An ABC transporter mutation is correlated with insect resistance to Bacillus thuringiensis Cry1Ac toxin

Structural and functional insights into the Diabrotica virgifera virgifera ATP-binding cassette transporter gene family

BACKGROUND

The western corn rootworm, Diabrotica virgifera virgifera, is a pervasive pest of maize in North America and Europe, which has adapted to current pest management strategies. In advance of an assembled and annotated D. v. virgifera genome, we developed transcriptomic resources to use in identifying candidate genes likely to be involved in the evolution of resistance, starting with members of the ATP-binding cassette (ABC) transporter family.

RESULTS

In this study, 65 putative D. v. virgifera ABC (DvvABC) transporters were identified within a combined transcriptome assembly generated from embryonic, larval, adult male, and adult female RNA-sequence libraries. Phylogenetic analysis placed the deduced amino-acid sequences of the DvvABC transporters into eight subfamilies (A to H). To supplement our sequence data with functional analysis, we identified orthologs of Tribolium castaneum ABC genes which had previously been shown to exhibit overt RNA interference (RNAi) phenotypes. We identified eight such D. v. virgifera genes, and found that they were functionally similar to their T. castaneum counterparts. Interestingly, depletion of DvvABCB_39715 and DvvABCG_3712 transcripts in adult females produced detrimental reproductive and developmental phenotypes, demonstrating the potential of these genes as targets for RNAi-mediated insect control tactics.

CONCLUSIONS

By combining sequence data from four libraries covering three distinct life stages, we have produced a relatively comprehensive de novo transcriptome assembly for D. v. virgifera. Moreover, we have identified 65 members of the ABC transporter family and provided the first insights into the developmental and physiological roles of ABC transporters in this pest species.

Insect Hsp90 Chaperone Assists Bacillus thuringiensis Cry Toxicity by Enhancing Protoxin Binding to the Receptor and by Protecting Protoxin from Gut Protease Degradation

Bacillus thuringiensis took advantage of important insect cellular proteins, such as chaperones, involved in maintaining protein homeostasis, to enhance its insecticidal activity. This constitutes a positive loop where the concentrations of Hsp90 and Hsp70 in the gut lumen are likely to increase as midgut cells burst due to Cry1A pore formation action. Hsp90 protects Cry1A protoxin from degradation and enhances receptor binding, resulting in increased toxicity. The effect of insect chaperones on Cry toxicity could have important biotechnological applications to enhance the toxicity of Cry proteins to insect pests, especially those that show low susceptibility to these toxins.

Bacillus thuringiensis Cry proteins are pore-forming insecticidal toxins with specificity against different crop pests and insect vectors of human diseases. Previous work suggested that the insect host Hsp90 chaperone could be involved in Cry toxin action. Here, we show that the interaction of Cry toxins with insect Hsp90 constitutes a positive loop to enhance the performance of these toxins. Plutella xylostella Hsp90 (PxHsp90) greatly enhanced Cry1Ab or Cry1Ac toxicity when fed together to P. xylostella larvae and also in the less susceptible Spodoptera frugiperda larvae. PxHsp90 bound Cry1Ab and Cry1Ac protoxins in an ATP- and chaperone activity-dependent interaction. The chaperone Hsp90 participates in the correct folding of proteins and may suppress mutations of some client proteins, and we show here that PxHsp90 recovered the toxicity of the Cry1AbG439D protoxin affected in receptor binding, in contrast to the Cry1AbR99E or Cry1AbE129K mutant, affected in oligomerization or membrane insertion, respectively, which showed a slight toxicity improvement. Specifically, PxHsp90 enhanced the binding of Cry1AbG439D protoxin to the cadherin receptor. Furthermore, PxHsp90 protected Cry1A protoxins from degradation by insect midgut proteases. Our data show that PxHsp90 assists Cry1A proteins by enhancing their binding to the receptor and by protecting Cry protoxin from gut protease degradation. Finally, we show that the insect cochaperone protein PxHsp70 also increases the toxicity of Cry1Ac in P. xylostella larvae, in contrast to a bacterial GroEL chaperone, which had a marginal effect, indicating that the use of insect chaperones along with Cry toxins could have important biotechnological applications for the improvement of Cry insecticidal activity, resulting in effective control of insect pests.

The progress in insect cross-resistance among Bacillus thuringiensis toxins

Bt crop pyramids produce two or more Bt proteins active to broaden the spectrum of action and to delay the development of resistance in exposed insect populations. The cross-resistance between Bt toxins is a vital restriction factor for Bt crop pyramids, which may reduce the effect of pyramid strategy. In this review, the status of the cross-resistance among more than 20 Bt toxins that are most commonly used against 13 insect pests was analyzed. The potential mechanisms of cross-resistance are discussed. The corresponding measures, including pyramid RNA interference and Bt toxin, "high dose/refuge," and so on are advised to be taken for adopting the pyramided strategy to delay the Bt evolution of resistance and control the target pest insect.

Knockout of the HaREase Gene Improves the Stability of dsRNA and Increases the Sensitivity of Helicoverpa armigera to Bacillus thuringiensis Toxin

Double-stranded RNA (dsRNA)-induced genes are usually related to RNA interference (RNAi) mechanisms and are involved in immune-related pathways. In a previous study, we found a lepidopteran-specific nuclease gene REase that was up-regulated by dsRNA and that affected RNAi efficiency in Asian corn borer (Ostrinia furnacalis). In this study, to verify the function of REase, the homologous gene HaREase in cotton bollworm (Helicoverpa armigera) was knocked out using CRISPR/Cas9 system. We found that the midgut epithelium structure was apparently not affected in the ΔHaREase mutant [Knock out (KO)]. Transcript sequencing results showed that most of the known insect immune-related genes were up-regulated in KO. When second instar larvae were fed artificial diet with Cry1Ac, a protoxin from Bacillus thuringiensis (Bt), in sublethal doses (2.5 or 4 μg/g), the growth rate of KO was repressed significantly. The dsRNA stability was also enhanced in midgut extraction of KO; however, RNAi efficiency was not obviously improved compared with the wild type (WT). The KO and WT were injected with dsEGFP (Enhanced green fluorescent protein) and subjected to transcriptome sequencing. The results showed that the expression levels of 14 nuclease genes were enhanced in KO after the dsRNA treatment. These findings revealed that HaREase expression level was not only related with dsRNA stability, but also with Bt resistance in cotton bollworm. When HaREase was knocked out, other immune- or nuclease-related genes were enhanced significantly. These results remind us that insect immune system is complex and pest control for cotton bollworm is an arduous task.

The Cadherin Cry1Ac Binding-Region is Necessary for the Cooperative Effect with ABCC2 Transporter Enhancing Insecticidal Activity of Bacillus thuringiensis Cry1Ac Toxin

Bacillus thuringiensis Cry1Ac toxin binds to midgut proteins, as cadherin (CAD) and ABCC2 transporter, to form pores leading to larval death. In cell lines, co-expression of CAD and ABCC2 enhance Cry1Ac toxicity significantly, but the mechanism remains elusive. Here, we show that the expression of Helicoverpa armigera CAD (HaCAD-GFP) in Hi5 cells induces susceptibility to Cry1Ac and enhanced Cry1Ac toxicity when co-expressed with H. armigera ABCC2 (HaABCC2-GFP), since Cry1Ac toxicity increased 735-fold compared to Hi5 cells expressing HaCAD-GFP alone or 28-fold compared to HaABCC2-GFP alone. In contrast, the expression of the Spodoptera litura CAD (SlCAD-GFP) in Hi5 cells did not induce susceptibility to Cry1Ac nor it potentiated Cry1Ac toxicity with HaABCC2-GFP. To identify the CAD regions involved in the enhancement of Cry1Ac toxicity with ABCC2, the different CAD domains were replaced between SlCAD-GFP and HaCad-GFP proteins, and cytotoxicity assays were performed in Hi5 cells in the absence or presence of HaABCC2-GFP. The HaCAD toxin-binding region (TB), specifically the CAD repeat-11, was necessary to enhance Cry1Ac toxicity with ABCC2. We propose that CAD TB is involved in recruiting Cry1Ac to localize it in a good position for its interaction with the ABCC2, resulting in efficient toxin membrane insertion enhancing Cry1Ac toxicity.

Mutation of ABC transporter ABCA2 confers resistance to Bt toxin Cry2Ab in Trichoplusia ni

Insecticidal proteins from Bacillus thuringiensis (Bt) are the primary recombinant proteins expressed in transgenic crops (Bt-crops) to confer insect resistance. Development of resistance to Bt toxins in insect populations threatens the sustainable application of Bt-crops in agriculture. The Bt toxin Cry2Ab is a major insecticidal protein used in current Bt-crops, and resistance to Cry2Ab has been selected in several insects, including the cabbage looper, Trichoplusia ni. In this study, the Cry2Ab resistance gene in T. ni was mapped to Chromosome 17 by genetic linkage analyses using a whole genome resequencing approach, and was then finely mapped using RNA-seq-based bulked segregant analysis (BSA) and amplicon sequencing (AmpSeq)-based fine linkage mapping to a locus containing two genes, ABCA1 and ABCA2. Mutations in ABCA1 and ABCA2 in Cry2Ab resistant T. ni were identified by both genomic DNA and cDNA sequencing. Analysis of the expression of ABCA1 and ABCA2 in T. ni larvae indicated that ABCA2 is abundantly expressed in the larval midgut, but ABCA1 is not a midgut-expressed gene. The mutation in ABCA2 in Cry2Ab resistant T. ni was identified to be an insertion of a transposon Tntransib in ABCA2. For confirmation of ABCA2 as the Cry2Ab-resistance gene, T. ni mutants with frameshift mutations in ABCA1 and ABCA2 were generated by CRISPR/Cas9 mutagenesis. Bioassays of the T. ni mutants with Cry2Ab verified that the mutations of ABCA1 did not change larval susceptibility to Cry2Ab, but the ABCA2 mutants were highly resistant to Cry2Ab. Genetic complementation test of the ABCA2 allele in Cry2Ab resistant T. ni with an ABCA2 mutant generated by CRISPR/Cas9 confirmed that the ABCA2 mutation in the Cry2Ab resistant strain confers the resistance. The results from this study confirmed that ABCA2 is essential for the toxicity of Cry2Ab in T. ni and mutation of ABCA2 confers the resistance to Cry2Ab in the resistant T. ni strain derived from a Bt resistant greenhouse population.

Synthesis and Characterization of Cry2Ab-AVM Bioconjugate: Enhanced Affinity to Binding Proteins and Insecticidal Activity

Bacillus thuringiensis insecticidal proteins (Bt toxins) have been widely used in crops for agricultural pest management and to reduce the use of chemical insecticides. Here, we have engineered Bt toxin Cry2Ab30 and bioconjugated it with 4"-O-succinyl avermectin (AVM) to synthesize Cry2Ab-AVM bioconjugate. It was found that Cry2Ab-AVM showed higher insecticidal activity against Plutella xylostella, up to 154.4 times compared to Cry2Ab30. The binding results showed that Cry2Ab-AVM binds to the cadherin-like binding protein fragments, the 10th and 11th cadherin repeat domains in the P. xylostella cadherin (PxCR_10-11), with a much higher affinity (dissociation equilibrium constant K_D = 3.44 nM) than Cry2Ab30 (K_D = 28.7 nM). Molecular docking suggested that the macrolide lactone group of Cry2Ab-AVM ligand docking into the PxCR_10-11 is a potential mechanism to enhance the binding affinity of Cry2Ab-AVM to PxCR_10-11. These findings offer scope for the engineering of Bt toxins by bioconjugation for improved pest management.

Transcriptome and differentially expressed genes of Busseola fusca (Lepidoptera: Noctuidae) larvae challenged with Cry1Ab toxin

Busseola fusca (Fuller) (Lepidoptera: Noctuidae), a major insect pest of maize in sub-Saharan Africa, has developed high levels of non-recessive resistance to Cry1Ab toxin expressed in genetically modified Bt maize. Multiple resistance mechanisms to various Cry toxins have been identified in Lepidoptera, but no study has yet been done to determine the mechanism of Cry1Ab resistance in B. fusca. Therefore, the larval transcriptome of B. fusca was sequenced, de novo assembled and characterized. Differential expression analysis was performed to compare gene expression profiles of Cry toxin challenged and unchallenged neonate larvae to assess the molecular basis of the defence mechanism employed by this insect. Several genes associated with Cry toxin resistance in other lepidopteran pests were detected in B. fusca. Results suggest that differential expression of metabolic and immune-related genes might explain Cry1Ab toxin defence in this pest (supplemental file). Transcript expression profiles of neonates demonstrated that 33.59% and 60.31% of the 131 differentially expressed genes were upregulated and downregulated in the toxin-challenged neonate larvae, respectively. Transcripts were grouped into two subclusters according to the similarity of their expression patterns. Transcripts in subcluster 1 were moderately upregulated in the toxin-challenged neonate larvae, and, conversely, downregulated in the unchallenged neonate larvae. The solute carrier organic anion transporter, which is involved in insecticide detoxification, was upregulated in the toxin-challenged neonate larvae. Conversely, most of the transcripts in subcluster 2 were moderately downregulated in the toxin-challenged neonate larvae, and upregulated for neonates feeding on non-challenged maize. Four unidentified transcripts were extremely down-regulated in the toxin-challenged neonate larvae, and upregulated in the unchallenged neonate larvae. Further studies are recommended to establish if there is a direct correlation between these differentially expressed genes and the observed resistance. Elucidation of such defence mechanisms is crucial for developing insect resistance management strategies to ensure sustainable use of genetically modified maize in Africa. Nevertheless, this is the first study on gene expression profiles of B. fusca strains challenged with Cry toxin. The transcriptome characterized in this study provides a significant resource base for future studies on B. fusca and contributes to understanding some of the gene regulation and signalling networks involved in the defence of B. fusca against Cry1Ab toxin.

The midgut V-ATPase subunit A gene is associated with toxicity to crystal 2Aa and crystal 1Ca-expressing transgenic rice in Chilo suppressalis

Insecticidal crystal (Cry) proteins produced by the bacterium Bacillus thuringiensis (Bt) are toxic to a diverse range of insects. Transgenic rice expressing Cry1A, Cry2A and Cry1C toxins have been developed that are lethal to Chilo suppressalis, a devastating insect pest of rice in China. Identifying the mechanisms underlying the interactions of Cry toxins with susceptible hosts will improve both our understanding of Cry protein toxicology and long-term efficacy of Bt crops. In this study, we tested the hypothesis that V-ATPase subunit A contributes to the action of Cry1Ab/1Ac, Cry2Aa and Cry1Ca toxins in C. suppressalis. The full-length V-ATPase subunit A transcript was initially cloned from the C. suppressalis larval midgut and then used to generate double-stranded RNA (dsRNA)-producing bacteria. Toxicity assays using transgenic rice lines TT51 (Cry1Ab and Cry1Ac fusion genes), T2A-1 (Cry2Aa), and T1C-19 (Cry1Ca) in conjunction with V-ATPase subunit A dsRNA-treated C. suppressalis larvae revealed significantly reduced larval susceptibility to T2A-1 and T1C-19 transgenic rice, but not to TT51 rice. These results suggest that the V-ATPase subunit A plays a crucial role in mediating Cry2Aa and Cry1Ca toxicity in C. suppressalis. These findings will have significant implications on the development of future resistance management tools.

A long non-coding RNA regulates cadherin transcription and susceptibility to Bt toxin Cry1Ac in pink bollworm, Pectinophora gossypiella

Extensive planting of transgenic crops producing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) has spurred increasingly rapid evolution of resistance in pests. In the pink bollworm, Pectinophora gossypiella, a devastating global pest, resistance to Bt toxin Cry1Ac produced by transgenic cotton is linked with mutations in a gene (PgCad1) encoding a cadherin protein that binds Cry1Ac in the larval midgut. We previously reported a long non-coding RNA (lncRNA) in intron 20 of cadherin alleles associated with both resistance and susceptibility to Cry1Ac. Here we tested the hypothesis that reducing expression of this lncRNA decreases transcription of PgCad1 and susceptibility to Cry1Ac. Quantitative RT-PCR showed that feeding susceptible neonates small interfering RNAs (siRNAs) targeting this lncRNA but not PgCad1 decreased the abundance of transcripts of both the lncRNA and PgCad1. Moreover, neonates fed the siRNAs had lower susceptibility to Cry1Ac. The results imply that the lncRNA increases transcription of PgCad1 and susceptibility of pink bollworm to Cry1Ac. The results suggest that disruption of lncRNA expression could be a novel mechanism of pest resistance to Bt toxins.

Cloning, expression and activity of ATP-binding protein in Bacillus thuringiensis toxicity modulation against Aedes aegypti

Background

Bacillus thuringiensis israelensis (Bti) is a widely used mosquitocidal microbial pesticide due to its high toxicity. ATP-binding proteins (ABP) are prevalently detected in insects and are related to reaction against Bti toxins. However, the function of ABP in mosquito biocontrol is little known, especially in Aedes aegypti. Therefore, this study aimed to clarify the function of ABP in Ae. aegypti against Bti toxin.

Results

Aedes aegypti ABP (GenBank: XM_001661856.2) was cloned, expressed and purified in this study. Far-western blotting and ELISA were also carried out to confirm the interaction between ABP and Cry11Aa. A bioassay of Cry11Aa was performed both in the presence and absence of ABP, which showed that the mortality of Ae. aegypti is increased with an increase in ABP.

Conclusions

Our results suggest that ABP in Ae. aegypti can modulate the toxicity of Cry11Aa toxin to mosquitoes by binding to Bti toxin. This could not only enrich the mechanism of Bt toxin, but also provide more data for the biocontrol of this transmission vector.

Insect ATP-Binding Cassette (ABC) Transporters: Roles in Xenobiotic Detoxification and Bt Insecticidal Activity

ATP-binding cassette (ABC) transporters, a large class of transmembrane proteins, are widely found in organisms and play an important role in the transport of xenobiotics. Insect ABC transporters are involved in insecticide detoxification and Bacillus thuringiensis (Bt) toxin perforation. The complete ABC transporter is composed of two hydrophobic transmembrane domains (TMDs) and two nucleotide binding domains (NBDs). Conformational changes that are needed for their action are mediated by ATP hydrolysis. According to the similarity among their sequences and organization of conserved ATP-binding cassette domains, insect ABC transporters have been divided into eight subfamilies (ABCA–ABCH). This review describes the functions and mechanisms of ABC transporters in insecticide detoxification, plant toxic secondary metabolites transport and insecticidal activity of Bt toxin. With improved understanding of the role and mechanisms of ABC transporter in resistance to insecticides and Bt toxins, we can identify valuable target sites for developing new strategies to control pests and manage resistance and achieve green pest control.

Molecular cloning, characterization, and expression profiling analysis of Cry toxin receptor genes from sugarcane shoot borer Chilo infuscatellus (Snellen)

The sugarcane shoot borer Chilo infuscatellus (Snellen) is known for causing severe damage to sugarcane yield in China. Methods have been developed to control this pest, including Cry toxin pesticide and transgenic Bt plants. In order to investigate the molecular mechanism of the Cry toxin binding process and provide a basis for understanding the insect's resistance mechanism, we used a high throughput sequencing platform to perform a de novo transcriptome assembly across different larval developmental stages and analyzed Cry toxin receptors based on our assembled transcripts. We cloned twelve Cry toxin receptor genes including 1 cadherin (Cad), 7 aminopeptidase-Ns (APNs), 3 alkaline phosphatases (ALPs), and 1 ATP-binding cassette transporter subfamily C2 (ABCC2), and three of them with full length. The sublethal dosage of Cry1Ac toxin was applied to sugarcane shoot borer and identified some Cry toxin receptor genes that were significantly induced after 48 h of exposure. Furthermore, quantitative RT-PCR was conducted to detect the expression profiles of these genes. Our transcriptome sequence data provided a valuable molecular resource for further study and the identified Cry toxin receptor data gave insights for improved research into the mechanism of Bt resistance.

HT-SuperSAGE of the gut tissue of a Vip3Aa-resistant Heliothis virescens (Lepidoptera: Noctuidae) strain provides insights into the basis of resistance

Multitoxin Bt-crops expressing insecticidal toxins with different modes of action, for example, Cry and Vip, are expected to improve resistance management in target pests. While Cry1A resistance has been relatively well characterized in some insect species, this is not the case for Vip3A, for which no mechanism of resistance has yet been identified. Here we applied HT-SuperSAGE to analyze the transcriptome of the gut tissue of tobacco budworm Heliothis virescens (F.) laboratory-selected for Vip3Aa resistance. From a total of 1 324 252 sequence reads, 5 895 126-bp tags were obtained representing 17 751 nonsingleton unique transcripts (UniTags) from genetically similar Vip3Aa-resistant (Vip-Sel) and susceptible control (Vip-Unsel) strains. Differential expression was significant (≥2.5 fold or ≤0.4; P < 0.05) for 1989 sequences (11.2% of total UniTags), where 420 represented overexpressed (OE) and 1569 underexpressed (UE) genes in Vip-Sel. BLASTN searches mapped 419 UniTags to H. virescens sequence contigs, of which, 416 (106 OE and 310 UE) were unambiguously annotated to proteins in NCBI nonredundant protein databases. Gene Ontology distributed 345 of annotated UniTags in 14 functional categories with metabolism (including serine-type hydrolases) and translation/ribosome biogenesis being the most prevalent. A UniTag homologous to a particular member of the REsponse to PAThogen (REPAT) family was found among most overexpressed, while UniTags related to the putative Vip3Aa-binding ribosomal protein S2 (RpS2) were underexpressed. qRT-PCR of a subset of UniTags validated the HT-SuperSAGE data. This study is the first providing lepidopteran gut transcriptome associated with Vip3Aa resistance and a foundation for future attempts to elucidate the resistance mechanism.

Mechanisms of resistance to commercially relevant entomopathogenic bacteria

Bacteria represent the most commercially successful entomopathogenic microbial group, with most commercialized insecticides containing gram-positive bacteria in the Bacillaceae family. Resistance to entomopathogenic bacteria threatens sustainable agriculture, and information on the mechanisms and genes involved is vital to develop management practices aimed at reducing this risk. We provide an integrative summary on mechanisms responsible for resistance to commercialized entomopathogenic bacteria, including information on resistance to transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt crops). The available experimental evidence identifies alterations in binding of insecticidal proteins to receptors in the host as the main mechanism for high levels of resistance to entomopathogenic bacteria.

Comparative Proteomics Analysis Between the Short-Term Stress and Long-Term Adaptation of the Blattella germanica (Blattodea: Blattellidae) in Response to Beta-Cypermethrin

A proteomic method combining two-dimensional polyacrylamide gel electrophoresis and tandem mass spectrometry was used to compare the hemolymph expression profiles of a beta-cypermethrin-resistant Blattella germanica L. strain (R) and a susceptible strain (S) after 24 h of beta-cypermethrin induction. The results showed that there were 42 differentially expressed proteins after induction of the R strain: 4 proteins were upregulated and 38 proteins were downregulated. One hundred one hemolymph proteins were differentially expressed after induction of the S strain: 53 proteins were upregulated and 48 proteins were downregulated. The identified proteins were mainly classified into the following categories: energy metabolism proteins such as arginine kinase and triose phosphate isomerase, detoxification-related proteins such as glutathione S-transferases (GSTs), signal molecule-regulated proteins such as nitric oxide synthase (NOS), and other proteins such as kinetic-related proteins and gene expression-related proteins. Several proteins show significant differences in response to short-term stress and long-term adaptation, and differential expression of these proteins reflects an overall change in cellular structure and metabolism associated with resistance to pyrethroid insecticides. In summary, our research has improved the understanding of the molecular mechanisms of beta-cypermethrin resistance in German cockroaches, which will facilitate the development of rational methods to improve the management of this pest.

CRISPR/Cas9-mediated knockout of both the PxABCC2 and PxABCC3 genes confers high-level resistance to Bacillus thuringiensis Cry1Ac toxin in the diamondback moth, Plutella xylostella (L.)

Rapid evolution of resistance by insect pests severely jeopardizes the sustainable utilization of biopesticides and transgenic crops that produce insecticidal crystal proteins derived from the entomopathogenic bacterium Bacillus thuringiensis (Bt). Recently, high levels of resistance to Bt Cry1 toxins have been reported to be genetically linked to the mutation or down-regulation of ABC transporter subfamily C genes ABCC2 and ABCC3 in seven lepidopteran insects, including Plutella xylostella (L.). To further determine the causal relationship between alterations in the PxABCC2 and PxABCC3 genes and Cry1Ac resistance in P. xylostella, the novel CRISPR/Cas9 genome engineering system was utilized to successfully construct two knockout strains: the ABCC2KO strain is homozygous for a 4-bp deletion in exon 3 of the PxABCC2 gene, and the ABCC3KO strain is homozygous for a 5-bp deletion in exon 3 of the PxABCC3 gene, both of which can produce only truncated ABCC proteins. Bioassay results indicated that high levels of resistance to the Cry1Ac protoxin were observed in both the ABCC2KO (724-fold) and ABCC3KO (413-fold) strains compared to the original susceptible DBM1Ac-S strain. Subsequently, dominance degree and genetic complementation tests demonstrated that Cry1Ac resistance in both the knockout strains was incompletely recessive, and Cry1Ac resistance alleles were located in the classic BtR-1 resistance locus that harbored the PxABCC2 and PxABCC3 genes, similar to the near-isogenic resistant NIL-R strain. Moreover, qualitative toxin binding assays revealed that the binding of the Cry1Ac toxin to midgut brush border membrane vesicles (BBMVs) in both knockout strains was dramatically reduced compared to that in the susceptible DBM1Ac-S strain. In summary, our CRISPR/Cas9-mediated genome editing study presents, for the first time, in vivo reverse genetics evidence for both the ABCC2 and ABCC3 proteins as midgut functional receptors for Bt Cry1 toxins in insects, which provides new insight into the pivotal roles of both the ABCC2 and ABCC3 proteins in the complex molecular mechanism of insect resistance to Bt Cry1 toxins.

The Spodoptera exigua ABCC2 Acts as a Cry1A Receptor Independently of its Nucleotide Binding Domain II

ABC proteins are primary-active transporters that require the binding and hydrolysis of ATP to transport substrates across the membrane. Since the first report of an ABCC2 transporter as receptor of Cry1A toxins, the number of ABC transporters known to be involved in the mode of action of Cry toxins has increased. In Spodoptera exigua, a mutation in the SeABCC2 gene is described as genetically linked to resistance to the Bt-product Xentari^TM. This mutation affects an intracellular domain involved in ATP binding, but not the extracellular loops. We analyzed whether this mutation affects the role of the SeABCC2 as a functional receptor to Cry1A toxins. The results show that Sf21 cells expressing the truncated form of the transporter were susceptible to Cry1A toxins. Moreover, specific Cry1Ac binding was observed in those cells expressing the truncated SeABCC2. Additionally, no differences in the irreversible Cry1Ac binding component (associated with the toxin insertion into the membrane) were observed when tested in Sf21 cells expressing either the full-length or the truncated form of the SeABCC2 transporter. Therefore, our results point out that the partial lack of the nucleotide binding domain II in the truncated transporter does not affect its functionality as a Cry1A receptor.

Differential gene expression in response to eCry3.1Ab ingestion in an unselected and eCry3.1Ab-selected western corn rootworm (Diabrotica virgifera virgifera LeConte) population

Diabrotica virgifera virgifera LeConte, the western corn rootworm (WCR) is one of the most destructive pests in the U.S. Corn Belt. Transgenic maize lines expressing various Cry toxins from Bacillus thuringiensis have been adopted as a management strategy. However, resistance to many Bt toxins has occurred. To investigate the mechanisms of Bt resistance we carried out RNA-seq using Illumina sequencing technology on resistant, eCry3.1Ab-selected and susceptible, unselected, whole WCR neonates which fed on seedling maize with and without eCry3.1Ab for 12 and 24 hours. In a parallel experiment RNA-seq experiments were conducted when only the midgut of neonate WCR was evaluated from the same treatments. After de novo transcriptome assembly we identified differentially expressed genes (DEGs). Results from the assemblies and annotation indicate that WCR neonates from the eCry3.1Ab-selected resistant colony expressed a small number of up and down-regulated genes following Bt intoxication. In contrast, unselected susceptible WCR neonates expressed a large number of up and down-regulated transcripts in response to intoxication. Annotation and pathway analysis of DEGs between susceptible and resistant whole WCR and their midgut tissue revealed genes associated with cell membrane, immune response, detoxification, and potential Bt receptors which are likely related to eCry3.1Ab resistance. This research provides a framework to study the toxicology of Bt toxins and mechanism of resistance in WCR, an economically important coleopteran pest species.

Recent progress on the interaction between insects and Bacillus thuringiensis crops

Extensive use of chemical pesticides poses a great threat to the environment and food safety. The discovery of Bacillus thuringiensis (Bt) toxins with effective insecticidal activity against pests and the development of transgenic technology of plants opened a new era of pest control. Transgenic Bt crops, including maize, cotton and soya bean, have now been produced and commercialized to protect against about 30 major coleopteran and lepidopteran pests, greatly benefiting the environment and the economy. However, with the long-term cultivation of Bt crops, some target pests have gradually developed resistance. Numerous studies have indicated that mutations in genes for toxins activation, toxin-binding and insect immunization are important sources in Bt resistance. An in-depth exploration of the corresponding Bt-resistance mechanisms will aid in the design of new strategies to prevent and control pests. Future research will focus on Bt crops expressing new genes and multiple genes to control a broader range of pests as part of an integrated pest management programme. This article is part of the theme issue 'Biotic signalling sheds light on smart pest management'.

Midgut metabolomic profiling of fall armyworm (Spodoptera frugiperda) with field-evolved resistance to Cry1F corn

Populations of the fall armyworm (Spodoptera frugiperda) have developed resistance to transgenic corn producing the Cry1F insecticidal protein from the bacterium Bacillus thuringiensis (Bt). Resistance in S. frugiperda from Puerto Rico is genetically linked to a mutation in an ATP Binding Cassette subfamily C2 gene (SfABCC2) that results in a truncated, non-functional Cry1F toxin receptor protein. Since ABCC2 proteins are involved in active export of xenobiotics and other metabolites from the cell, we hypothesized that Cry1F-resistant fall armyworm with a non-functional SfABCC2 protein would display altered gut metabolome composition when compared to susceptible insects. Mass spectrometry and multivariate statistical analyses identified 126 unique metabolites from larval guts, of which 7 were found to display statistically significant altered levels between midguts from susceptible and Cry1F-resistant S. frugiperda larvae when feeding on meridic diet. Among these 7 differentially present metabolites, 6 were found to significantly accumulate (1.3-3.5-fold) in midguts from Cry1F-resistant larvae, including nucleosides, asparagine, and carbohydrates such as trehalose 6-phosphate and sedoheptulose 1/7-phosphate. In contrast, metabolomic comparisons of larvae fed on non-transgenic corn identified 5 metabolites with statistically significant altered levels and only 2 of them, 2-isopropylmalate and 3-phosphoserine, that significantly accumulated (2.3- and 3.5-fold, respectively) in midguts from Cry1F-resistant compared to susceptible larvae. These results identify a short list of candidate metabolites that may be transported by SfABCC2 and that may have the potential to be used as resistance markers.

Function and Role of ATP-Binding Cassette Transporters as Receptors for 3D-Cry Toxins

When ABC transporter family C2 (ABCC2) and ABC transporter family B1 (ABCB1) were heterologously expressed in non-susceptible cultured cells, the cells swelled in response to Cry1A and Cry3 toxins, respectively. Consistent with the notion that 3D-Cry toxins form cation-permeable pores, Bombyx mori ABCC2 (BmABCC2) facilitated cation-permeable pore formation by Cry1A when expressed in Xenopus oocytes. Furthermore, BmABCC2 had a high binding affinity (KD) to Cry1Aa of 3.1 × 10-10 M. These findings suggest that ABC transporters, including ABCC2 and ABCB1, are functional receptors for 3D-Cry toxins. In addition, the Cry2 toxins most distant from Cry1A toxins on the phylogenetic tree used ABC transporter A2 as a receptor. These data suggest that 3D-Cry toxins use ABC transporters as receptors. In terms of inducing cell swelling, ABCC2 has greater activity than cadherin-like receptor. The pore opening of ABC transporters was hypothesized to be linked to their receptor function, but this was repudiated by experiments using mutants deficient in export activity. The synergistic relationship between ABCC2 and cadherin-like receptor explains their ability to cause resistance in one species of insect.

Transcriptome and Proteome Alternation With Resistance to Bacillus thuringiensis Cry1Ah Toxin in Ostrinia furnacalis

Background: Asian corn borer (ACB), Ostrinia furnacalis can develop resistance to transgenic Bacillus thuringiensis (Bt) maize expressing Cry1Ah-toxin. However, the mechanisms that regulate the resistance of ACB to Cry1Ah-toxin are unknown.

Objective: In order to understand the molecular basis of the Cry1Ah-toxin resistance in ACB, “omics” analyses were performed to examine the difference between Cry1Ah-resistant (ACB-AhR) and susceptible (ACB-BtS) strains of ACB at both transcriptional and translational levels.

Results: A total of 7,007 differentially expressed genes (DEGs) and 182 differentially expressed proteins (DEPs) were identified between ACB-AhR and ACB-BtS and 90 genes had simultaneous transcription and translation profiles. Down-regulated genes associated with Cry1Ah resistance included aminopeptidase N, ABCC3, DIMBOA-induced cytochrome P450, alkaline phosphatase, glutathione S-transferase, cadherin-like protein, and V-ATPase. Whereas, anti-stress genes, such as heat shock protein 70 and carboxylesterase were up-regulated in ACB-AhR, displaying that a higher proportion of genes/proteins related to resistance was down-regulated compared to up-regulated. The Kyoto encyclopedia of genes and genomes (KEGG) analysis mapped 578 and 29 DEGs and DEPs, to 27 and 10 pathways, respectively (P < 0.05). Furthermore, real-time quantitative (qRT-PCR) results based on relative expression levels of randomly selected genes confirmed the “omics” response.

Conclusion: Despite the previous studies, this is the first combination of a study using RNA-Seq and iTRAQ approaches on Cry1Ah-toxin binding, which led to the identification of longer length of unigenes in ACB. The DEGs and DEPs results are valuable for further clarifying Cry1Ah-mediated resistance.

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.

Going global - genomic insights into insect invasions

The spread of invasive insect pests is becoming an increasing problem for agriculture globally. We discuss a number of invasive insects, already of major economic significance that have recently expanded their range to become truly global threats. These include the noctuid moths Helicoverpa and Spodoptera, whose caterpillars have long been among the worst pests in their native Old and New World habitats, respectively, and the whitefly Bemisia, a major vector of plant virus diseases. Importantly, genomic resources for these species have recently become available, allowing research to move beyond the restrictions imposed by earlier approaches limited to a single or few mitochondrial and nuclear markers, to employ genome-wide genotyping and resequencing protocols. These studies have shown hybridisation within the various species complexes, identified regions under selection in agricultural environments, and enable monitoring of genes important as biosecurity risks through introgression into established populations free of the genes. In all cases studied, global trade has emerged as the probable cause of insect spread, making it ever more important that biosecurity protocols and agencies work with researchers to make the most effective use of emerging genomic resources and tools.

The C-terminal protoxin region of Bacillus thuringiensis Cry1Ab toxin has a functional role in binding to GPI-anchored receptors in the insect midgut

Bacillus thuringiensis Cry toxins are used worldwide for controlling insects. Cry1Ab is produced as a 130-kDa protoxin that is activated by proteolytic removal of an inert 500 amino-acid-long C-terminal region, enabling the activated toxin to bind to insect midgut receptor proteins, leading to its membrane insertion and pore formation. It has been proposed that the C-terminal region is only involved in toxin crystallization, but its role in receptor binding is undefined. Here we show that the C-terminal region of Cry1Ab protoxin provides additional binding sites for alkaline phosphatase (ALP) and aminopeptidase N (APN) insect receptors. ELISA, ligand blot, surface plasmon resonance, and pulldown assays revealed that the Cry1Ab C-terminal region binds to both ALP and APN but not to cadherin. Thus, the C-terminal region provided a higher binding affinity of the protoxin to the gut membrane that correlated with higher toxicity of protoxin than activated toxin. Moreover, Cry1Ab domain II loop 2 or 3 mutations reduced binding of the protoxin to cadherin but not to ALP or APN, supporting the idea that protoxins have additional binding sites. These results imply that two different regions mediate the binding of Cry1Ab protoxin to membrane receptors, one located in domain II-III of the toxin and another in its C-terminal region, suggesting an active role of the C-terminal protoxin fragment in the mode of action of Cry toxins. These results suggest that future manipulations of the C-terminal protoxin region could alter the specificity and increase the toxicity of B. thuringiensis proteins.

Population differentiation between Australian and Chinese Helicoverpa armigera occurs in distinct blocks on the Z-chromosome

Over the last 40 years, many types of population genetic markers have been used to assess the population structure of the pest moth species Helicoverpa armigera. While this species is highly vagile, there is evidence of inter-continental population structure. Here, we examine Z-chromosome molecular markers within and between Chinese and Australian populations. Using 1352 polymorphic sites from 40 Z-linked loci, we compared two Chinese populations of moths separated by 700 km and found virtually no population structure (n = 41 and n = 54, with <1% of variation discriminating between populations). The levels of nucleotide diversity within these populations were consistent with previous estimates from introns in Z-linked genes of Australian samples (π = 0.028 vs. 0.03). Furthermore, all loci surveyed in these Chinese populations showed a skew toward rare variants, with ten loci having a significant Tajima's D statistic, suggesting that this species could have undergone a population expansion. Eight of the 40 loci had been examined in a previous study of Australian moths, of which six revealed very little inter-continental population structure. However, the two markers associated with the Cyp303a1 locus that has previously been proposed to be a target of a selective sweep, exhibited allele structuring between countries. Using a separate dataset of 19 Australian and four Chinese moths, we scanned the molecular variation distributed across the entire Z-chromosome and found distinct blocks of differentiation that include the region containing Cyp303a1. We recommend some of these loci join those associated with insecticide resistance to form a set of genes best suited to analyzing population structure in this global pest.

Transcriptomic Responses to Different Cry1Ac Selection Stresses in Helicoverpa armigera

Helicoverpa armigera can develop resistance to Bacillus thuringiensis (Bt), which threaten the long-term success of Bt crops. In the present study, RNAseq was employed to investigate the midgut genes response to strains with different levels of resistance (LF5, LF10, LF20, LF30, LF60, and LF120) in H. armigera. Results revealed that a series of differentially expressed unigenes (DEGs) were expressed significantly in resistant strains compared with the LF-susceptible strain. Nine trypsin genes, ALP2, were downregulated significantly in all the six resistant strains and further verified by qRT-PCR, indicating that these genes may be used as markers to monitor and manage pest resistance in transgenic crops. Most importantly, the differences in DEG functions in the different resistant strains revealed that different resistance mechanisms may develop during the evolution of resistance. The immune and detoxification processes appear to be associated with the low-level resistance (LF5 strain). Metabolic process-related macromolecules possibly lead to resistance to Cry1Ac in the LF10 and LF20 strains. The DEGs involved in the “proton-transporting V-type ATPase complex” and the “proton-transporting two-sector ATPase complex” were significantly expressed in the LF30 strain, probably causing resistance to Cry1Ac in the LF30 strain. The DEGs involved in binding and iron ion homeostasis appear to lead to high-level resistance in the LF60 and LF120 strains, respectively. The multiple genes and different pathways seem to be involved in Cry1Ac resistance depending on the levels of resistance. Although the mechanisms of resistance are very complex in H. armigera, a main pathway seemingly exists, which contributes to resistance in each level of resistant strain. Altogether, the findings in the current study provide a transcriptome-based foundation for identifying the functional genes involved in Cry1Ac resistance in H. armigera.

Efficacy and Resistance Management Potential of a Modified Vip3C Protein for Control of Spodoptera frugiperda in Maize

A modified Vip3C protein has been developed that has a spectrum of activity that has the potential to be commercially useful for pest control, and shows good efficacy against Spodoptera frugiperda in insect bioassays and field trials. For the first time Vip3A and Vip3C proteins have been compared to Cry1 and Cry2 proteins in a complete set of experiments from insect bioassays to competition binding assays to field trials, and the results of these complementary experiments are in agreement with each other. Binding assays with radiolabelled toxins and brush border membrane vesicles from S. frugiperda and Helicoverpa armigera show that the modified Vip3C protein shares binding sites with Vip3A, and does not share sites with Cry1F or Cry2A. In agreement with the resulting binding site model, Vip3A-resistant insects were also cross-resistant to the modified Vip3C protein. Furthermore, maize plants expressing the modified Vip3C protein, but not those expressing Cry1F protein, were protected against Cry1F-resistant S. frugiperda in field trials.

Bacillus thuringiensis Cry1A toxins exert toxicity by multiple pathways in insects

Adoption of biotech crops engineered to express insecticidal toxins from Bacillus thuringiensis (Bt) has revolutionized insect pest control in agriculture. For continuing effective application and development of the environmentally friendly Bt biotechnology, it is fundamental to understand pathways of toxicity of Bt toxins in insects. In this study, mutations were introduced in the midgut cadherin gene in the cabbage looper, Trichoplusia ni, by CRISPR/Cas9 mutagenesis. T. ni strains with mutations in the genes of two major receptors for Bt toxins, the midgut cadherin and ABCC2, and three Cry1A toxins with shared and differential midgut binding sites were used as an experimental system to dissect the roles of the cadherin and ABCC2 in the pathways of toxicity of Bt toxins. Results from assays of responses of the T. ni strains to different Bt toxins revealed that the cadherin and ABCC2 play independent roles in the mode of action of Cry1A toxins and that Bt toxins exert insecticidal activity through multiple redundant pathways of toxicity in insects. Besides the cadherin and ABCC2 pathways, there exists an additional major pathway of toxicity to be identified for Cry1Aa. The results also confirmed that the toxicity of Cry2Ab involves neither the cadherin nor the ABCC2 protein. The multiple pathway model for Bt toxins clarified from this study provided new insights into the molecular modes of action of Bt toxins and mechanisms of insect resistance to Bt toxins.

Role of Bacillus thuringiensis Cry1A toxins domains in the binding to the ABCC2 receptor from Spodoptera exigua

Cry proteins from Bacillus thuringiensis (Bt) have been used to control insect pests either as formulated sprays or as in Bt-crops. However, field-evolved resistance to Bt proteins is threatening the long-term use of Bt products. The SeABCC2 locus has been genetically linked to resistance to a Bt bioinsecticide (Xentari™) in Spodoptera exigua (a mutation producing a truncated form of the transporter lacking an ATP binding domain was found in the resistant insects). Here, we investigated the role of SeABCC2 in the mode of action of Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ca, and two Cry1A-1Ca hybrids by expressing the receptor in Sf21 and HEK293T cell lines. Cell toxicity assays showed that Sf21 cells expressing SeABCC2 become susceptible to Cry1A proteins. HEK293T cells expressing the transporter were found susceptible to Cry1A proteins but not to Cry1Ca. The results with the Cry1A-1Ca hybrids suggest that domain II from Cry1Ab/c is crucial for the toxicity to Sf21 cells, whereas domain III from Cry1Aa/b is crucial for the toxicity to HEK293T cells. Binding assays showed that the Cry1Ac binding is of high affinity and specific to cells expressing the SeABCC2 transporter. Heterologous competition experiments support a model in which domain II of Cry1Ab/c has a common binding site in the SeABCC2 protein, whereas domain III of Cry1Aa/b binds to a different binding site in the SeABCC2 protein.

A single amino acid polymorphism in ABCC2 loop 1 is responsible for differential toxicity of Bacillus thuringiensis Cry1Ac toxin in different Spodoptera (Noctuidae) species

Bacillus thuringiensis Cry toxins exert their toxicity by forming membrane pores after binding with larval midgut membrane proteins known as receptors. Spodoptera litura and Spodoptera frugiperda belong to the same genus, but S. litura is tolerant to Cry1Ac, while S. frugiperda is susceptible. The mechanism involved in the differential toxicity of Cry1Ac to these insect species is not understood. Amino acid sequences analysis of ABCC2, a well-recognized Cry1Ac receptor, from both species showed high sequence identity. Hi5 insect cells expressing SfABCC2 from S. frugiperda were 65-fold more susceptible than those expressing the SlABCC2 from S. litura. Substitution of fragments, point mutations and deletions between the ABCC2 of the two species revealed that ABCC2 amino acid Q¹²⁵ from SfABCC2 or E¹²⁵ from SlABCC2 was key factor for the differential Cry1Ac toxicity to Hi5 cells expressing these receptors. Consistently with this, cells expressing Helicoverpa armigera HaABCC2^Q122-GFP, were more susceptible to Cry1Ac than cells expressing HaABCC2^E122-GFP mutant. Q¹²⁵ or E¹²⁵ is located in a predicted exposed loop 1 region of ABCC2 indicating that this region could be important for Cry1Ac binding. These findings identified a single amino acid residue located in loop 1 of ABCC2 transporter as responsible for the different levels of susceptibility to Cry1Ac among various lepidopteran species.

Expansions of key protein families in the German cockroach highlight the molecular basis of its remarkable success as a global indoor pest

The German cockroach, Blattella germanica, is a worldwide pest that infests buildings, including homes, restaurants, and hospitals, often living in unsanitary conditions. As a disease vector and producer of allergens, this species has major health and economic impacts on humans. Factors contributing to the success of the German cockroach include its resistance to a broad range of insecticides, immunity to many pathogens, and its ability, as an extreme generalist omnivore, to survive on most food sources. The recently published genome shows that B. germanica has an exceptionally high number of protein coding genes. In this study, we investigate the functions of the 93 significantly expanded gene families with the aim to better understand the success of B. germanica as a major pest despite such inhospitable conditions. We find major expansions in gene families with functions related to the detoxification of insecticides and allelochemicals, defense against pathogens, digestion, sensory perception, and gene regulation. These expansions might have allowed B. germanica to develop multiple resistance mechanisms to insecticides and pathogens, and enabled a broad, flexible diet, thus explaining its success in unsanitary conditions and under recurrent chemical control. The findings and resources presented here provide insights for better understanding molecular mechanisms that will facilitate more effective cockroach control.

Specific Binding Protein ABCC1 Is Associated With Cry2Ab Toxicity in Helicoverpa armigera

A pyramid strategy combining the crystal (Cry) 1A and 2A toxins in Bacillus thuringiensis (Bt) crops are active against many species of insects and nematode larvae. It has been widely used to delay pest adaption to genetically modified plants and broaden the insecticidal spectrum in many countries. Unfortunately, Cry2A can also bind with the specific receptor proteins of Cry1A. ATP-binding cassette (ABC) transporters can interact with Cry1A toxins as receptors in the insect midgut, and ABC transporter mutations result in resistance to Bt proteins. However, there is limited knowledge of the ABC transporters that specifically bind to Cry2Ab. Here, we cloned the ABCC1 gene in Helicoverpa armigera, which expressed at all larval stages and in nine different tissues. Expression levels were particularly high in fifth-instar larvae and Malpighian tubules. The two heterologously expressed HaABCC1 transmembrane domain peptides could specifically bind to Cry2Ab with high affinity levels. Moreover, transfecting HaABCC1 into the Spodoptera frugiperda nine insect cell significantly increased its mortality when exposed to Cry2Ab in vitro, and silencing HaABCC1 in H. armigera by RNA interference significantly reduced the mortality of larvae exposed to Cry2Ab in vivo. Altogether current results suggest that HaABCC1 serves as a functional receptor for Cry2Ab.

Epistasis confers resistance to Bt toxin Cry1Ac in the cotton bollworm

Evolution of resistance by insect pests reduces the benefits of extensively cultivated transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt). Previous work showed that resistance to Bt toxin Cry1Ac, which is produced by transgenic cotton, can be conferred by mutations disrupting a cadherin protein that binds this Bt toxin in the larval midgut. However, the potential for epistatic interactions between the cadherin gene and other genes has received little attention. Here, we report evidence of epistasis conferring resistance to Cry1Ac in the cotton bollworm, Helicoverpa armigera, one of the world's most devastating crop pests. Resistance to Cry1Ac in strain LF256 originated from a field-captured male and was autosomal, recessive, and 220-fold relative to susceptible strain SCD. We conducted complementation tests for allelism by crossing LF256 with a strain in which resistance to Cry1Ac is conferred by a recessive allele at the cadherin locus HaCad. The resulting F1 offspring were resistant, suggesting that resistance to Cry1Ac in LF256 is also conferred by resistance alleles at this locus. However, the HaCad amino acid sequence in LF256 lacked insertions and deletions, and did not differ consistently between LF256 and a susceptible strain. In addition, most of the cadherin alleles in LF256 were not derived from the field-captured male. Moreover, Cry1Ac resistance was not genetically linked with the HaCad locus in LF256. Furthermore, LF256 and the susceptible strain were similar in levels of HaCad transcript, cadherin protein, and binding of Cry1Ac to cadherin. Overall, the results imply that epistasis between HaCad and an unknown second locus in LF256 yielded the observed resistance in the F1 progeny from the complementation test. The observed epistasis has important implications for interpreting results of the F1 screen used widely to monitor and analyze resistance, as well as the potential to accelerate evolution of resistance.

microRNA profiling between Bacillus thuringiensis Cry1Ab-susceptible and -resistant European corn borer, Ostrinia nubilalis (Hübner)

Transgenic maize hybrids that express insecticidal Bacillus thuringiensis (Bt) crystalline (Cry) protein toxins effectively protect against the European corn borer, Ostrinia nubilalis, a devastating maize pest. Field monitoring and laboratory selections have detected varying levels of O. nubilalis resistance to Cry1Ab toxin. MicroRNAs (miRNAs) are short noncoding RNAs that are involved in post-transcriptional gene regulation. Their potential roles in the evolution of Bt resistance, however, remain largely unknown. Sequencing of small RNA libraries from the midgut of Cry1Ab-susceptible and resistant O. nubilalis larvae resulted in the discovery of 277 miRNAs, including 248 conserved and 29 novel. Comparative analyses of miRNA expression profiles between the laboratory strains predicted 26 and nine significantly up- and down-regulated transcripts, respectively, in the midgut of Cry1Ab resistant larvae. Amongst 15 differentially regulated miRNAs examined by quantitative real-time PCR, nine (60%) were validated as cosegregating with Cry1Ab resistance in a backcross progeny. Differentially expressed miRNAs were predicted to affect transcripts involved in cell membrane components with functions in metabolism and binding, and the putative Bt-resistance genes aminopeptidase N and cadherin. These results lay the foundation for future investigation of the potential role of miRNAs in the evolution of Bt resistance.

Alimentary Tract Transcriptome Analysis of the Tea Geometrid, Ectropis oblique (Lepidoptera: Geometridae)

Ectropis oblique Prout (Lepidoptera: Geometridae) is one of the main pests that damages the tea crop in Southeast Asia. To understand the molecular mechanisms of its feeding biology, transcriptomes of the alimentary tract (AT) and of the body minus the AT of E. oblique were successfully sequenced and analyzed in this study. A total of 36,950 unigenes from de novo sequences were assembled. After analysis using six annotation databases (e.g., Gene Ontology, Kyoto Encyclopedia of Genes and Genome, and NCBI nr), a series of putative genes were found for this insect species that were related to digestion, detoxification, the immune system, and Bacillus thuringiensis (Bt) receptors. From this series of genes, 21 were randomly selected to verify the relative expression levels of transcripts using quantitative real-time polymerase chain reaction. These results will provide an invaluable genomic resource for future studies on the molecular mechanisms of E. oblique, which will be useful in developing biological control strategies for this pest.

Mutational disruption of the ABCC2 gene in fall armyworm, Spodoptera frugiperda, confers resistance to the Cry1Fa and Cry1A.105 insecticidal proteins

The use of Bt proteins in crops has revolutionized insect pest management by offering effective season-long control. However, field-evolved resistance to Bt proteins threatens their utility and durability. A recent example is field-evolved resistance to Cry1Fa and Cry1A.105 in fall armyworm (Spodoptera frugiperda). This resistance has been detected in Puerto Rico, mainland USA, and Brazil. A S. frugiperda population with suspected resistance to Cry1Fa was sampled from a maize field in Puerto Rico and used to develop a resistant lab colony. The colony demonstrated resistance to Cry1Fa and partial cross-resistance to Cry1A.105 in diet bioassays. Using genetic crosses and proteomics, we show that this resistance is due to loss-of-function mutations in the ABCC2 gene. We characterize two novel mutant alleles from Puerto Rico. We also find that these alleles are absent in a broad screen of partially resistant Brazilian populations. These findings confirm that ABCC2 is a receptor for Cry1Fa and Cry1A.105 in S. frugiperda, and lay the groundwork for genetically enabled resistance management in this species, with the caution that there may be several distinct ABCC2 resistances alleles in nature.

Isolating, characterising and identifying a Cry1Ac resistance mutation in field populations of Helicoverpa punctigera

Transgenic cotton expressing insecticidal proteins from Bacillus thuringiensis (Bt) has been grown in Australia for over 20 years and resistance remains the biggest threat. The native moth, Helicoverpa punctigera is a significant pest of cotton. A genotype causing resistance to Cry1Ac in H. punctigera was isolated from the field and a homozygous line established. The phenotype is recessive and homozygous individuals possess 113 fold resistance to Cry1Ac. Individuals that carry Cry1Ac resistance genes are rare in Australia with a frequency of 0.033 being detected in field populations. RNAseq, RT-PCR and DNA sequencing reveals a single nucleotide polymorphism at a splice site in the cadherin gene as the causal mutation, resulting in the partial transcription of the intron and a premature stop codon. Analysis of Cry1Ac binding to H. punctigera brush border membrane vesicles showed that it is unaffected by the disrupted cadherin gene. This suggests that the major Cry1Ac target is not cadherin but that this molecule plays a key role in resistance and therefore the mode of action. This work adds to our knowledge of resistance mechanisms in H. punctigera and the growing literature around the role of cadherin in the mode of action of Cry1 type Bt proteins.

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

P-glycoprotein [P-gp or the ATP-binding cassette transporter B1 (ABCB1)] is an important participant in multidrug resistance of cancer cells, yet the precise function of this arthropod transporter is unknown. The aim of this study was to determine the importance of P-gp for susceptibility to insecticides in the beet armyworm (Spodoptera exigua) using clustered regularly interspaced short palindromic repeats/CRISPR-associated 9 (CRISPR/Cas9) gene-editing technology. We cloned an open reading frame (ORF) encoding the S. exigua P-gp protein (SeP-gp) predicted to display structural characteristics common to P-gp and other insect ABCB1 transporters. A knockout line with a frame shift deletion of four nucleotides in the SeP-gp ORF was established using the CRISPR/Cas9 gene-editing system to test its potential role in determining susceptibility to chemical insecticides or insecticidal proteins from the bacterium Bacillus thuringiensis (Bt). Results from comparative bioassays demonstrate that knockout of SeP-gp significantly increases susceptibility of S. exigua by around threefold to abamectin and emamectin benzoate (EB), but not to spinosad, chlorfenapyr, beta-cypermethrin, carbosulfan indoxacarb, chlorpyrifos, phoxim, diafenthiuron, chlorfluazuron, chlorantraniliprole or two Bt toxins (Cry1Ca and Cry1Fa). Our data support an important role for SeP-gp in susceptibility of S. exigua to abamectin and EB and imply that overexpression of SeP-gp may contribute to abamectin and EB resistance in S. exigua.

Cell lines as models for the study of Cry toxins from Bacillus thuringiensis

Cell lines have been use extensively for the study of the mode of action of different pore forming toxins produced by different bacterial species. Bacillus thuringiensis Cry toxins are not the exception and their mechanism of action has been analyzed in different cell lines. Here we review the data obtained with different cell lines, including those that are naturally susceptible to the three domain Cry toxins (3d-Cry) and other non-susceptible cell lines that have been transformed with 3d-Cry toxin binding molecules cloned from the susceptible insects. The effects on Cry toxin action after expressing different insect gut proteins, such as glycosyl-phosphatidyl-inositol (GPI) anchored proteins (like alkaline phosphatase (ALP) aminopeptidase (APN)), or trans-membrane proteins (like cadherin (CAD) or ATP-binding cassette subfamily C member 2 (ABCC2) transporter) in cell lines showed that, with few exceptions, expression of GPI-anchored proteins do not correlated with increased susceptibility to the toxin, while the expression of CAD or ABCC2 proteins correlated with induced susceptibility to Cry toxins in the transformed cells lines. Also, that the co-expression of CAD and ABCC2 transporter induced a synergistic effect in the toxicity of 3d-Cry toxins. Overall the data show that in susceptible cell lines, the 3d-Cry toxins induce pore formation that correlates with toxicity. However, the intracellular responses remain controversial since it was shown that the same 3d-Cry toxin in different cell lines activated different responses such as adenylate cyclase-PKA death response or apoptosis. Parasporins are Cry toxins that are toxic to cancer cell lines that have structural similarities with the insecticidal Cry toxins. They belong to the 3d-Cry toxin or to MTX-like Cry toxin families but also show important differences with the insecticidal Cry proteins. Some parasporins are pore-forming toxins, and some activate apoptosis. In this review we summarized the results of the different studies about the Cry toxins mode of action using cultured cell lines and discuss their relation with the studies performed in insect larvae.

Midgut transcriptomal response of the rice leaffolder, Cnaphalocrocis medinalis (Guenée) to Cry1C toxin

Cnaphalocrocis medinalis (Guenée) is one of the important insect pests in rice field. Bt agents were recommended in the C. medinalis control and Bt rice is bred as a tactic to control this insect. However, the tolerance or resistance of insect to Bt protein is a main threat to the application of Bt protein. In order to investigate the response of C. medinalis transcriptome in defending a Cry1C toxin, high-through RNA-sequencing was carried in the C. medinalis larvae treated with and without Cry1C toxin. A total of 35,586 high-quality unigenes was annotated in the transcriptome of C. medinalis midgut. The comparative analysis identified 6,966 differently expressed unigenes (DEGs) between the two treatments. GO analysis showed that these genes involved in proteolysis and extracellular region. Among these DEGs, carboxylesterase, glutathione S-transferase and P450 were differently expressed in the treated C. medinalis midgut. Furthermore, trypsin, chymotrypsin, and carboxypeptidase were identified in DEGs, and most of them up-regulated. In addition, thirteen ABC transporters were downregulated and three upregulated in Cry1C-treated C. medinalis midgut. Based on the pathway analysis, antigen processing and presentation pathway, and chronic myeloid leukemia pathway were significant in C. medinalis treated with Cry1C toxin. These results indicated that serine protease, detoxification enzymes and ABC transporter, antigen processing and presentation pathway, and chronic myeloid leukemia pathway may involved in the response of C. medinalis to Cry1C toxin. This study provides a transcriptomal foundation for the identification and functional characterization of genes involved in the toxicity of Bt Cry protein against C. medinalis, and provides potential clues to the studies on the tolerance or resistance of an agriculturally important insect pest C. medinalis to Cry1C toxin.

Cry11Aa Interacts with the ATP-Binding Protein from Culex quinquefasciatus To Improve the Toxicity

Cry11Aa displays high toxicity to the larvae of several mosquito species, including Aedes, Culex, and Anopheles. To study its binding characterization against Culex quinquefasciatus, Cry11Aa was purified and western blot results showed that Cry11Aa could bind successfully to the brush border membrane vesicles. To identify Cry11Aa-binding proteins in C. quinquefasciatus, a biotin-based protein pull-down experiment was performed and seven Cry11Aa-binding proteins were isolated from the midgut of C. quinquefasciatus larvae. Analysis of liquid chromatography-tandem mass spectrometry showed that one of the Cry11Aa-binding proteins is the ATP-binding domain 1 family member B. To investigate its binding property and effect on the toxicity of Cry11Aa, western blot, far-western blot, enzyme-linked immunosorbent assay, and bioassays of Cry11Aa in the presence and absence of the recombinant ATP-binding protein were performed. Our results showed that the ATP-binding protein interacted with Cry11Aa and increased the toxicity of Cry11Aa against C. quinquefasciatus. Our study suggests that midgut proteins other than the toxin receptors may modulate the toxicity of Cry toxins against mosquitoes.

Identification of Aminopeptidase-N2 as a Cry2Ab binding protein in Manduca sexta

Bacillus thuringiensis Cry2Ab toxin has been used in combination with Cry1Ac for resistance management on the Bt-cotton that is widely planted worldwide. However, little is known regarding Cry2Ab mode of action. Particularly, there is a gap of knowledge on the identification of insect midgut proteins that bind Cry2Ab and mediate toxicity. In the case of Cry1Ab toxin, a transmembrane cadherin protein and glycosyl-phosphatidylinositol (GPI) anchored proteins like aminopeptidase-N1 (APN1) or alkaline-phosphatase (ALP) from Manduca sexta, have been shown to be important for oligomer formation and insertion into the membrane. Binding competition experiments showed that Cry2Ab toxin does not share binding sites with Cry1Ab toxin in M. sexta brush border membrane vesicles (BBMV). Also, that Cry2Ab shows reduced binding to the Cry1Ab binding molecules cadherin, APN1 or ALP. Finally, ligand blot experiments and protein sequence by LC-MS/MS identified APN2 isoform as a Cry2Ab binding protein. Cloning and expression of APN2 confirmed that APN2 is a Cry2Ab binding protein.

Functional characterization of Aedes aegypti alkaline phosphatase ALP1 involved in the toxicity of Cry toxins from Bacillus thuringiensis subsp. israelensis and jegathesan

Presently three major groups of proteins from Aedes aegypti, cadherin, alkaline phosphatases (ALP) and aminopeptidases N (APN), have been identified as Cry11Aa toxin receptors. To further characterize their role on toxicity, transgenic mosquitoes with silenced Aedes cadherin expression were previously generated and the role of cadherin in mediating the toxicity of four different mosquitocidal toxins (Cry11Aa, Cry11Ba, Cry4Aa and Cry4Ba) was demonstrated. Here, we investigated the role of another reported Cry11Aa receptor, ALP1. As with Aedes cadherin, this protein is localized in the apical cell membrane of distal and proximal gastric caecae and the posterior midgut. We also successfully generated transgenic mosquitoes that knockdowned ALP1 transcript levels using an inducible Aedes heat shock promoter, Hsp70A driving dsALP1RNA. Four different mosquitocidal toxins were used for larval bioassays against this transgenic mosquito. Bioassay results show thatCry11Aa toxicity to these transgenic larvae following a heat shock decreased (4.4 fold) and Cry11Ba toxicity is slightly attenuated. But Cry4Aa and Cry4Ba toxicity to ALP1 silenced larvae is unchanged. Without heat shock, toxicity of all four toxins does not change, suggesting this heat shock promoter is heat-inducible. Notably, transgenic mosquitoes with ALP1 knockdown are about 3.7 times less resistant to Cry11Aa toxin than those with Aedes cadherin knockdown. These results demonstrate that the ALP1 is an important secondary receptor for Cry11Aa and Cry11Ba, but it might not be involved in Cry4Aa and Cry4Ba toxicity.

Bombyx mori ABC transporter C2 structures responsible for the receptor function of Bacillus thuringiensis Cry1Aa toxin

Because Bombyx mori ABC transporter C2 (BmABCC2) has 1000-fold higher potential than B. mori cadherin-like protein as a receptor for Bacillus thuringiensis Cry1Aa toxin (Tanaka et al., 2013), the gate-opening ability of the latent pore under six extracellular loops (ECLs) of BmABCC2 was expected to be the reason for its higher potential (Heckel, 2012). In this study, cell swelling assays in Sf9 cells showed that BmABCC2 mutants lacking substrate-excreting activity retained receptor activity, indicating that the gate-opening activity of BmABCC2 is not responsible for Cry1Aa toxicity. The analysis of 29 BmABCC2 mutants demonstrated that ⁷⁷⁰DYWL⁷⁷³ of ECL 4 comprise a putative binding site to Cry1Aa. This suggests that specific toxicity of Cry1Aa toxin to a restricted range of lepidopteran insects is dependent on conservation and variation in the amino acid residues around ⁷⁷⁰DYWL⁷⁷³ of ECL 4 in the ABCC2.

Identification of Bacillus thuringiensis Cry1AbMod binding-proteins from Spodoptera frugiperda

Bacillus thuringiensis Cry toxins are currently used for pest control in transgenic crops but evolution of resistance by the insect pests threatens the use of this technology. The Cry1AbMod toxin was engineered to lack the alpha helix-1 of the parental Cry1Ab toxin and was shown to counter resistance to Cry1Ab and Cry1Ac toxins in different insect species including the fall armyworm Spodoptera frugiperda. In addition, Cry1AbMod showed enhanced toxicity to Cry1Ab-susceptible S. frugiperda populations. To gain insights into the mechanisms of this Cry1AbMod-enhanced toxicity, we isolated the Cry1AbMod toxin binding proteins from S. frugiperda brush border membrane vesicles (BBMV), which were identified by pull-down assay and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The LC-MS/MS results indicated that Cry1AbMod toxin could bind to four classes of aminopeptidase (N1, N3, N4 y N5) and actin, with the highest amino acid sequence coverage acquired for APN 1 and APN4. In addition to these proteins, we found other proteins not previously described as Cry toxin binding proteins. This is the first report that suggests the interaction between Cry1AbMod and APN in S. frugiperda.

Cry toxin specificities of insect ABCC transporters closely related to lepidopteran ABCC2 transporters

In this study, we examined insect and human ABCC transporters closely related to the lepidopteran ABC transporter C2 (ABCC2), a powerful receptor for the Bacillus thuringiensis Cry toxin, for their responses to various Cry toxins. ABCC2 and the lepidopteran ABC transporter C3 (ABCC3) conferred cultured cells with susceptibility to a lepidopteran-specific Cry1Aa toxin but not to lepidopteran-specific Cry1Ca and Cry1Da. One coleopteran ABCC transporter specifically responded to a coleopteran-specific Cry8Ca toxin. ABCC transporters from a dipteran insect and humans did not respond to any of the tested Cry toxins that are active to lepidopteran and coleopteran insects. These results yield important information for our understanding of insect specificity of Cry toxins and provide the first demonstration of a coleopteran ABCC transporter that serves as a Cry toxin receptor.

Using phage display technology to obtain Crybodies active against non-target insects

The insecticidal Cry toxins produced by Bacillus thuringiensis (Bt) are increasingly important in the biological control of insect pests and vectors of human disease. Markets for Bt products and transgenic plants expressing their toxins are driven by their specificity, safety and the move away from chemical control agents. However, the high specificity of Cry toxins can also prove to be a limitation when there is no known Cry toxin active against a particular target. Novel activities can be discovered by screening natural Bt isolates or through modifications of the Cry proteins. Here we demonstrate the use of λ-phage displaying Cry1Aa13 toxin variants modified in domain II loop 2 (Crybodies) to select retargeted toxins. Through biopanning using gut tissue from larvae of the non-target insect Aedes aegypti, we isolated a number of phage for further testing. Two of the overexpressed Cry toxin variants showed significant activity against A. aegypti larvae while another induced mortality at the pupal stage. We present the first report of the use of phage display to identify novel activities toward insects from distant taxonomic Orders and establish this technology based on the use of Crybodies as a powerful tool for developing tailor-made insecticides against new target insects.