME genes and gene families. In this mini-review, we give an overview of the CRISPR-Cas9 technology and summarize recent applications of CRISPR-Cas9 within the ADME field. We also speculate about future applications of CRISPR-Cas9 in ADME research.