Intestinal regeneration as an insect resistance mechanism to entomopathogenic bacteria

Disruption of HaVipR1 confers Vip3Aa resistance in the moth crop pest Helicoverpa armigera

The global reliance on Bacillus thuringiensis (Bt) proteins for controlling lepidopteran pests in cotton, corn, and soybean crops underscores the critical need to understand resistance mechanisms. Vip3Aa, one of the most widely deployed and currently effective Bt proteins in genetically modified crops, plays a pivotal role in pest management. This study investigates the molecular basis of Vip3Aa resistance in Australian Helicoverpa armigera through genetic crosses, and integrated genomic and transcriptomic analyses. We identified a previously uncharacterized gene, LOC110373801 (designated HaVipR1), as potentially important in Vip3Aa resistance in two field-derived resistant lines. Functional validation using CRISPR/Cas9 knockout in susceptible lines confirmed the gene's role in conferring high-level resistance to Vip3Aa. Despite extensive laboratory selection of Vip3Aa-resistant colonies in Lepidoptera, the biochemical mechanisms underlying resistance have remained elusive. Our research identifies HaVipR1 as a potential contributor to resistance, adding to our understanding of how insects may develop resistance to this important Bt protein. The identification of HaVipR1 contributes to our understanding of potential resistance mechanisms and may inform future resistance management strategies. Future work should explore the biochemical pathways influenced by HaVipR1 and assess its interactions with other resistance mechanisms. The approach utilized here underscores the value of field-derived resistant lines for understanding resistance in agricultural pests and highlights the need for targeted approaches to manage resistance sustainably.

Potential of Lambda-Cyhalothrin for Controlling the Black Armyworm Spodoptera cosmioides (Walker) (Lepidoptera: Noctuidae): Toxicity and Midgut Histopathological Effects

Caterpillars of the genus Spodoptera are the main pests in soybean and cotton crops and Spodoptera cosmioides causes more severe losses than other caterpillars in these agricultural crops. However, there are few recommended insecticides for controlling this pest. Lambda-cyhalothrin is a pyrethroid used to control a wide spectrum of arthropods including lepidopterans. Therefore, the objective of this study was to evaluate the potential of lambda-cyhalothrin for the control of S. cosmioides. Specifically, toxicity and histopathological changes in the midgut were evaluated. The effectiveness of the insecticide was determined by estimating the different lethal concentrations (LCs) ​​in the laboratory upon S. cosmioides. Lambda-cyhalothrin was found toxic to S. cosmioides (LC50 = 23.03 mg L-1 and LC90 = 174.8 mg L-1), with the survival reduced from 83.33% in the control to 37.89%, 16.66%, 0%, and 0% after 72 h of exposure to the LC25, LC50, LC75, and LC90 of lambda-cyhalothrin, respectively. Histopathological studies revealed that lambda-cyhalothrin caused damage to midgut cells, including epithelial disorganization, increased cytoplasmic vacuolization, brush border degeneration, nuclear chromatin condensation, and cell fragmentation, indicating cell death by apoptosis. It was concluded that lambda-cyhalothrin, a neurotoxic insecticide, caused damage to the midgut of S. cosmioides, compromising its physiology and indicating that it has potential to be used to control this pest.

Effect of partial and total replacement of fishmeal by soybean meal in feed on growth and gut performance of Penaeus vannamei

This study evaluated the growth and gut performance of shrimp fed three isonitrogenous diets (37% crude protein) with varying inclusions of fish meal (FM) and soybean meal (SBM): F1 (27.5% FM), F2 (10% FM + 23.5% SBM), and F3 (38% SBM). Over a 28-day period, feed intake, feed conversion ratio (FCR), and survival rates showed no significant differences among the groups. However, shrimp fed F2 and F3 exhibited significantly higher weight gain and average daily growth (ADG) compared to those fed F1 (P < 0.05). Gut performance analysis revealed that F3 consistently had the highest gut passage time (GPT), while F1 had the lowest. By day 28, shrimp fed F2 displayed elevated gut retention time (GRT). F1-fed shrimp showed a high gut passage rate (GPR), whereas F3-fed shrimp had a low GPR until day 21, with differences becoming negligible by day 28. Histological examination of the hepatopancreas revealed an increased R-cell population in shrimp fed F3. These findings highlight the adaptability of shrimp to different dietary compositions and underscore the importance of considering multiple factors when assessing the impacts of feed on growth and physiology.

Knockdown of an ATP-binding cassette transporter in resistant western corn rootworm larvae partially reverses resistance to eCry3.1Ab protein

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, has evolved resistance to nearly every management tactic utilized in the field. This study investigated the resistance mechanisms in a WCR strain resistant to the Bacillus thuringiensis (Bt) protein eCry3.1Ab using dsRNA to knockdown WCR midgut genes previously documented to be associated with the resistance. ATP-binding cassette transporter (ABCC4), aminopeptidase-N, cadherin, and cathepsin-B were previously found to be differentially expressed in eCry3.1Ab-resistant WCR larvae when compared to susceptible larvae after feeding on maize expressing eCry3.1Ab and its near-isoline. Here we compared the susceptibility of resistant and susceptible WCR larvae to eCry3.1Ab protein in presence or absence of dsRNA targeting the above genes using 10-day diet overlay toxicity assays. Combining ABCC4 dsRNA with eCry3.1Ab protein increased susceptibility to Bt protein in WCR-resistant larvae, but the other three genes had no such effect. Among 65 ABC transport genes identified, several were expressed differently in resistant or susceptible WCR larvae, fed on eCry3.1Ab-expressing maize versus its isoline, that may be involved in Bt resistance. Our findings provide strong evidence that ABCC4 is indirectly involved in WCR resistance to eCry3.1Ab protein by enhancing the effects of Bt-induced toxicity.

Single-nucleus RNA sequencing reveals midgut cellular heterogeneity and transcriptional profiles in Bombyx mori cytoplasmic polyhedrosis virus infection

The gut is not only used by insects as an organ for the digestion of food and absorption of nutrients but also as an important barrier against the invasion and proliferation of pathogenic microorganisms. Bombyx mori cytoplasmic polyhedrosis virus (BmCPV), an insect-specific virus, predominantly colonizes the midgut epithelial cells of the silkworm, thereby jeopardizing its normal growth. However, there is limited knowledge of the cellular immune responses to viral infection and whether the infection is promoted or inhibited by different types of cells in the silkworm midgut. In this study, we used single-nucleus RNA sequencing to identify representative enteroendocrine cells, enterocytes, and muscle cell types in the silkworm midgut. In addition, by analyzing the transcriptional profiles of various subpopulations in the infected and uninfected groups, we found that BmCPV infection suppresses the response of the antiviral pathways and induces the expression of BmHSP70, which plays a role in promoting BmCPV replication. However, certain immune genes in the midgut of the silkworm, such as BmLebocin3, were induced upon viral infection, and downregulation of BmLEB3 using RNA interference promoted BmCPV replication in the midgut of B. mori. These results suggest that viral immune evasion and active host resistance coexist in BmCPV-infected silkworms. We reveal the richness of cellular diversity in the midgut of B. mori larvae by single-nucleus RNA sequencing analysis and provide new insights into the complex interactions between the host and the virus at the single-cell level.

Insights into midgut cell types and their crucial role in antiviral immunity in the lepidopteran model Bombyx mori

The midgut, a vital component of the digestive system in arthropods, serves as an interface between ingested food and the insect's physiology, playing a pivotal role in nutrient absorption and immune defense mechanisms. Distinct cell types, including columnar, enteroendocrine, goblet and regenerative cells, comprise the midgut in insects and contribute to its robust immune response. Enterocytes/columnar cells, the primary absorptive cells, facilitate the immune response through enzyme secretions, while regenerative cells play a crucial role in maintaining midgut integrity by continuously replenishing damaged cells and maintaining the continuity of the immune defense. The peritrophic membrane is vital to the insect's innate immunity, shielding the midgut from pathogens and abrasive food particles. Midgut juice, a mixture of digestive enzymes and antimicrobial factors, further contributes to the insect's immune defense, helping the insect to combat invading pathogens and regulate the midgut microbial community. The cutting-edge single-cell transcriptomics also unveiled previously unrecognized subpopulations within the insect midgut cells and elucidated the striking similarities between the gastrointestinal tracts of insects and higher mammals. Understanding the intricate interplay between midgut cell types provides valuable insights into insect immunity. This review provides a solid foundation for unraveling the complex roles of the midgut, not only in digestion but also in immunity. Moreover, this review will discuss the novel immune strategies led by the midgut employed by insects to combat invading pathogens, ultimately contributing to the broader understanding of insect physiology and defense mechanisms.

JAK/STAT signaling regulated intestinal regeneration defends insect pests against pore-forming toxins produced by Bacillus thuringiensis

A variety of coordinated host-cell responses are activated as defense mechanisms against pore-forming toxins (PFTs). Bacillus thuringiensis (Bt) is a worldwide used biopesticide whose efficacy and precise application methods limits its use to replace synthetic pesticides in agricultural settings. Here, we analyzed the intestinal defense mechanisms of two lepidopteran insect pests after intoxication with sublethal dose of Bt PFTs to find out potential functional genes. We show that larval intestinal epithelium was initially damaged by the PFTs and that larval survival was observed after intestinal epithelium regeneration. Further analyses showed that the intestinal regeneration caused by Cry9A protein is regulated through c-Jun NH (2) terminal kinase (JNK) and Janus tyrosine kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathways. JAK/STAT signaling regulates intestinal regeneration through proliferation and differentiation of intestinal stem cells to defend three different Bt proteins including Cry9A, Cry1F or Vip3A in both insect pests, Chilo suppressalis and Spodoptera frugiperda. Consequently, a nano-biopesticide was designed to improve pesticidal efficacy based on the combination of Stat double stranded RNA (dsRNA)-nanoparticles and Bt strain. This formulation controlled insect pests with better effect suggesting its potential use to reduce the use of synthetic pesticides in agricultural settings for pest control.

Mechanisms of feeding cessation in Helicoverpa armigera larvae exposed to Bacillus thuringiensis Cry1Ac toxin

Insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have been applied in sprayable formulations and expressed in transgenic crops for the control of pests in the field. When exposed to Bt proteins insect larvae display feeding cessation, yet the mechanism for this phenomenon remains unknown. In this study, we investigated the feeding behavior and underlying mechanisms of cotton bollworm (Helicoverpa armigera) larvae after exposure to the Cry1Ac protein from Bt. Three H. armigera strains were studied: the susceptible SCD strain, the C2/3-KO strain with HaABCC2 and HaABCC3 knocked out and high-level resistance to Cry1Ac (>15,000-fold), and the SCD-KI strain with a T92C point mutation in tetraspanin (HaTSPAN1) and medium-level resistance to Cry1Ac (125-fold). When determining the percentage of insects that continued feeding after various exposure times to Cry1Ac, we observed quick cessation of feeding in larvae from the susceptible SCD strain, whereas larvae from the C2/3-KO strain did not display feeding cessation. In contrast, larvae from the SCD-KI strain rapidly recovered from the initial feeding cessation. Histopathological analyses and qRT-PCR in midguts of SCD larvae after Cry1Ac exposure detected serious epithelial damage and significantly reduced expression of the neuropeptide F gene (NPF) and its potential receptor gene NPFR, which are reported to promote insect feeding. Neither epithelial damage nor altered NPF and NPFR expression appeared in midguts of C2/3-KO larvae after Cry1Ac treatment. The same treatment in SCD-KI larvae resulted in milder epithelial damage and subsequent repair, and a decrease followed by an initial increase in NPF and NPFR expression. These results demonstrate that the feeding cessation response to Cry1Ac in cotton bollworm larvae is closely associated with midgut epithelial damage and downregulation of NPF and NPFR expression. This information provides clues to the mechanism of feeding cessation in response to Bt intoxication and contributes to the mode of action of the Cry1Ac toxin in target pests.

Bacillus thuringiensis toxins divert progenitor cells toward enteroendocrine fate by decreasing cell adhesion with intestinal stem cells in Drosophila

Bacillus thuringiensis subsp. kurstaki (Btk) is a strong pathogen toward lepidopteran larvae thanks to specific Cry toxins causing leaky gut phenotypes. Hence, Btk and its toxins are used worldwide as microbial insecticide and in genetically modified crops, respectively, to fight crop pests. However, Btk belongs to the B. cereus group, some strains of which are well known human opportunistic pathogens. Therefore, ingestion of Btk along with food may threaten organisms not susceptible to Btk infection. Here we show that Cry1A toxins induce enterocyte death and intestinal stem cell (ISC) proliferation in the midgut of Drosophila melanogaster, an organism non-susceptible to Btk. Surprisingly, a high proportion of the ISC daughter cells differentiate into enteroendocrine cells instead of their initial enterocyte destiny. We show that Cry1A toxins weaken the E-Cadherin-dependent adherens junction between the ISC and its immediate daughter progenitor, leading the latter to adopt an enteroendocrine fate. Hence, although not lethal to non-susceptible organisms, Cry toxins can interfere with conserved cell adhesion mechanisms, thereby disrupting intestinal homeostasis and endocrine functions.

eCry3.1Ab-resistant Western Corn Rootworm Larval Midgut Epithelia Respond Minimally to Bt Intoxication

Insect resistance to toxins derived from Bacillus thuringiensis (Bt) is a major issue in agriculture. Resistance to Bt has been linked to the loss of toxin binding sites within the insect, changes within the gut microbiota, and midgut tissue regeneration. Histopathological documentation of intoxication and resistance to Bt is lacking for rootworms in the genus Diabrotica (Coleoptera: Chrysomelidae), a major target of Bt corn. Here, we document the morphological response of both Bt-resistant and Bt-susceptible larval western corn rootworm, Diabrotica virgifera virgifera LeConte, to intoxication with eCry3.1Ab. Gut lumen structural differences are subtle between the two colonies when feeding on non-Bt corn. However, upon ingestion of Bt-corn roots, susceptible larvae develop symptoms indicative of gut disruption by Bt, whereas resistant larvae incur milder effects. Mild disruption of the peritrophic matrix and gut lumen is accompanied by stem cell proliferation that may lead to midgut tissue regeneration. These results help contextualize the multifaceted nature of Bt-resistance in western corn rootworm for the first time from a histopathological perspective.

Bt Cry1Ab/2Ab toxins disrupt the structure of the gut bacterial community of Locusta migratoria through host immune responses

The gut microbiota of insects plays a vital role in digestion, nutrient acquisition, metabolism of dietary toxins, pathogen immunity and maintenance of gut homeostasis. Bacillus thuringinensis (Bt) poisons target insects through its toxins that are activated in the insect gut. The effects of Bt toxins on gut microbiota of insects and their underlying mechanisms are not well understood. In this study, we found that Cry1Ab/2Ab toxins significantly changed the gut bacterial community's structure and reduced the total load of gut bacteria in the Locusta migratoria. In addition, Cry toxins significantly increased the level of reactive oxygen species (ROS) in the gut of locusts. Our results also showed that Cry1Ab/2Ab toxins induced the host gut's immune response by up-regulating of key genes in the Immune deficiency (IMD) and Toll pathway. RNA interference showed that knocking down Relish could narrow the difference in the load, diversity, and composition in gut bacteria caused by Cry toxins. Our findings suggest that Bt potentially influences the gut bacterial community of L. migratoria through host immune response.

The Effect of Silicon Dioxide Nanoparticles Combined with Entomopathogenic Bacteria or Fungus on the Survival of Colorado Potato Beetle and Cabbage Beetles

Three types of modified silicon dioxide nanoparticles (SiO2, 10−20 nm) with additives of epoxy, silane and amino groups, used independently and in combination with the entomopathogenic bacteria Bacillus thuringiensis subsp. morrisoni and fungus Metarhizium robertsii were tested against Colorado potato beetle (Leptinotarsa decemlineata) and cabbage beetles (Phyllotreta spp.). All three nanoparticles were found to have an entomocidal effect on Colorado potato beetle larvae and crucifer flea beetles when ingested. Increased susceptibility of insects to B. thuringiensis or M. robertsii blastospores and their metabolites was shown after exposure to the modified silicon dioxide nanoparticles. The potential of modified silicon dioxide nanoparticles to enhance the efficiency of biopesticides based on the bacteria B. thuringiensis and fungi M. robertsii is considered in the paper.

Insect Gut Regeneration

In adult insects, as in vertebrates, the gut epithelium is a highly regenerative tissue that can renew itself rapidly in response to changing inputs from nutrition, the gut microbiota, ingested toxins, and signals from other organs. Because of its cellular and genetic similarities to the mammalian intestine, and its relevance as a target for the control of insect pests and disease vectors, many researchers have used insect intestines to address fundamental questions about stem cell functions during tissue maintenance and regeneration. In Drosophila, where most of the experimental work has been performed, not only are intestinal cell types and behaviors well characterized, but numerous cell signaling interactions have been detailed that mediate gut epithelial regeneration. A prevailing model for regenerative responses in the insect gut invokes stress sensing by damaged enterocytes (ECs) as a principal source for signaling that activates the division of intestinal stem cells (ISCs) and the growth and differentiation of their progeny. However, extant data also reveal alternative mechanisms for regeneration that involve ISC-intrinsic functions, active culling of healthy epithelial cells, enhanced EC growth, and even cytoplasmic shedding by infected ECs. This article reviews current knowledge of the molecular mechanisms involved in gut regeneration in several insect models (Drosophila and Aedes of the order Diptera, and several Lepidoptera).

Bacillus thuringiensis Spores and Cry3A Toxins Act Synergistically to Expedite Colorado Potato Beetle Mortality

The insect integument (exoskeleton) is an effective physiochemical barrier that limits disease-causing agents to a few portals of entry, including the gastrointestinal and reproductive tracts. The bacterial biopesticide Bacillus thuringiensis (Bt) enters the insect host via the mouth and must thwart gut-based defences to make its way into the body cavity (haemocoel) and establish infection. We sought to uncover the main antibacterial defences of the midgut and the pathophysiological features of Bt in a notable insect pest, the Colorado potato beetle Leptinotarsa decemlineata (CPB). Exposing the beetles to both Bt spores and their Cry3A toxins (crystalline δ-endotoxins) via oral inoculation led to higher mortality levels when compared to either spores or Cry3A toxins alone. Within 12 h post-exposure, Cry3A toxins caused a 1.5-fold increase in the levels of reactive oxygen species (ROS) and malondialdehyde (lipid peroxidation) within the midgut - key indicators of tissue damage. When Cry3A toxins are combined with spores, gross redox imbalance and 'oxidation stress' is apparent in beetle larvae. The insect detoxification system is activated when Bt spores and Cry3A toxins are administered alone or in combination to mitigate toxicosis, in addition to elevated mRNA levels of candidate defence genes (pattern-recognition receptor, stress-regulation, serine proteases, and prosaposin-like protein). The presence of bacterial spores and/or Cry3A toxins coincides with subtle changes in microbial community composition of the midgut, such as decreased Pseudomonas abundance at 48 h post inoculation. Both Bt spores and Cry3A toxins have negative impacts on larval health, and when combined, likely cause metabolic derangement, due to multiple tissue targets being compromised.

Up-regulation of apoptotic- and cell survival-related gene pathways following exposures of western corn rootworm to B. thuringiensis crystalline pesticidal proteins in transgenic maize roots

BACKGROUND

Resistance of pest insect species to insecticides, including B. thuringiensis (Bt) pesticidal proteins expressed by transgenic plants, is a threat to global food security. Despite the western corn rootworm, Diabrotica virgifera virgifera, being a major pest of maize and having populations showing increasing levels of resistance to hybrids expressing Bt pesticidal proteins, the cell mechanisms leading to mortality are not fully understood.

RESULTS

Twenty unique RNA-seq libraries from the Bt susceptible D. v. virgifera inbred line Ped12, representing all growth stages and a range of different adult and larval exposures, were assembled into a reference transcriptome. Ten-day exposures of Ped12 larvae to transgenic Bt Cry3Bb1 and Gpp34/Tpp35Ab1 maize roots showed significant differential expression of 1055 and 1374 transcripts, respectively, compared to cohorts on non-Bt maize. Among these, 696 were differentially expressed in both Cry3Bb1 and Gpp34/Tpp35Ab1 maize exposures. Differentially-expressed transcripts encoded protein domains putatively involved in detoxification, metabolism, binding, and transport, were, in part, shared among transcripts that changed significantly following exposures to the entomopathogens Heterorhabditis bacteriophora and Metarhizium anisopliae. Differentially expressed transcripts in common between Bt and entomopathogen treatments encode proteins in general stress response pathways, including putative Bt binding receptors from the ATP binding cassette transporter superfamily. Putative caspases, pro- and anti-apoptotic factors, as well as endoplasmic reticulum (ER) stress-response factors were identified among transcripts uniquely up-regulated following exposure to either Bt protein.

CONCLUSIONS

Our study suggests that the up-regulation of genes involved in ER stress management and apoptotic progression may be important in determining cell fate following exposure of susceptible D. v. virgifera larvae to Bt maize roots. This study provides novel insights into insect response to Bt intoxication, and a possible framework for future investigations of resistance mechanisms.

Transcriptome reveal the response to Cry1Ac toxin in susceptible Bombyx mori

Bombyx mori as a representative in Lepidoptera is an important economic insect in agriculture production. Bacillus thuringiensis (Bt) is a bacterial pathogen in silkworm production. Understanding how silkworm respond to Bt-toxin can provide guidance to cultivate resistant silkworm strains. Cry1Ac is one type of Bt-toxin. In current research, Dazao, a susceptible B. mori strain to Bt-toxin, was treated by Cry1Ac toxin and compared its transcriptome with untreated samples. This analysis detected 1234 differentially expressed genes (DEGs). Gene Ontology, KEGG, and UniProt keyword enrichment analysis showed that DEGs include ATP-binding cassette (ABC) transporter, stress response, cuticle, and protein synthesis, and folding process. Five ABC genes were upregulated after Cry1Ac treatment including ABCA2, ABCA3, and ABCC4. They are also known as the transporters of Bt-toxin in lepidopteran insect. Expression of cuticle proteins was significantly increased at 6 h after Cry1Ac treatment. Sex-specific storage-proteins and heat shock protein were also upregulated in Cry1Ac treated samples. Our data provide an expression profile about the response of Cry1Ac toxin in susceptible B. mori strain.

Response Mechanisms of Invertebrates to Bacillus thuringiensis and Its Pesticidal Proteins

Extensive use of chemical insecticides adversely affects both environment and human health. One of the most popular biological pest control alternatives is bioinsecticides based on Bacillus thuringiensis This entomopathogenic bacterium produces different protein types which are toxic to several insect, mite, and nematode species. Currently, insecticidal proteins belonging to the Cry and Vip3 groups are widely used to control insect pests both in formulated sprays and in transgenic crops. However, the benefits of B. thuringiensis-based products are threatened by insect resistance evolution. Numerous studies have highlighted that mutations in genes coding for surrogate receptors are responsible for conferring resistance to B. thuringiensis Nevertheless, other mechanisms may also contribute to the reduction of the effectiveness of B. thuringiensis-based products for managing insect pests and even to the acquisition of resistance. Here, we review the relevant literature reporting how invertebrates (mainly insects and Caenorhabditis elegans) respond to exposure to B. thuringiensis as either whole bacteria, spores, and/or its pesticidal proteins.

Bacillus thuringiensis Bioinsecticides Induce Developmental Defects in Non-Target Drosophila melanogaster Larvae

Bioinsecticides made from the bacterium Bacillus thuringiensis (Bt) are the bestselling bioinsecticide worldwide. Among Bt bioinsecticides, those based on the strain Bt subsp. kurstaki (Btk) are widely used in farming to specifically control pest lepidopteran larvae. Although there is much evidence of the lack of acute lethality of Btk products for non-target animals, only scarce data are available on their potential non-lethal developmental adverse effects. Using a concentration that could be reached in the field upon sprayings, we show that Btk products impair growth and developmental time of the non-target dipteran Drosophila melanogaster. We demonstrate that these effects are mediated by the synergy between Btk bacteria and Btk insecticidal toxins. We further show that Btk bioinsecticides trigger intestinal cell death and alter protein digestion without modifying the food intake and feeding behavior of the larvae. Interestingly, these harmful effects can be mitigated by a protein-rich diet or by adding the probiotic bacterium Lactobacillus plantarum into the food. Finally, we unravel two new cellular mechanisms allowing the larval midgut to maintain its integrity upon Btk aggression: First the flattening of surviving enterocytes and second, the generation of new immature cells arising from the adult midgut precursor cells. Together, these mechanisms participate to quickly fill in the holes left by the dying enterocytes.

The Tripartite Interaction of Host Immunity-Bacillus thuringiensis Infection-Gut Microbiota

Bacillus thuringiensis (Bt) is an important cosmopolitan bacterial entomopathogen, which produces various protein toxins that have been expressed in transgenic crops. The evolved molecular interaction between the insect immune system and gut microbiota is changed during the Bt infection process. The host immune response, such as the expression of induced antimicrobial peptides (AMPs), the melanization response, and the production of reactive oxygen species (ROS), varies with different doses of Bt infection. Moreover, B. thuringiensis infection changes the abundance and structural composition of the intestinal bacteria community. The activated immune response, together with dysbiosis of the gut microbiota, also has an important effect on Bt pathogenicity and insect resistance to Bt. In this review, we attempt to clarify this tripartite interaction of host immunity, Bt infection, and gut microbiota, especially the important role of key immune regulators and symbiotic bacteria in the Bt killing activity. Increasing the effectiveness of biocontrol agents by interfering with insect resistance and controlling symbiotic bacteria can be important steps for the successful application of microbial biopesticides.

Comparative Mortality and Adaptation of a Smurf Assay in two Species of Tenebrionid Beetles Exposed to Bacillus thuringiensis

Bacillus thuringiensis is a spore-forming bacterium which infects insect larvae naturally via the oral route. Its virulence factors interact with the epithelium of the digestive tract of insect larvae, disrupting its function and eventually leading to the death of susceptible hosts. The most cited B. thuringiensis killing mechanism is the extensive damage caused to the insect midgut, leading to its leakage. The mortality caused by B. thuringiensis has been shown to vary between serovars and isolates, as well as between host life stages. Moreover, whether susceptibility to B. thuringiensis-induced gut leakage is generalized to all host species and whether there is individual variation within species is unclear. In this study, we adapted a non-invasive “Smurf” assay from Drosophila melanogaster to two species of tenebrionid beetles: The mealworm beetle Tenebrio molitor and the red flour beetle Tribolium castaneum, during exposure to B. thuringiensis. We highlight a differential mortality between two age/size classes of T. molitor larvae, as well as different killing dynamics between B. thuringiensis var. tenebrionis and var. tolworthi in T. castaneum. The Smurf assay did not reveal a high occurrence of extensive gut disintegration in both host species upon ingestion during B. thuringiensis exposure.

Toxicity and cytopathology mediated by Bacillus thuringiensis in the midgut of Anticarsia gemmatalis (Lepidoptera: Noctuidae)

Bioinsecticides and transgenic plants, based on Bacillus thuringiensis (Bt) toxins are important when managing Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae), a soybean defoliator pest. The interaction of these toxins with the caterpillar’s midgut cells determines their efficacy as an insecticide. The objective was to evaluate the toxicity of B. thuringiensis, subsp. kurstaki strain HD-1 and cytopathological changes mediated by these bacterial toxins in the midgut of A. gemmatalis caterpillars. Insecticidal efficacy was determined by calculating lethal concentration values (LC₂₅, LC₅₀, LC₇₅, LC₉₀ and LC₉₉) in the laboratory. Midgut fragments from A. gemmatalis were extracted after bacterial ingestion and evaluated by light, transmission electron and confocal microscopy. The Bt median lethal concentrations showed toxicity [LC₅₀ = 0.46 (0.43–0.49) mg mL⁻¹] to fourth instar A. gemmatalis caterpillars after 108 hours. Bt induces severe cytotoxicity to A. gemmatalis midgut epithelial cells with increasing exposure over time, causing cellular disorganization, microvillus degeneration, cell fragmentation and protrusion, peritrophic membrane rupture, and cell vacuolization. The cell nuclei presented condensed chromatin and an increase in lysosome numbers. Apoptosis occurred in the midgut cells of caterpillars exposed to Bt. A regenerative response in A. gemmatalis caterpillars was observed 8 hours after exposure to Bt, however this response was not continuous. Toxins produced by Bt are harmful to A. gemmatalis at median concentration with structural damage and death of the midgut epithelial cells of this insect.

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.

De Novo DNA Synthesis in Aedes aegypti Midgut Cells as a Complementary Strategy to Limit Dengue Viral Replication

Aedes aegypti is the main vector of Dengue Virus, carrying the virus during the whole mosquito life post-infection. Few mosquito fitness costs have been associated to the virus infection, thereby allowing for a swift dissemination. In order to diminish the mosquito population, public health agency use persistent chemicals with environmental impact for disease control. Most countries barely use biological controls, if at all. With the purpose of developing novel Dengue control strategies, a detailed understanding of the unexplored virus-vector interactions is urgently needed. Damage induced (through tissue injury or bacterial invasion) DNA duplication (endoreplication) has been described in insects during epithelial cells renewal. Here, we delved into the mosquito midgut tissue ability to synthesize DNA de novo; postulating that Dengue virus infection could trigger a protective endoreplication mechanism in some mosquito cells. We hypothesized that the Aedes aegypti orthologue of the Drosophila melanogaster hindsight gene (not previously annotated in Aedes aegypti transcriptome/genome) is part of the Delta-Notch pathway. The activation of this transcriptional cascade leads to genomic DNA endoreplication. The amplification of the genomic copies of specific genes ultimately limits the viral spreading during infection. Conversely, inhibiting DNA synthesis capacity, hence endoreplication, leads to a higher viral replication.

Two Phytoplasmas Elicit Different Responses in the Insect Vector Euscelidius variegatus Kirschbaum

Phytoplasmas are plant-pathogenic bacteria transmitted by hemipteran insects. The leafhopper Euscelidius variegatus is a natural vector of chrysanthemum yellows phytoplasma (CYp) and a laboratory vector of flavescence dorée phytoplasma (FDp). The two phytoplasmas induce different effects on this species: CYp slightly improves whereas FDp negatively affects insect fitness. To investigate the molecular bases of these different responses, transcriptome sequencing (RNA-seq) analysis of E. variegatus infected with either CYp or FDp was performed. The sequencing provided the first de novo transcriptome assembly for a phytoplasma vector and a starting point for further analyses on differentially regulated genes, mainly related to immune system and energy metabolism. Insect phenoloxidase activity, immunocompetence, and body pigmentation were measured to investigate the immune response, while respiration and movement rates were quantified to confirm the effects on energy metabolism. The activation of the insect immune response upon infection with FDp, which is not naturally transmitted by E. variegatus, confirmed that this bacterium is mostly perceived as a potential pathogen. Conversely, the acquisition of CYp, which is naturally transmitted by E. variegatus, seems to increase the insect fitness by inducing a prompt response to stress. This long-term relationship is likely to improve survival and dispersal of the infected insect, thus enhancing the opportunity of phytoplasma transmission.

Bacterial microbiota of Aedes aegypti mosquito larvae is altered by intoxication with Bacillus thuringiensis israelensis

Background

Insect microbiota is a dynamic microbial community that can actively participate in defense against pathogens. Bacillus thuringiensis (Bt) is a natural entomopathogen widely used as a bioinsecticide for pest control. Although Bt’s mode of action has been extensively studied, whether the presence of microbiota is mandatory for Bt to effectively kill the insect is still under debate. An association between a higher tolerance and a modified microbiota was already evidenced but a critical point remained to be solved: is the modified microbiota a cause or a consequence of a higher tolerance to Bt?

Methods

In this study we focused on the mosquito species Aedes aegypti, as no work has been performed on Diptera on this topic to date, and on B. thuringiensis israelensis (Bti), which is used worldwide for mosquito control. To avoid using antibiotics to cure bacterial microbiota, mosquito larvae were exposed to an hourly increasing dose of Bti during 25 hours to separate the most susceptible larvae dying quickly from more tolerant individuals, with longer survival.

Results

Denaturing gradient gel electrophoresis (DGGE) fingerprinting revealed that mosquito larval bacterial microbiota was strongly affected by Bti infection after only a few hours of exposure. Bacterial microbiota from the most tolerant larvae showed the lowest diversity but the highest inter-individual differences. The proportion of Bti in the host tissue was reduced in the most tolerant larvae as compared to the most susceptible ones, suggesting an active control of Bti infection by the host.

Conclusions

Here we show that a modified microbiota is associated with a higher tolerance of mosquitoes to Bti, but that it is rather a consequence of Bti infection than the cause of the higher tolerance. This study paves the way to future investigations aiming at unraveling the role of host immunity, inter-species bacterial competition and kinetics of host colonization by Bti that could be at the basis of the phenotype observed in this study.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2741-8) contains supplementary material, which is available to authorized users.

Changes in gene expression and apoptotic response in Spodoptera exigua larvae exposed to sublethal concentrations of Vip3 insecticidal proteins

The insecticidal Vip3 proteins from Bacillus thuringiensis (Bt), along with the classical Bt Cry proteins, are currently used in Bt-crops to control insect pests, since they do not share the same mode of action. Here we characterized the response of Spodoptera exigua larvae after Vip3 challenge. The expression profile of 47 genes was analyzed in larvae challenged with three concentrations of Vip3Ca. Results showed that the up-regulated genes were mainly involved in immune response, whereas the down-regulated genes were mainly involved in the digestion process. Other mechanisms of cellular response to the damage such as apoptosis were analyzed. For this analysis, sections from the midguts were examined by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. The nuclei of the midgut epithelial cells were stained at the highest concentration of the Vip3Ca protein and at lower concentrations of Vip3Aa in agreement with the different potency of the two proteins. In addition, apoptosis was also examined by the analysis of the expression of five caspase genes. The present study shows that exposure of S. exigua larvae to sublethal concentrations of Vip3 proteins activates different insect response pathways which trigger the regulation of some genes, APN shedding, and apoptotic cell death.

Larval Mid-Gut Responses to Sub-Lethal Dose of Cry Toxin in Lepidopteran Pest Achaea janata

The lack of homogeneity in field application of Bacillus thuringiensis formulation often results in ingestion of sub-lethal doses of the biopesticide by a fraction of pest population and there by promotes the toxin tolerance and resistance in long term. Gut regeneration seems to be one of the possible mechanism by which this is accomplished. However, the existing information is primarily derived from in vitro studies using mid-gut cell cultures. Present study illustrates cellular and molecular changes in mid-gut epithelium of a Bt-susceptible polyphagous insect pest castor semilooper, Achaea janata in response to a Cry toxin formulation. The present report showed that prolonged exposure to sub-lethal doses of Cry toxin formulation has deleterious effect on larval growth and development. Histological analysis of mid-gut tissue exhibits epithelial cell degeneration, which is due to necrotic form of cell death followed by regeneration through enhanced proliferation of mid-gut stem cells. Cell death is demonstrated by confocal microscopy, flow-cytometry, and DNA fragmentation analysis. Cell proliferation in control vs. toxin-exposed larvae is evaluated by bromodeoxyuridine (BrdU) labeling and toluidine blue staining. Intriguingly, in situ mRNA analysis detected the presence of arylphorin transcripts in larval mid-gut epithelial cells. Quantitative PCR analysis further demonstrates altered expression of arylphorin gene in toxin-exposed larvae when compared with the control. The coincidence of enhanced mid-gut cell proliferation coincides with the elevated arylphorin expression upon Cry intoxication suggests that it might play a role in the regeneration of mid-gut epithelial cells.