Identification and Validation of ATP-Binding Cassette Transporters Involved in the Detoxification of Abamectin in Rice Stem Borer, Chilo suppressalis

Chromosome-level genome assembly of the medicinal insect Blaps rhynchopetera using Nanopore and Hi-C technologies

Blaps rhynchopetera Fairmaire is a significant medicinal resource in southwestern China. We utilized Nanopore and Hi-C technologies in combination to generate a high-quality, chromosome-level assembly of the B. rhynchopetera genome and described its genetic features. Genome surveys revealed that B. rhynchopetera is a highly heterozygous species. The assembled genome was 379.24 Mb in size, of which 96.03% was assigned to 20 pseudochromosomes. A total of 212.93 Mb of repeat sequences were annotated, and 26,824 protein-coding genes and 837 noncoding RNAs were identified. Phylogenetic analysis indicated the divergence of the ancestors of B. rhynchopetera and its closely related species Tenebrio molitor at about 85.6 million years ago. The colinearity analysis showed that some chromosomes of B. rhynchopetera may have had fission events, and it has a good synteny relationship with Tribolium castaneum. Furthermore, in the enrichment analyses, the gene families related to detoxification and immunity of B. rhynchopetera facilitated the understanding of its environmental adaptations, which will serve as a valuable research resource for pest control strategies and conservation efforts of beneficial insects. This high-quality reference genome will also contribute to the conservation of insect species diversity and genetic resources.

Flavin-Dependent Monooxgenase Confers Resistance to Chlorantraniliprole and Spinetoram in the Rice Stem Borer Chilo suppressalis Walker (Lepidoptera: Crambidae)

Understanding the role of flavin-containing monooxygenases (FMOs) in the genetic mechanisms of insecticide resistance is essential for developing effective management strategies against the rice stem borer, Chilo suppressalis. In this study, we identified five FMO genes in C. suppressalis, examined their expression patterns, and revealed overexpression of FMO3B and FMO3C in field populations resistant to multiple insecticides, including chlorantraniliprole and spinetoram. Functional characterization using transgenic Drosophila indicated that FMO3B and FMO3C do not confer resistance to abamectin or methoxyfenozide but do mediate resistance to chlorantraniliprole and spinetoram. Knockdown of FMO3B and FMO3C increased sensitivity to these insecticides in C. suppressalis. Molecular docking studies indicated direct binding of chlorantraniliprole and spinetoram to these FMOs, underscoring their role in metabolic resistance. These findings indicate that FMOs are key enzymes in the metabolic resistance of C. suppressalis to chlorantraniliprole and spinetoram, enhancing our understanding of insecticide resistance and aiding the development of management strategies.

Functional Analysis of Insecticide Inhibition and Metabolism of Six Glutathione S-Transferases in the Rice Stem Borer, Chilo suppressalis

The glutathione S-transferases (GSTs) are important detoxifying enzymes in insects. Our previous studies found that the susceptibility of Chilo suppressalis to abamectin was significantly increased when the CsGST activity was inhibited by glutathione (GSH) depletory. In this study, the potential detoxification mechanisms of CsGSTs to abamectin were explored. Six CsGSTs of C. suppressalis were expressed in vitro. Enzymatic kinetic parameters including Km and Vmax of recombinant CsGSTs were determined, and results showed that all of the six CsGSTs were catalytically active and displaying glutathione transferase activity. Insecticide inhibitions revealed that a low concentration of abamectin could effectively inhibit the activities of CsGSTs including CsGSTd1, CsGSTe4, CsGSTo2, CsGSTs3, and CsGSTu1. However, the in vitro metabolism assay found that the six CsGSTs could not metabolize abamectin directly. Additionally, the glutathione transferase activity of CsGSTs in C. suppressalis was significantly increased post-treatment with abamectin. Comprehensive analysis of the results in present and our previous studies demonstrated that CsGSTs play an important role in detoxification of abamectin by catalyzing the conjugation of GSH to abamectin in C. suppressalis, and the high binding affinities of CsGSTd1, CsGSTe4, CsGSTo2, CsGSTs3, and CsGSTu1 with abamectin might also suggest the involvement of CsGSTs in detoxification of abamectin via the noncatalytic passive binding and sequestration instead of direct metabolism. These studies are helpful to better understand the detoxification mechanisms of GSTs in insects.

Functional Characterization and Putative Regulatory Mechanism of an RNAi Efficiency-Related Nuclease (REase) in the Fall Armyworm, Spodoptera frugiperda

The lepidopteran-specific RNAi efficiency-related nuclease (REase) has been shown to contribute to double-strand RNA (dsRNA) degradation in several lepidopteran insects. However, little is known about its regulatory mechanism. In this study, we identified and characterized SfREase in Spodoptera frugiperda. The exposure of the third-instar larvae to dsEGFP and high temperature led to the upregulation of SfREase, whereas starvation treatment resulted in the downregulation of SfREase. Further experiments revealed that dsRNA degraded more slowly in the hemolymph or midgut fluid extracted from dsSfREase-injected or dsSfREase-ingested larvae compared with those from dsEGFP-treated larvae, and the recombinant SfREase degraded dsRNA in a concentration-dependent manner. Additionally, the knockdown of SfREase improved RNAi efficiency. Finally, both RNAi and dual-luciferase reporter assay in Sf9 cells revealed that SfREase is negatively regulated by FOXO. These data provide insights into the function and regulatory mechanism of REase and have applied implications for the development of an RNAi-based control strategy of S. frugiperda.

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.

Transcriptomic Analysis Reveals the Detoxification Mechanism of Chilo suppressalis in Response to the Novel Pesticide Cyproflanilide

Chilo suppressalis is one of the most damaging rice pests in China’s rice-growing regions. Chemical pesticides are the primary method for pest control; the excessive use of insecticides has resulted in pesticide resistance. C. suppressalis is highly susceptible to cyproflanilide, a novel pesticide with high efficacy. However, the acute toxicity and detoxification mechanisms remain unclear. We carried out a bioassay experiment with C. suppressalis larvae and found that the LD10, LD30 and LD50 of cyproflanilide for 3rd instar larvae was 1.7 ng/per larvae, 6.62 ng/per larvae and 16.92 ng/per larvae, respectively. Moreover, our field trial results showed that cyproflanilide had a 91.24% control efficiency against C. suppressalis. We investigated the effect of cyproflanilide (LD30) treatment on the transcriptome profiles of C. suppressalis larvae and found that 483 genes were up-regulated and 305 genes were down-regulated in response to cyproflanilide exposure, with significantly higher CYP4G90 and CYP4AU10 expression in the treatment group. The RNA interference knockdown of CYP4G90 and CYP4AU10 increased mortality by 20% and 18%, respectively, compared to the control. Our results indicate that cyproflanilide has effective insecticidal toxicological activity, and that the CYP4G90 and CYP4AU10 genes are involved in detoxification metabolism. These findings provide an insight into the toxicological basis of cyproflanilide and the means to develop efficient resistance management tools for C. suppressalis.

Molecular Characterization of Spodoptera frugiperda Heme Oxygenase and Its Involvement in Susceptibility to Chlorantraniliprole

The mammalian heme oxygenase (HO) plays an important role in cytoprotection against oxidative-stress-induced cell damage; however, functional characterization of insect HO is still limited. In this study, cDNA encoding a HO, named SfHO, was cloned from Spodoptera frugiperda. Analysis of the transcription level and enzymatic activity showed that exposure of the LC₃₀ concentration of chlorantraniliprole to the third instar larvae significantly upregulated both the mRNA level and enzymatic activity of SfHO at 24 h after treatment. Further injection of the HO activator, hemin, into the third instar larvae led to the upregulation of SfHO as well as decreased susceptibility of S. frugiperda to chlorantraniliprole. Consistently, overexpression of SfHO increased the Sf9 cell viability under chlorantraniliprole treatment. Strikingly, both RNAi and the dual-luciferase reporter assay in Sf9 cells revealed that, unlike mammalian HO that is regulated by the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2), SfHO was not subject to the regulation by cap 'n' collar isoform C (CncC), the Nrf2 homologue in insects. These data provide insights into the function and regulatory mechanism of insect HOs and had applied implications for the control of S. frugiperda.

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.