Different Effects of Bacillus thuringiensis Toxin Cry1Ab on Midgut Cell Transmembrane Potential of Mythimna separata and Agrotis ipsilon Larvae

V-ATPase E mediates Cry2Ab binding and toxicity in Helicoverpa armigera

Cry2Ab is one of the important alternative Bt proteins that can be used to manage insect pests resistant to Cry1A toxins and to expand the insecticidal spectrum of pyramided Bt crops. Previous studies have showed that vacuolar H+-ATPase subunits A and B (V-ATPase A and B) may be involved in Bt insecticidal activities. The present study investigated the role of V-ATPases subunit E in the toxicity of Cry2Ab in Helicoverpa amigera. RT-PCR analysis revealed that oral exposure of H. amigera larvae to Cry2Ab led to a significant reduction in the expression of H. armigera V-ATPase E (HaV-ATPase E). Ligand blot, homologous and heterologous competition experiments confirmed that HaV-ATPases E physically and specifically bound to activated Cry2Ab toxin. Heterologous expressing of HaV-ATPase E in Sf9 cells made the cell line more susceptible to Cry2Ab, whereas knockdown of the endogenous V-ATPase E in H. zea midgut cells decreased Cry2Ab's cytotoxicity against this cell line. Further in vivo bioassay showed that H. armigera larvae fed a diet overlaid with both Cry2Ab and E. coli-expressed HaV-ATPase E protein suffered significantly higher mortality than those fed Cry2Ab alone. These results support that V-ATPases E is a putative receptor of Cry2Ab and can be used to improve Cry2Ab toxicity and manage Cry2Ab resistance at least in H. armigera.

Identification and RNAi-based functional analysis of chitinase family genes in Agrotis ipsilon

BACKGROUND

Chitin is a major component in the extracellular matrix of insects, and its metabolism largely affects insect development and molting. As essential degradative enzymes, chitinases are encoded by multiple genes that differ in size, expression pattern and function in insects. However, our limited knowledge on the functions of different chitinases in Agrotis ipsilon has prevented our application of new technologies to target these genes as new pest management strategies.

RESULTS

We revealed 11 full-length complementary DNA sequences of chitinase genes (AiChts) from A. ipsilon transcriptome. Although the domain architecture of these chitinases varied greatly, they all contained at least one chitinase catalytic domain. Developmental stage- and tissue-dependent expression profiles showed that most AiChts had the highest expression in the pupal stage. Furthermore, AiCht2, AiCht6, AiCht7 and AiCht10 were mainly expressed in the integument, whereas AiCht8 and AiCht-h had the highest expression in the midgut. The RNA interference (RNAi) experiment revealed that knockdown of AiCht10 or the imaginal disc growth factor gene (AiIDGF) induced high larval mortality. Larvae failed to shed the old cuticle during molting after the injection of double-stranded RNA targeting AiCht10 (dsAiCht10), whereas the larval bodies shrunk and blackened after the injection of dsRNA targeting AiIDGF (dsAiIDGF).

CONCLUSION

Our results revealed for the first time the important functions of AiCht10 and AiIDGF in A. ipsilon. These genes are essential for larval development, and can potentially serve as new targets for RNAi-based pest management. © 2022 Society of Chemical Industry.

The Masc gene product controls masculinization in the black cutworm, Agrotis ipsilon

Sex determination has been studied in the model lepidopteran species Bombyx mori, but it remains poorly understood in lepidopteran pests. In the present study, we identified and characterized the Masculinizer (Masc) gene in a Noctuidae pest species, Agrotis ipsilon. Sequence analysis revealed that AiMasc encodes a protein of 658 amino acids that has two CCCH-type zinc finger domains and two conserved cysteine residues (Cys-277 and Cys-280). We assessed the masculinizing activity of AiMasc in BmN cells and found that AiMasc induced expression of the male-specific doublesex isoform. Disruption of Masc via clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) in A. ipsilon caused abnormalities in abdominal segments and external genitalia, resulting in male-specific sterility. These results suggest that Masc participates in the process of sex determination in A. ipsilon. Successful identification of sex-determination gene in a pest species may enable the development of novel genetic approaches for pest control.

Transcriptome profiling analysis of the intoxication response in midgut tissue of Agrotis ipsilon larvae to Bacillus thuringiensis Vip3Aa protoxin

Vip insecticidal proteins are produced by Bacillus thuringiensis (Bt) during its vegetative growth phase. In the present study, Vip3Aa11 and Vip3Aa39 proteins were investigated. These two proteins present 39 amino acid differential sites and they shared 95.06% amino acid sequence similarity. They are effective against some Lepidoptera insect larvae. In a previous study, using artificial diet bioassays, we estimated the LC₅₀ of Vip3Aa11 and Vip3Aa39 strains against Agrotis ipsilon larvae were 73.41 μg/mL (with 95% confidence interval of 2.34-11.19) and 5.43 μg/mL (with 95% confidence interval of 43.20-115.03), respectively. To investigate the response of Agrotis ipsilon transcriptome in defending against Vip3Aa11 and Vip3Aa39 toxins, we performed high-throughput RNA-sequencing on cDNA generated from the midguts of Agrotis ipsilon larvae that consumed a control diet (CK-M-A), Vip3Aa11 (Vip3Aa11-M-A) and Vip3Aa39 (Vip3Aa39-M-A) proteins. We generated about 98.87 Gb bases in total on BGISEQ-500 sequencing platform. After assembling all samples together and filtering the abundance, we got 51,887 unigenes, the total length, average length, N50 and GC content of unigenes are 64,523,651 bp, 1243 bp, 2330 bp and 41.81% respectively. We revealed 558 midgut genes differential expressed in Vip3Aa11-M-A and 65 midgut genes differentially expressed in Vip3Aa39-M-A. The differentially expressed genes were enriched for serine proteases and potential Bt Vip toxin midgut receptor genes. Eleven serine proteases related genes and 13 Bt toxin potential receptor genes with differential expression were found. Based on transcriptome profiling, we focused on validation the sensitivity of these two Vip3Aa proteins to trypsin and their binding properties to Agrotis ipsilon midgut BBMV (Brush Border Membrane Vesicles). The results show that the sensitivity of the two proteins to trypsin is similar. Binding experiments revealed that both proteins can bind to Agrotis ipsilon midgut BBMV, and there is a competitive binding between them. This transcriptome dataset provided a comprehensive sequence resource of Agrotis ipsilon and provides a foundation for comparative studies with other species of insects.

Identification of yellow gene family in Agrotis ipsilon and functional analysis of Aiyellow-y by CRISPR/Cas9

The yellow gene family has been identified in several model insects, but yellow genes were poorly identified in non-model insects and the functions of yellow genes are largely unknown. In this study, we identified seven yellow genes in an important agricultural pest Agrotis ipsilon. Each gene encodes a protein containing a major royal jelly domain. Phylogenetic analysis defined these genes as yellow-y, -b, -b2, -c, -d, -e, and -h, respectively. The A. ipsilon yellow genes yellow-b, -b2, and -c were stably expressed in all developmental stages and tissues analyzed, whereas the other four yellow genes had unique expression patterns, suggesting distinct physiological roles of each gene. Using the CRISPR/Cas9 system, we successfully disrupted yellow-y in A. ipsilon and obtained G₀ insects with somatic mutations. Unlike the black of wild-type newly hatched larvae and of adults, the mutants were yellow, although in the pupal stage mutant coloration did not differ from wild-type coloration. This phenotype was inherited by G₁ offspring. The G₀ mutants did not show any growth deficiency compared with control insects; however, a dehydration-like phenotype was observed in newly hatched G₁ larvae from sibling crossed mutants. Our results indicate that A. ipsilon yellow-y gene plays a role in body pigmentation and also might function in waterproofing.

Insight into the Mode of Action of Celangulin V on the Transmembrane Potential of Midgut Cells in Lepidopteran Larvae

Celangulin V (CV) is the main insecticidal constituent of Celastrus angulatus. The V-ATPase H subunit of the midgut cells of lepidopteran larvae is the putative target protein of CV. Here, we compared the effects of CV on the midgut membrane potentials of Mythimna separata and Agrotis ipsilon larvae with those of the Cry1Ab toxin from Bacillus thuringiensis and with those of inactive CV-MIA, a synthetic derivative of CV. We investigated the changes in the apical membrane potentials (V_am) and basolateral membrane potentials (V_bm) of the midguts of sixth-instar larvae force-fed with the test toxins. We also measured the V_am and V_bm of larval midguts that were directly incubated with the test toxins. Similar to the effect of Cry1Ab, the V_am of CV-treated midguts rapidly decayed over time in a dose-dependent manner. By contrast, CV-MIA did not influence V_am. Meanwhile, the V_am of A. ipsilon larval midguts directly incubated with CV decayed less than that of M. separata larval midguts, whereas that of larvae force-fed with CV did not significantly change. Similar to Cry1Ab, CV did not affect the V_bm of isolated midguts. CV significantly inhibited V-ATPase activity in a dose-dependent manner. Therefore, CV initially inhibits V-ATPase in the apical membrane and affects intracellular pH, homeostasis, and nutrient transport mechanisms in lepidopteran midgut cells.

Effects of Periplocoside P from Periploca sepium on the Midgut Transmembrane Potential of Mythimna separata Larvae

Periplocoside P (PSP) isolated from the root bark of Periploca sepium contains a pregnane glycoside skeleton and possesses high insecticidal properties. Preliminary studies indicated that PSP disrupts epithelial functions in the midgut of lepidopteran larvae. In the present study, we examined the effects of PSP on the apical and basolateral membrane voltages, V_a and V_bl, respectively, of cells from (1) midguts isolated from the larvae of the oriental armyworm Mythimna separata that were in vitro incubated with toxins and (2) midguts isolated from M. separata larvae force-fed with PSP. We compared the effects of PSP with the effects of the Bacillus thuringiensis toxin Cry1Ab and inactive periplocoside E (PSE) on the midgut epithelial cells. The results showed that V_a rapidly decreased in the presence of PSP in a time- and dose-dependent manner, similar to the effects of Cry1Ab. By contrast, PSE did not affect the V_a and V_bl. Additionally, PSP did not influence the V_bl. Given these results, we speculate that PSP may modulate transport mechanisms at the apical membrane of the midgut epithelial cells by inhibiting the V-type H⁺ ATPase.