Transcriptomic analyses uncover emerging roles of mucins, lysosome/secretory addressing and detoxification pathways in insect midguts

MCMV-infected maize attracts its insect vector Frankliniella occidentalis by inducing β-myrcene

Maize lethal necrosis is attributed to the accumulation of maize chlorotic mottle virus (MCMV), an invasive virus transmitted by insect vectors. The western flower thrips (WFT) can shift host to maize, thus promoting the spread of MCMV. However, our understanding of the characteristics and interactions involved in the transmission of MCMV is still limited. This study finds that non-viruliferous WFTs showed a 57.56% higher preference for MCMV-infected maize plants compared to healthy maize plants, while viruliferous WFTs showed a 53.70% higher preference for healthy maize plants compared to MCMV-infected maize plants. We also show for the first time that both adults and larvae of WFT could successfully acquire MCMV after 1 min of acquisition access period (AAP), and after 48 h of AAP, WFT could transmit MCMV in an inoculation access period of 1 h without a latent period. Both adults and larvae of WFT can transmit MCMV for up to 2 days. Furthermore, the decreasing number of viruliferous WFTs and transmission rates as time progressed, together with the transcriptomic evidence, collectively suggest that WFTs transmit MCMV in a semi-persistent method, a mode of transmission requiring minutes to several hours for acquisition access and having a retention time of several hours to a few days. Additionally, β-myrcene can attract WFTs significantly and is detected in Nicotiana benthamiana plants transiently expressing MCMV CP (coat protein), which is consistent with results in MCMV-infected maize plants through the metabolomic profiling and the preference analyses of WFT. Therefore, this study demonstrates the indirect interaction between MCMV and WFT by inducing maize to synthesize β-myrcene to attract insect vectors. The exploration of specific interactions between MCMV and WFT could help to expand the mechanism studies of virus-vector-host plant interaction and put forward a new insight for the combined control of MCMV and WFT through the manipulation of plant volatiles and key insect genes.

Molecular mechanisms of cytochrome P450-mediated detoxification of tetraniliprole, spinetoram, and emamectin benzoate in the fall armyworm, Spodoptera frugiperda (J.E. Smith)

The fall armyworm (FAW) Spodoptera frugiperda (J.E. Smith) is a highly damaging invasive omnivorous pest that has developed varying degrees of resistance to commonly used insecticides. To investigate the molecular mechanisms of tolerance to tetraniliprole, spinetoram, and emamectin benzoate, the enzyme activity, synergistic effect, and RNA interference were implemented in S. frugiperda. The functions of cytochrome P450 monooxygenase (P450) in the tolerance to tetraniliprole, spinetoram, and emamectin benzoate in S. frugiperda was determined by analysing changes in detoxification metabolic enzyme activity and the effects of enzyme inhibitors on susceptibility to the three insecticides. 102 P450 genes were screened via transcriptome and genome, of which 67 P450 genes were differentially expressed in response to tetraniliprole, spinetoram, and emamectin benzoate and validated by quantitative real-time PCR. The expression patterns of CYP9A75, CYP340AA4, CYP340AX8v2, CYP340L16, CYP341B15v2, and CYP341B17v2 were analysed in different tissues and at different developmental stages in S. frugiperda. Silencing CYP340L16 significantly increased the susceptibility of S. frugiperda to tetraniliprole, spinetoram, and emamectin benzoate. Furthermore, knockdown of CYP340AX8v2, CYP9A75, and CYP341B17v2 significantly increased the sensitivity of S. frugiperda to tetraniliprole. Knockdown of CYP340AX8v2 and CYP340AA4 significantly increased mortality of S. frugiperda to spinetoram. Knockdown of CYP9A75 and CYP341B15v2 significantly increased the susceptibility of S. frugiperda to emamectin benzoate. These results may help to elucidate the mechanisms of tolerance to tetraniliprole, spinetoram and emamectin benzoate in S. frugiperda.

Visualizing the intracellular aggregation behavior of gold nanoclusters via structured illumination microscopy and scanning transmission electron microscopy

Given the growing concerns about nanotoxicity, numerous studies have focused on providing mechanistic insights into nanotoxicity by imaging the intracellular fate of nanoparticles. A suitable imaging strategy is necessary to uncover the intracellular behavior of nanoparticles. Although each conventional technique has its own limitations, scanning transmission electron microscopy (STEM) and three-dimensional structured illumination microscopy (3D-SIM) combine the advantages of chemical element mapping, ultrastructural analysis, and cell dynamic tracking. Gold nanoclusters (AuNCs), synthesized using 6-aza-2 thiothymine (ATT) and L-arginine (Arg) as reducing and protecting ligands, referred to as Arg@ATT-AuNCs, have been widely used in biological sensing and imaging, medicine, and catalyst yield. Based on their intrinsic fluorescence and high electron density, Arg@ATT-AuNCs were selected as a model. STEM imaging showed that both the single-particle and aggregated states of Arg@ATT-AuNCs were compartmentally distributed within a single cell. Real-time 3D-SIM imaging showed that the fluorescent Arg@ATT-AuNCs gradually aggregated after being located in the lysosomes of living cells, causing lysosomal damage. The aggregate formation of Arg@ATT-AuNCs was triggered by the low-pH medium, particularly in the lysosomal acidic environment. The proposed dual imaging strategy was verified using other types of AuNCs, which is valuable for studying nano-cell interactions and any associated cytotoxicity, and has the potential to be a useful approach for exploring the interaction of cells with various nanoparticles.

Functional Characterization of CYP6QE1 and CYP6FV21 in Resistance to λ-Cyhalothrin and Imidacloprid in Bradysia odoriphaga

Cytochrome P450 monooxygenases (P450s) belong to a family of metabolic enzymes that are involved in the detoxification of insecticides. In this study, our bioassay results showed that a field-collected population of Bradysia odoriphaga displayed a moderate resistance to λ-cyhalothrin and imidacloprid. Compared to susceptible population, CYP6QE1 and CYP6FV21 were significantly overexpressed in the field population. The expression of CYP6QE1 and CYP6FV21 was more abundant in the third and fourth larval stages, and CYP6QE1 and CYP6FV21 were most highly expressed in the midgut and Malpighian tubules. Exposure to λ-cyhalothrin and imidacloprid significantly increased the expression levels of CYP6QE1 and CYP6FV21. Furthermore, the silencing of CYP6QE1 and CYP6FV21 significantly increased the susceptibility of B. odoriphaga larvae to λ-cyhalothrin and imidacloprid. The overexpression of CYP6QE1 and CYP6FV21 significantly enhanced the tolerance of transgenic Drosophila melanogaster lines to λ-cyhalothrin and imidacloprid. In addition, molecular docking revealed that these two P450 proteins have strong binding affinity toward λ-cyhalothrin and imidacloprid insecticides. Taken together, these results indicate that the overexpression of CYP6QE1 and CYP6FV21 is responsible for resistance to λ-cyhalothrin and imidacloprid in B. odoriphaga.

Identification of G Protein-Coupled Receptors (GPCRs) Associated with Lambda-Cyhalothrin Detoxification in Cydia pomonella

While previous studies have reported G protein-coupled receptor (GPCR)-mediated insecticide resistance in various arthropods, the understanding of GPCR-associated resistance mechanisms in Cydia pomonella remains limited. In this study, a total of 95 CpGPCR genes categorized into four families were identified in C. pomonella. Results revealed high expression levels of the majority of the CpGPCRs during the first larval stage and in the head of C. pomonella. Exposure to lambda-cyhalothrin significantly increased the expression of 15 CpGPCRs, including CpGPCR70, which is highly expressed in all larval stages and shows the highest expression in the midgut. RNA interference (RNAi) demonstrated that downregulation of CpGPCR70 leads to reduced expression of key resistance-related genes and a decreased tolerance of larvae to lambda-cyhalothrin. These findings indicate that CpGPCR70 plays a crucial role in regulating the expression of detoxifying genes involved in lambda-cyhalothrin resistance, offering valuable insights for the development of more effective pest control strategies.

Histochemistry and transcriptomics of mucins and peritrophic membrane (PM) proteins along the midgut of a beetle with incomplete PM and their complementary function

The midgut of Zabrotes subfasciatus (Coleoptera) and other insects may have regions lacking a peritrophic membrane (matrix, PM) and covered with a jelly-like material known as peritrophic gel. This work was undertaken to test the hypothesis that the peritrophic gel is a vertebrate-like mucus. By histochemistry we identified mucins along the whole midgut, which contrasts with the known occurrence of PM only at the posterior midgut. We also analyzed the expression of the genes coding for mucus-forming mucins (Mf-mucins), peritrophins, chitin synthases and chitin deacetylases along the midgut and carcass (insect without midgut) by RNA-seq. Mf-mucins were identified as proteins with high O-glycosylation and multiple tandem repeats of Pro/Thr/Ser residues. Peritrophins were separated into PM proteins, cuticular proteins analogous to peritrophins (CPAPs) and ubiquitous-chitin-binding domain-(CBD)-containing proteins (UCBPs). PM proteins have at least 3, CPAP one or 3, and UCBPs have a varied number of CBDs. PM proteins are more expressed at midgut, CPAP at the carcass, and UCBP at both. The results showed that most PM proteins are mainly expressed at the posterior midgut, together with midgut chitin synthase and chitin deacetylase, and in agreement with the presence of PM only at the posterior midgut by visual inspection. The excretion of most midgut chitinase is avoided, suggesting that the shortened PM is functional. Mf-mucins are expressed along the whole midgut, probably forming the extracellular mucus layer observed by histochemistry. Thus, the lack of PM at anterior and middle midgut causes the exposure of a mucus, which may correspond to the previously described peritrophic gel. The putative functional interplay of mucus and PM is discussed. The major role of mucus is proposed to be tissue protection and of PM to enhancing digestive efficiency by allowing enzyme recycling.

Role of the epsilon glutathione S-transferases in xanthotoxin tolerance in Spodoptera litura

Spodoptera litura, a polyphagous lepidopteran pest, demonstrates a remarkable capacity to adapt to varying host plants by efficiently detoxifying phytochemicals. However, the underlying mechanism for this adaptation is not well understood. Herein, twenty eplison glutathione S-transferase genes (GSTes) were characterized and their roles in phytochemical tolerance were analyzed in S. litura. Most of the GSTe genes were mainly expressed in the larval midgut and fat body. Exposure to the phytochemicals, especially xanthotoxin, induced the expression of most GSTe genes. Molecular docking analysis revealed that xanthotoxin could form stable bonds with six xanthotoxin-responsive GSTes, with binding free energies ranging from -36.44 to -68.83 kcal mol-1. Knockdown of these six GSTe genes increased the larval susceptibility to xanthotoxin. Furthermore, xanthotoxin exposure significantly upregulated the expression of two transcription factor genes CncC and MafK. Silencing of either CncC or MafK reduced the expression of GSTe16, which exhibited the largest change in response to xanthotoxin. Additionally, analysis of the promoter sequence of GSTe16 revealed the presence of seven CncC/Maf binding sites. Luciferase reporter assays showed that CncC and MafK enhanced the expression of GSTe16, leading to the increased xanthotoxin tolerance in S. litura. These findings provide insight into the functions and transcriptional regulatory mechanisms of GSTes, thereby enhancing our understanding of the role of GSTs in the adaptation of lepidopteran pests to phytochemicals.

Discovery of 3-Formyl-N-(un)Substituted Benzylindole Pyrimidines as an Acaricidal Agent and Their Mechanism of Action

To discover the pronounced acaricide candidate, herein, a series of 3-formyl-N-(un)substituted benzylindole pyrimidines were prepared by structural modification of indoles at the N-1 and C-3 positions via the successive Vilsmeier-Haack-Arnold (VHA), aldol condensation, and cyclization reactions. The steric structures of nine compounds were undoubtedly confirmed by X-ray single-crystallography. Against Tetranychus cinnabarinus Boisduval, compounds V-15, V-31, V-34, V-42, V-44, and V-60 exhibited promising acaricidal activity with LC50 values of 0.299-0.481 mg/mL. In particular, compound V-34 displayed 4.2 times the acaricidal activity of its precursor 6-methylindole. Scanning electron microscopy (SEM) imaging revealed that the construction of the cuticle layer of V-34-treated T. cinnabarinus was seriously destroyed. Furthermore, RNA-Seq analysis indicated that compound V-34 could regulate the homeostasis metabolism of T. cinnabarinus through arachidonic acid and linoleic acid metabolism and lysosome pathways. These results suggested that compound V-34 can be further studied as a lead acaricidal agent.

Functional analysis of UDP-glycosyltransferase genes conferring indoxacarb resistance in Spodoptera litura

UDP-glycosyltransferase (UGT) is the major detoxification enzymes of phase II involved in xenobiotics metabolism, which potentially mediates the formation of insect resistance. Previous transcriptome sequencing studies have found that several UGT genes were upregulated in indoxacarb resistant strains of Spodoptera litura, but whether these UGT genes were involved in indoxacarb resistance and their functions in resistance were unclear. In this study, the UGTs inhibitor, 5-nitrouracil, enhanced the toxicity of indoxacarb against S. litura, preliminarily suggesting that UGTs were participated in indoxacarb resistance. Two UGT genes, UGT33J17 and UGT41D10 were upregulated in the resistant strains and could be induced by indoxacarb. Alignment of UGT protein sequences revealed two conserved donor-binding regions with several key residues that interact with catalytic sites and sugar donors. Further molecular modeling and docking analysis indicated that two UGT proteins were able to stably bind indoxacarb and N-decarbomethoxylated metabolite (DCJW). Furthermore, knockdown of UGT33J17 and UGT41D10 decreased viability of Spli-221 cells and enhanced susceptibility of larvae to indoxacarb. Transgenic overexpression of these genes reduced the toxicity of indoxacarb in Drosophila melanogaster. This work revealed that upregulation of UGT genes significantly contributes to indoxacarb resistance in S. litura, and is of great significance for the development of integrated and sustainable management strategies for resistant pests in the field.

Toxicity assessment and detoxification metabolism of sodium pentachlorophenol (PCP-Na) on marine economic species: a case study of Moerella iridescens and Exopalaemon carinicauda

Sodium pentachlorophenol (PCP-Na) is widespread in the marine environment; however, its impact on marine organisms remains under-researched. Moerella iridescens and Exopalaemon carinicauda are marine species of economic importance in China and under threat from PCP-Na pollution. Thus, this study aimed to assess the toxicity and detoxification metabolism of PCP-Na on M. iridescens and E. carinicauda. The study revealed that the 96 h median lethal concentration (LC50) of PCP-Na for M. iridescens and E. carinicauda were 9.895 mg/L and 14.143 mg/L, respectively. A species sensitivity distribution (SSD) for PCP-Na was developed specifically for marine organisms, determining a hazardous concentration to 5% of the species (HC5) of 0.047 mg/L. During the sub-chronic exposure period, PCP-Na accumulated significantly in M. iridescens and E. carinicauda, with highest concentrations of 41.22 mg/kg in the soft tissues of M. iridescens, 42.58 mg/kg in the hepatopancreas of E. carinicauda, and only 0.85 mg/kg in the muscle of E. carinicauda. Furthermore, the study demonstrated that detoxifying metabolic enzymes and antioxidant defense system enzymes of E. carinicauda responded stronger to PCP-Na compared to M. iridescens, suggesting that E. carinicauda may possess a stronger detoxification capacity. Notably, five biomarkers were identified and proposed for monitoring and evaluating PCP-Na contamination. Overall, the results indicated that M. iridescens and E. carinicauda exhibit greater tolerance to PCP-Na than other marine species, but they are susceptible to accumulating PCP-Na in their tissues, posing a significant health risk. Consequently, conducting aquatic health risk assessments in areas with potential PCP-Na contamination is strongly recommended.

Characterization of the physiological, histopathological, and gene expression alterations in Spodoptera frugiperda larval midguts affected by toosendanin exposure

The fall armyworm, Spodoptera frugiperda, is a polyphagous pest worldwide and feeds on many grain and cash crops, which threatens the safety of agriculture and forestry production. Toosendanin (TSN) is a commercial insecticidal active ingredient used to manage various pests in the field and showed adverse effects against S. frugiperda, while the effects of TSN on the larval midguts are not yet known. In this study, the effects of 10 and 20 mg/kg TSN exposures on the larval midguts were analyzed. The structural changes of the larval midgut induced by TSN treatments were also determined by hematoxylin-eosin staining. Besides, TSN treatments also changed the enzyme activities of three digestive enzymes (α-amylase, lipase, and trypsin) and two detoxification enzymes (CarE and GST). A total of 2868 differentially expressed genes (DEGs) were identified by RNA-Seq in the larval midguts with 20 mg/kg TSN treatment, and the DEGs responsible for food digestion and detoxification were further examined. Our findings revealed the preliminary modes of action of TSN on the larval midguts of S. frugiperda, which provide a preliminary rationale for controlling S. frugiperda with TSN in the field.

Contribution of UDP-glycosyltransferases to chlorpyrifos resistance in Nilaparvata lugens

As a multigene superfamily of Phase II detoxification enzymes, uridine diphosphate (UDP)-glycosyltransferases (UGTs) play important roles in the metabolism of xenobiotics including insecticides. In this study, 5-nitrouracil, an inhibitor of UGT enzyme activity, effectively increased the toxicity of chlorpyrifos to the chlorpyrifos-resistant strain of Nilaparvata lugens, one of the most resistant rice pests. The enzyme content of UGT in the resistant strain was significantly higher than that in the susceptible strain. Among 20 identified UGT genes, UGT386H2, UGT386J2, UGT386N2 and UGT386P1 were found significantly overexpressed in the resistant strain and can be effectively induced by chlorpyrifos. These four UGT genes were most highly expressed in the midgut and/or fat body, two main insect detoxification tissues. Amino acid sequence alignments revealed that these four UGTs contained a variable N-terminal substrate-binding domain and a conserved C-terminal sugar donor-binding domain. Furthermore, homology modeling and molecular docking analyses showed that these UGTs could stably bind to chlorpyrifos and chlorpyrifos oxon, with the binding free energies from -19.4 to -110.62 kcal mol^-1. Knockdown of UGT386H2 or UGT386P1 by RNA interference dramatically increased the susceptibility of the resistant strain to chlorpyrifos. These findings suggest that overexpression of these two UGT genes contributes to chlorpyrifos resistance in N. lugens.

The Aedes aegypti peritrophic matrix controls arbovirus vector competence through HPx1, a heme-induced peroxidase

Aedes aegypti mosquitoes are the main vectors of arboviruses. The peritrophic matrix (PM) is an extracellular layer that surrounds the blood bolus. It acts as an immune barrier that prevents direct contact of bacteria with midgut epithelial cells during blood digestion. Here, we describe a heme-dependent peroxidase, hereafter referred to as heme peroxidase 1 (HPx1). HPx1 promotes PM assembly and antioxidant ability, modulating vector competence. Mechanistically, the heme presence in a blood meal induces HPx1 transcriptional activation mediated by the E75 transcription factor. HPx1 knockdown increases midgut reactive oxygen species (ROS) production by the DUOX NADPH oxidase. Elevated ROS levels reduce microbiota growth while enhancing epithelial mitosis, a response to tissue damage. However, simultaneous HPx1 and DUOX silencing was not able to rescue bacterial population growth, as explained by increased expression of antimicrobial peptides (AMPs), which occurred only after double knockdown. This result revealed hierarchical activation of ROS and AMPs to control microbiota. HPx1 knockdown produced a 100-fold decrease in Zika and dengue 2 midgut infection, demonstrating the essential role of the mosquito PM in the modulation of arbovirus vector competence. Our data show that the PM connects blood digestion to midgut immunological sensing of the microbiota and viral infections.

Parallel meta-transcriptome analysis reveals degradation of plant secondary metabolites by beetles and their gut symbionts

Switching to a new host plant is a driving force for divergence and speciation in herbivorous insects. This process of incorporating a novel host plant into the diet may require a number of adaptations in the insect herbivores that allow them to consume host plant tissue that may contain toxic secondary chemicals. As a result, herbivorous insects are predicted to have evolved efficient ways to detoxify major plant defenses and increase fitness by either relying on their own genomes or by recruiting other organisms such as microbial gut symbionts. In the present study we used parallel meta-transcriptomic analyses of Altica flea beetles and their gut symbionts to explore the contributions of beetle detoxification mechanisms versus detoxification by their gut consortium. We compared the gut meta-transcriptomes of two sympatric Altica species that feed exclusively on different host plant species as well as their F₁ hybrids that were fed one of the two host plant species. These comparisons revealed that gene expression patterns of Altica are dependent on both beetle species identity and diet. The community structure of gut symbionts was also dependent on the identity of the beetle species, and the gene expression patterns of the gut symbionts were significantly correlated with beetle species and plant diet. Some of the enriched genes identified in the beetles and gut symbionts are involved in the degradation of secondary metabolites produced by plants, suggesting that Altica flea beetles may use their gut microbiota to help them feed on and adapt to their host plants.

A mucin protein predominantly expressed in the female-specific symbiotic organ of the stinkbug Plautia stali

Diverse insects are obligatorily associated with microbial symbionts, wherein the host often develops special symbiotic organs and vertically transmits the symbiont to the next generation. What molecular factors underpin the host-symbiont relationship is of great interest but poorly understood. Here we report a novel protein preferentially produced in a female-specific symbiotic organ of the stinkbug Plautia stali, whose posterior midgut develops numerous crypts to host a Pantoea-allied bacterial mutualist. In adult females, several posteriormost crypts are conspicuously enlarged, presumably specialized for vertical symbiont transmission. We detected conspicuous protein bands specific to the female’s swollen crypts by gel electrophoresis, and identified them as representing a novel mucin-like glycoprotein. Histological inspections confirmed that the mucin protein is localized to the female’s swollen crypts, coexisting with a substantial population of the symbiotic bacteria, and excreted from the swollen crypts to the midgut main tract together with the symbiotic bacteria. Using RNA interference, we successfully suppressed production of the mucin protein in adult females of P. stali. However, although the mucin protein was depleted, the symbiont population persisted in the swollen crypts, and vertical symbiont transmission to the next generation occurred. Possible biological roles and evolutionary trajectory of the symbiosis-related mucin protein are discussed.

Recruitment of lysosomal cathepsins B, L and D as digestive enzymes in Coleoptera

The recruitment of the lysosomal cathepsins B (CAB), L (CAL) and D (CAD) as luminal digestive enzymes was investigated in 3 species of beetles. Gene expression was determined by RNA-seq in different regions of the midgut and in the carcasses from the transcriptomes of Dermestes maculatus, Tenebrio molitor and Zabrotes subfasciatus. These data together with phylogenetic analyses, allowed us to identify the sequences of the gene coding for digestive and lysosomal CABs, CADs and CALs in T. molitor and Z. subfasciatus and observe the absence of digestive cathepsins in D. maculatus. Comparisons of structures based on the overall similarity of modelled structures were performed and subsite residues in the lysosomal and digestive CALs were identified by molecular docking. The data showed that S2 subsites are very variable, probably as an adaption to a luminal digestive role. The survey of sequences of the gene coding for cathepsins in the genomes of 13 beetle species from different phylogenetic groups showed that expansion of CAL and CAB genes occurred only in the Cucujiformia clade. Several digestive CABs have a reduced occluding loop, probably to act as digestive enzymes. Pollen-feeding was proposed to be the selective pressure to recruit cathepsins as digestive enzymes in Cucujiformia beetles.

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.

Transcriptome analysis of Chironomus sulfurosus larvae living in acidic environments: Insights into molecular mechanisms for acid tolerance

Larvae of chironomid Chironomus sulfurosus mainly live in acidic rivers near hot springs, suggesting that they naturally select acidic environments as preferred habitats. Here we showed that C. sulfurosus larvae moved toward acidic areas and stayed alive on agar gels with a pH gradient of H₂SO₄, and the body fluid pH of the homogenized larvae was near neutral even acclimated under the acidic conditions, indicating mechanisms for acid tolerance. In order to gain insights into this mechanism at the molecular level, de novo RNA-seq analysis was performed on C. sulfurosus larvae. As a result, 1,208 genes were found to be significantly up-regulated in larvae acclimated at pH 2.0 compared to controls at pH 7.0. Among the up-regulated genes, ones encoding cuticle proteins, peritrophic matrix proteins, mucus-forming proteins, F-type ATPase subunits, glutathione S transferases, β-1,3-D-glucan synthetase, hemoglobin, and cytochrome P450 were identified. This transcriptome analysis in conjunction with behavioral and biochemical assays expands our knowledge of gene expression in C. sulfurosus larvae living in acidic environments, which will provide a basis for further studies to elucidate the molecular mechanisms for acid tolerance employed by organisms in nature.

Growth inhibition of Spodoptera frugiperda larvae by camptothecin correlates with alteration of the structures and gene expression profiles of the midgut

Background

Spodoptera frugiperda is a serious pest that causes devastating losses to many major crops, including corn, rice, sugarcane, and peanut. Camptothecin (CPT) is a bioactive secondary metabolite of the woody plant Camptotheca acuminata, which has shown high toxicity to various pests. However, the effect of CPT against S. frugiperda remains unknown.

Results

In this study, bioassays have been conducted on the growth inhibition of CPT on S. frugiperda larvae. Histological and cytological changes were examined in the midgut of larvae fed on an artificial diet supplemented with 1.0 and 5.0 µg/g CPT. The potential molecular mechanism was explored by comparative transcriptomic analyses among midgut samples obtained from larvae under different treatments. A total of 915 and 3560 differentially expressed genes (DEGs) were identified from samples treated with 1.0 and 5.0 µg/g CPT, respectively. Among the identified genes were those encoding detoxification-related proteins and components of peritrophic membrane such as mucins and cuticle proteins. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses indicated that part of DEGs were involved in DNA replication, digestion, immunity, endocrine system, and metabolism.

Conclusions

Our results provide useful information on the molecular basis for the impact of CPT on S. frugiperda and for future studies on potential practical application.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07726-8.

Non-immune Traits Triggered by Blood Intake Impact Vectorial Competence

Blood-feeding arthropods are considered an enormous public health threat. They are vectors of a plethora of infectious agents that cause potentially fatal diseases like Malaria, Dengue fever, Leishmaniasis, and Lyme disease. These vectors shine due to their own physiological idiosyncrasies, but one biological aspect brings them all together: the requirement of blood intake for development and reproduction. It is through blood-feeding that they acquire pathogens and during blood digestion that they summon a collection of multisystemic events critical for vector competence. The literature is focused on how classical immune pathways (Toll, IMD, and JAK/Stat) are elicited throughout the course of vector infection. Still, they are not the sole determinants of host permissiveness. The dramatic changes that are the hallmark of the insect physiology after a blood meal intake are the landscape where a successful infection takes place. Dominant processes that occur in response to a blood meal are not canonical immunological traits yet are critical in establishing vector competence. These include hormonal circuitries and reproductive physiology, midgut permeability barriers, midgut homeostasis, energy metabolism, and proteolytic activity. On the other hand, the parasites themselves have a role in the outcome of these blood triggered physiological events, consistently using them in their favor. Here, to enlighten the knowledge on vector-pathogen interaction beyond the immune pathways, we will explore different aspects of the vector physiology, discussing how they give support to these long-dated host-parasite relationships.

Cathepsins L and B in Dysdercus peruvianus, Rhodnius prolixus, and Mahanarva fimbriolata. Looking for enzyme adaptations to digestion

Cysteine peptidases (CP) play a role as digestive enzymes in hemipterans similar to serine peptidases in most other insects. There are two major CPs: cathepsin L (CAL), which is an endopeptidase and cathepsin B (CAB) that is both an exopeptidase and a minor endopeptidase. There are thirteen putative CALs in Dysdercus peruvianus, which in some cases were confirmed by cloning their encoding genes. RNA-seq data showed that DpCAL5 is mainly expressed in the anterior midgut (AM), DpCAL10 in carcass (whole body less midgut), suggesting it is a lysosomal enzyme, and the other DpCALs are expressed in middle (MM) and posterior (PM) midgut. The expression data were confirmed by qPCR and enzyme secretion to midgut lumen by a proteomic approach. Two CAL activities were isolated by chromatography from midgut samples with similar kinetic properties toward small substrates. Docking analysis of a long peptide with several DpCALs modeled with digestive Tenebrio molitor CAL (TmCAL3) as template showed that on adapting to luminal digestion DpCALs (chiefly DpCAL5) changed in relation to their ancestral lysosomal enzyme (DpCAL10) mainly at its S2 subsite. A similar conclusion arrived from structure alignment-based clustering of DpCALs based on structural similarity of the modeled structures. Changes mostly on S2 subsite could mean the enzymes turn out less peptide-bond selective, as described in TmCALs. R. prolixus CALs changed on adapting to luminal digestion, although less than DpCALs. Both D. peruvianus and R. prolixus have two digestive CABs which are expressed in the same extension as CALs, in the first digestive section of the midgut, but less than in the other midgut sections. Mahanarva fimbriolata does not seem to have digestive CALs and their digestive CABs are mainly expressed in the first digestive section of the midgut and do not diverge much from their lysosomal counterparts. The data suggest that CABs are necessary at the initial stage of digestion in CP-dependent Hemipterans, which action is completed by CALs with low peptide-bond selectivity in Heteroptera species. In M. fimbriolata protein digestion is supposed to be associated with the inactivation of sap noxious proteins, making CAB sufficient as digestive CP. Hemipteran genomes and transcriptome data showed that CALs have been recruited as digestive enzymes only in heteropterans, whereas digestive CABs occur in all hemipterans.

PGE2 upregulates gene expression of dual oxidase in a lepidopteran insect midgut via cAMP signalling pathway

In insect midgut, prostaglandins (PGs) play a crucial role in defending bacterial and malarial pathogens. However, little is known about the PG signalling pathway in the midgut. A dual oxidase (Se-Duox) with presumed function of catalysing reactive oxygen species (ROS) production in the midgut was identified in beet armyworm, Spodoptera exigua. Se-Duox was expressed in all developmental stages, exhibiting relatively high expression levels in the midgut of late larval instars. Se-Duox expression was upregulated upon bacterial challenge. RNA interference (RNAi) of Se-Duox expression significantly suppressed ROS levels in the midgut lumen. The suppression of ROS levels increased insecticidal activity of Serratia marcescens after oral infection. Interestingly, treatment with a PLA₂ inhibitor prevented the induction of Se-Duox expression in response to bacterial challenge. On the other hand, addition of its catalytic product rescued the induction of Se-Duox expression. Especially, PG synthesis inhibitor significantly suppressed Se-Duox expression, while the addition of PGE₂ or PGD₂ rescued the inhibition. Subsequent PG signals involved cAMP and downstream components because specific inhibitors of cAMP signal components such as adenylate cyclase (AC) and protein kinase A (PKA) significantly inhibited Se-Duox expression. Indeed, addition of a cAMP analogue stimulated Se-Duox expression in the midgut. Furthermore, individual RNAi specific to PGE₂ receptor (a trimeric G-protein subunit), AC, PKA or cAMP-responsive element-binding protein resulted in suppression of Se-Duox expression. These results suggest that PGs can activate midgut immunity via cAMP signalling pathway by inducing Se-Duox expression along with increased ROS levels.

Mechanisms of Novel Host Use by Bactrocera tau (Tephritid: Diptera) Revealed by RNA Transcriptomes

Use of novel plant hosts can facilitate the establishment and range expansion of herbivorous invasive species. However, the inherent mechanisms of novel host use are still unclear in many herbivorous species. Here, we examine mechanisms of novel host use in the invasive tephritid fruit fly Bactrocera tau (Walker)(Diptera: Tephritidae) by documenting changes in the RNA transcriptomes associated with a novel host. RNA transcripts of B. tau were obtained with high-throughput sequencing from samples continuously reared on two traditional Cucurbitaceae hosts and a novel host (banana). We found transcriptome variation was strongly associated with feeding on banana. Moreover, B. tau feeding on banana contained more differentially expressed genes (DEGs) and more annotated categories of DEGs in the Kyoto Encyclopedia of Genes and Genomes (KEGG) database with 1,595 DEGs and 21 major annotated pathways. The annotated categories of DEGs in individuals reared on banana differed with from those individuals feeding on other hosts and were enriched in oxidative phosphorylation, citrate cycle pathway, and four other carbohydrate pathways. For B. tau feeding on banana, the predominant numbers of upregulated genes in the mitochondrial NADH (56 on average) and a relatively higher numbers of upregulated genes (13 on average) were found in oxidative phosphorylation and the TCA pathway, respectively. Changes in RNA transcriptomes associated with novel host use, especially for genes related to energy and carbohydrate metabolism, help to explain how B. tau can be successful in use of novel hosts and may be useful in developing novel strategies for control of tephritid flies.

Chlorantraniliprole degenerates microvilli goblet cells of the Anticarsia gemmatalis (Lepidoptera: Noctuidae) midgut

Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae) is mainly controlled with synthetic insecticides such as chlorantraniliprole. However, these compounds may affect non-target organs of insect metabolism. The objective of this study was to evaluate the toxic effect in the midgut goblet cells of A. gemmatalis caterpillars exposed to chlorantraniliprole. The midgut of these caterpillars, which ingested the insecticide in medium-lethal dose (LD₅₀), was dissected and evaluated by transmission electron microscopy. The goblet cells microvilli, after exposure to the insecticide, were disorganized and degenerated. This can compromise ionic homeostasis and nutrient absorption, impair physiological mechanisms of detoxification, and reduce the movement of food boluses throughout the insect midgut.

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.

Recruited lysosomal enzymes as major digestive enzymes in insects

The mass recruitment to the midgut contents of lysosomal proteolytic enzymes occurred in insects under three major selective pressures. Hemipteran (true bugs, aphids, and cicadas) ancestors lost their serine peptidases (SP) on adapting to feed on protein-free plant sap. When they returned to protein diets, their cathepsins L and B were recruited to replace their lost SP. Among beetles of the series Cucujiformia, cathepsins L were recruited to hydrolyze ingested plant inhibitors that affect their major SP and/or to deal with special seed proteins, such as prolamins. Larval flies have a very acid middle midgut and use cathepsin D to digest bacteria from their infected food. All the recruited enzymes originated from duplicated genes. The recruited digestive enzymes differ from their lysosomal counterparts in critical regions of their amino acid sequences that resulted in changes in substrate specificities and other kinetic properties. The discharge of digestive cathepsins in the midgut contents, instead of lysosomes, seems to be a consequence of their overexpression or the existence of new targeting signals. Their activation at the midgut contents occurs by an autoactivation mechanism or with the help of other enzymes or by a combination of both. The targeting to lysosomes of the insect lysosomal enzymes does not follow the mammalian mannose 6-phosphate route, but an incompletely known mechanism.