Role of proteases in extra-oral digestion of a predatory bug, Andrallus spinidens

Identification of salivary proteins of the cowpea aphid Aphis craccivora by transcriptome and LC-MS/MS analyses

Aphid salivary proteins mediate the interaction between aphids and their host plants. Moreover, these proteins facilitate digestion, detoxification of secondary metabolites, as well as activation and suppression of plant defenses. The cowpea aphid, Aphis craccivora, is an important sucking pest of leguminous crops worldwide. Although aphid saliva plays an important role in aphid plant interactions, knowledge of the cowpea aphid salivary proteins is limited. In this study, we performed transcriptomic and LC-MS/MS analyses to identify the proteins present in the salivary glands and saliva of A. craccivora. A total of 1,08,275 assembled transcripts were identified in the salivary glands of aphids. Of all these assembled transcripts, 53,714 (49.11%) and 53,577 (49.48%) transcripts showed high similarity to known proteins in the Nr and UniProt databases, respectively. A total of 2159 proteins were predicted as secretory proteins from the salivary gland transcriptome dataset, which contain digestive enzymes, detoxification enzymes, previously known effectors and elicitors, and potential proteins whose functions have yet to be determined. The proteomic analysis of aphid saliva resulted in the identification of 171 proteins. Tissue-specific expression of selected genes using RT-PCR showed that three genes were expressed only in the salivary glands. Overall, our results provide a comprehensive repertoire of cowpea aphid salivary proteins from the salivary gland and saliva, which will be a good resource for future effector functional studies and might also be useful for sustainable aphid management.

Comparative transcriptomic analysis of salivary glands between the zoophytophagous Cyrtorhinus lividipennis and the phytozoophagous Apolygus lucorum

BACKGROUND

Saliva plays a crucial role in shaping the feeding behavior of insects, involving processes such as food digestion and the regulation of interactions between insects and their hosts. Cyrtorhinus lividipennis serves as a predominant natural enemy of rice pests, while Apolygus lucorum, exhibiting phytozoophagous feeding behavior, is a destructive agricultural pest. In this study, a comparative transcriptome analysis, incorporating the published genomes of C.lividipennis and A.lucorum, was conducted to reveal the role of salivary secretion in host adaptation.

RESULTS

In contrast to A.lucorum, C.lividipennis is a zoophytophagous insect. A de novo genome analysis of C.lividipennis yielded 19,706 unigenes, including 16,217 annotated ones. On the other hand, A.lucorum had altogether 20,111 annotated genes, as obtained from the published official gene set (20,353 unigenes). Functional analysis of the top 1,000 salivary gland (SG)-abundant genes in both insects revealed that the SG was a dynamically active tissue engaged in protein synthesis and secretion. Predictions of other tissues and signal peptides were compared. As a result, 94 and 157 salivary proteins were identified in C.lividipennis and A.lucorum, respectively, and were categorized into 68 and 81 orthogroups. Among them, 26 orthogroups were shared, potentially playing common roles in digestion and detoxification, including several venom serine proteases. Furthermore, 42 and 55 orthogroups were exclusive in C.lividipennis and A.lucorum, respectively, which were exemplified by a hyaluronidase in C.lividipennis that was associated with predation, while polygalacturonases in A.lucorum were involved in mesophyll-feeding patterns.

CONCLUSIONS

Findings in this study provide a comprehensive insight into saliva secretions in C.lividipennis and A.lucorum via a transcriptome approach, reflecting the intricate connections between saliva secretions and feeding behaviors. It is found that conserved salivary secretions are involved in shaping the overlapping feeding patterns, while a plethora of unique salivary secretions may drive the evolution of specific feeding behaviors crucial for their survival. These results enhance our understanding of the feeding mechanisms in different insects from the perspective of saliva and contribute to future environmentally friendly pest control by utilizing predatory insects.

The Predatory Stink Bug Arma custos (Hemiptera: Pentatomidae) Produces a Complex Proteinaceous Venom to Overcome Caterpillar Prey

Predatory stink bugs capture prey by injecting salivary venom from their venom glands using specialized stylets. Understanding venom function has been impeded by a scarcity of knowledge of their venom composition. We therefore examined the proteinaceous components of the salivary venom of the predatory stink bug Arma custos (Fabricius, 1794) (Hemiptera: Pentatomidae). We used gland extracts and venoms from fifth-instar nymphs or adult females to perform shotgun proteomics combined with venom gland transcriptomics. We found that the venom of A. custos comprised a complex suite of over a hundred individual proteins, including oxidoreductases, transferases, hydrolases, ligases, protease inhibitors, and recognition, transport and binding proteins. Besides the uncharacterized proteins, hydrolases such as venom serine proteases, cathepsins, phospholipase A₂, phosphatases, nucleases, alpha-amylases, and chitinases constitute the most abundant protein families. However, salivary proteins shared by and unique to other predatory heteropterans were not detected in the A. custos venom. Injection of the proteinaceous (>3 kDa) venom fraction of A. custos gland extracts or venom into its prey, the larvae of the oriental armyworm Mythimna separata (Walker, 1865), revealed insecticidal activity against lepidopterans. Our data expand the knowledge of heteropteran salivary proteins and suggest predatory asopine bugs as a novel source for bioinsecticides.

Altered microbiomes of aquatic macroinvertebrates and riparian spiders downstream of municipal wastewater effluents

Municipal wastewater treatment plants (WWTPs) contain numerous contaminants, including antimicrobials, that could affect the composition of the beneficial bacterial communities associated with host aquatic organisms. There is also potential for these effects to transfer to terrestrial predators. Riparian spiders and five families of aquatic macroinvertebrates were collected from sites upstream and downstream of two WWTPs, Waterloo and Kitchener, discharging to the Grand River, Ontario, Canada. Whole-body microbiota were analyzed following the extraction, PCR amplification, and sequencing of bacterial DNA using the V3-V4 hypervariable regions of the 16S rRNA genetic barcode. Changes in the relative abundance of major microbiome phyla were observed in all downstream aquatic insects except Hydropsychidae caddisflies, which exhibited little site variation. Shannon alpha diversity differed among sites for Tetragnathidae spiders, Perlidae, Hydropsychidae, and Heptageniidae. Downstream of the Waterloo WWTP alpha diversity decreased in spiders, while downstream of the Kitchener WWTP this measure decreased in Perlidae and increased in spiders. Bray-Curtis beta diversity was dissimilar among sites in all invertebrate taxa; upstream sites differed from those downstream of Waterloo in spiders, Perlidae, and Hydropsychidae, and from those downstream of Kitchener in spiders, Perlidae, and Hydropsychidae. Finally, effluent-derived bacteria were found in the microbiomes of downstream spiders and aquatic insects and not upstream. Overall, results indicated that the microbiomes of invertebrates collected downstream differed from those collected upstream of WWTPs, which has implications for altered host health and transport of WWTP-derived bacteria through aquatic ecosystems.

Screening Candidate Effectors of the Bean Bug Riptortus pedestris by Proteomic and Transcriptomic Analyses

The damage of Riptortus pedestris is exceptional by leading soybean plants to keep green in late autumn. Identification of the salivary proteins is essential to understand how the pest-plant interaction occurs. Here, we have tried to identify them by a combination of proteomic and transcriptomic analyses. The transcriptomes of salivary glands from R. pedestris males, females and nymphs showed about 28,000 unigenes, in which about 40% had open reading frames (ORFs). Therefore, the predicted proteins in the transcriptomes with secretion signals were obtained. Many of the top 1,000 expressed transcripts were involved in protein biosynthesis and transport, suggesting that the salivary glands produce a rich repertoire of proteins. In addition, saliva of R. pedestris males, females and nymphs was collected and proteins inside were identified. In total, 155, 20, and 11 proteins were, respectively, found in their saliva. We have tested the tissue-specific expression of 68 genes that are likely to be effectors, either because they are homologs of reported effectors of other sap-feeding arthropods, or because they are within the top 1,000 expressed genes or found in the salivary proteomes. Their potential functions in regulating plant defenses were discussed. The datasets reported here represent the first step in identifying effectors of R. pedestris.

Extraoral digestion: outsourcing the role of the hemipteran midgut

Extraoral digestion allows for breakdown of dietary components before they reach the midgut for final enzymatic degradation and absorption. In the Hemiptera, this is achieved by the secretion of enzyme-rich fluids from the salivary gland, with the combination of protein and mRNA from these tissues termed the sialome. Separate channels within the hemipteran stylets allow for secretion of saliva and ingestion of predigested material in a non-reflux mechanism. Both feeding mode and diet type influence the composition of the hemipteran sialome, as illustrated by 1) differences in protease abundance between hematophagous and predatory heteropteran sialomes, 2) diet specific aminopeptidase-N genes among aphid biotypes, and 3) adaptation-induced sialome variation in related cicada populations. Despite challenges associated with incomplete genome annotation, -omics analysis of the sialomes of diverse hemipteran species will enhance understanding of both sialome function and the evolution of extraoral digestion within the order.

Establishment of a dietary exposure assay for evaluating the toxicity of insecticidal compounds to Apolygus lucorum (Hemiptera: Miridae)

With the commercialization of transgenic cotton that expresses Bt (Bacillus thuringiensis) insecticidal proteins, mirid bugs have become key pests in cotton and maize fields in China. Genetically engineered (GE) crops for controlling mirids are unavailable owing to a lack of suitable insecticidal genes. In this study, we developed and validated a dietary exposure assay for screening insecticidal compounds and for assessing the potential effects of insecticidal proteins produced by GE plants on Apolygus lucorum, one of the main mirid pests of Bt cotton and Bt maize. Diets containing potassium arsenate (PA) or the cysteine protease inhibitor E-64 were used as positive controls for validating the efficacy of the dietary exposure assay. The results showed that with increasing concentrations of PA or E-64, A. lucorum larval development time was prolonged and adult weight and fecundity were decreased, suggesting that the dietary exposure assay was useful for detecting the toxicity of insecticidal compounds to A. lucorum. This assay was then used to assess the toxicity of Cry1Ab, Cry1Ac, Cry1F, Cry2Aa, and Cry2Ab proteins, which have been transformed into several crops, against A. lucorum. The results showed that A. lucorum did not show a negative effect by feeding on an artificial diet containing any of the purified Cry proteins. No significant changes in the activities of digestive, detoxifying, or antioxidant enzymes were detected in A. lucorum that fed on a diet containing Cry proteins, but A. lucorum fitness was reduced when the insect fed on a diet containing E-64 or PA. These results demonstrate that A. lucorum is not sensitive to the tested Cry proteins and that the dietary exposure assay is useful for evaluating the toxicity of insecticidal compounds to this species.

Activities of Digestive Enzymes in the Omnivorous Pest Apolygus lucorum (Hemiptera: Miridae)

The mirid bug Apolygus lucorum Meyer-Dür, 1843, an omnivorous species that feeds on plants and animals, has become a major pest in China as production of Bt-cotton has grown to such a large scale. Its omnivory is likely to be critical for its success, but the digestive mechanism(s) underlying processing and adsorption of such diverse foods is relatively unknown. Here, we examined the activities of digestive enzymes of A. lucorum in the salivary gland complex and midgut and the effect of sex, age, and food source on these activities. Amylase and protease were present in the salivary gland complex and the midgut, but were higher in the salivary gland complex. Trypsin-like enzyme was also present in both organs, but chymotrypsin-like enzyme was present only in the midgut. Sex, age, and food source affected the activities of these digestive enzymes. In general, the activities of these enzymes peaked at 10 d after emergence, and amylase and protease activities were higher in female adults than in males. Of the food sources tested, green bean pods (Gb) induced the highest amylase activity, whereas Helicoverpa armigera Hübner, 1809 eggs (He) and a mixture of Gb and He induced higher activities of the trypsin-like and chymotrypsin-like enzymes. The results from food switching experiments confirmed that amylase activity could be induced by plant sources, and animal sources induced protease activity. Thus, the types and activities of digestive enzymes in A. lucorum provide the physiological basis of the pest's omnivory.

Venoms of Heteropteran Insects: A Treasure Trove of Diverse Pharmacological Toolkits

The piercing-sucking mouthparts of the true bugs (Insecta: Hemiptera: Heteroptera) have allowed diversification from a plant-feeding ancestor into a wide range of trophic strategies that include predation and blood-feeding. Crucial to the success of each of these strategies is the injection of venom. Here we review the current state of knowledge with regard to heteropteran venoms. Predaceous species produce venoms that induce rapid paralysis and liquefaction. These venoms are powerfully insecticidal, and may cause paralysis or death when injected into vertebrates. Disulfide-rich peptides, bioactive phospholipids, small molecules such as N,N-dimethylaniline and 1,2,5-trithiepane, and toxic enzymes such as phospholipase A2, have been reported in predatory venoms. However, the detailed composition and molecular targets of predatory venoms are largely unknown. In contrast, recent research into blood-feeding heteropterans has revealed the structure and function of many protein and non-protein components that facilitate acquisition of blood meals. Blood-feeding venoms lack paralytic or liquefying activity but instead are cocktails of pharmacological modulators that disable the host haemostatic systems simultaneously at multiple points. The multiple ways venom is used by heteropterans suggests that further study will reveal heteropteran venom components with a wide range of bioactivities that may be recruited for use as bioinsecticides, human therapeutics, and pharmacological tools.

The Effect of Protease Inhibitors on Digestive Proteolytic Activity in the Raspberry Weevil, Aegorhinus superciliosus (Guérin) (Coleoptera: Curculionidae)

The raspberry weevil, Aegorhinus superciliosus (Guérin) (Coleoptera: Curculionidae), is an economically important pest of blueberry in southern Chile. The digestive protease activity of adult insects was investigated using general and specific substrates and inhibitors. Enzymatic assays demonstrated the presence of trypsin- and chymotrypsin-like serine proteinases. Furthermore, in vitro assays using phenylmethylsulfonyl fluoride (PMSF) and soybean trypsin inhibitor (SBTI) at 0.01 and 0.1 mM showed percentages of enzymatic inhibition between 0 and 16% for PMSF and 67 to 76% for SBTI, whereas in vivo assays indicated that SBTI caused between 50 and 90% mortality in males and between 80 and 100% in females. Our data indicate the presence of serine proteases and suggest that digestive proteases could be a target for the design and development of strategies to control the raspberry weevil.

Prey size affects the costs and benefits of group predation in nymphs of the predatory stink bug Andrallus spinidens (Heteroptera: Pentatomidae)

Group predation promotes foraging efficiency because it increases the size of prey that can be killed and improves hunting success compared to solitary predation. However, group predation may increase competition among group members during feeding. Earlier studies have focused on the advantages of group predation, but little is known about the costs and benefits of group predation for individual members of the group. Here, we show that the costs and benefits of group predation for individuals of the predatory stink bug Andrallus spinidens vary with prey size in laboratory experiments. We found that when A. spinidens fed on small prey, group predation did not significantly increase foraging efficiency but did increase competition for food among group members. In contrast, when prey was large, group predation promoted foraging efficiency, and competition over food was not detected. Our results suggest that group predation by A. spinidens nymphs is advantageous for individual members because it enables each member to hunt larger prey that could not be hunted alone. However, when group size was large or prey size was small, group predation increased competition among group members.

Purification and characterization of midgut α-amylase in a predatory bug, Andralus spinidens

α-Amylases are widespread enzymes that catalyze endohydrolysis of long α-1,4-glucan chains such as starch and glycogen. The highest amylolytic activity was found in 5th instar nymphs and midgut of the predatory bug, Andrallus spinidens F. (Hemiptera: Pentatomidae). The α-amylase was purified following a three-step procedure. The purified α-amylase had a specific activity of 13.46 U/mg protein, recovery of 4.21, purification fold of 13.87, and molecular weight of 21.3 kDa. The enzyme had optimal pH and temperature of 7 and 45°C, respectively. Na+, Mn+, Mg2+, and Zn2+ significantly decreased activity of the purified α-amylase, but some concentrations of K+, Ca2+, and Cu2+ had the opposite effect. EDTA, EGTA, and DTC significantly decreased enzymatic activity, showing the presence of metal ions in the catalytic site of the enzyme. Kinetic parameters of the purified α-amylase showed a Km of 3.71% in starch and 4.96% for glycogen, suggesting that the enzyme had a higher affinity for starch.

Immune response of Chilo suppressalis Walker (Lepidoptera: Crambidae) larvae to different entomopathogenic fungi

The current study reports mortality and effects on cellular immune response of several entomopathogenic fungi including isoleates BB1, BB2 and BB3 of Beauveria bassiana, Metarhizium anisopliae, Isaria fumosoroseus and Lecanicilium lecanii against larvae of Chilo suppressalis. Prohemocytes, granulocytes, plasmatocytes and oenocytoids were identified as the main circulating hemocytes in the hemolymph of larvae using Giemsa staining solution. Entomopathogenic fungi caused differential mortality on larvae: BB1, BB3, M. anisopliae lead to the highest mortality on larvae and L. lecanii caused the lowest mortality. The highest numbers of total hemocytes were observed 3 h post-injection of B. bassiana isolates and 6 h for the other treatments. The highest numbers of plasmatocytes were observed 3 h post-injection of BB1 and Tween 80, whereas BB2, BB3, M. anisopliae, I. fumosoroseus and L. lecani caused plasmatocyte increase 6 h post-injection. Similar results were obtained in case of granulocytes but only Tween 80 showed the highest number of hemocytes 3 h post-injection. The highest numbers of nodules were found at various time intervals after injection of fungal isolates and latex bead. The highest activities of phenoloxidase were observed 12 h post-injection by BbB1, BbB3, M. anisopliae and latex bead; 3-6 h post-injection by BbB2, 6 h post-injection by I. fumosoroseus and 3-6 h post-injection by L. lecanii. Our data demonstrate the possibility of utilizing different fungal extracts in the field to help reduce the risk of resistance evolution in C. suppressalis and encourage experimentations aimed to increase the number of biological control agent for insect pests such as the striped rice stem borer C. suppressalis.

Digestion in adult females of the leaf-footed bug Leptoglossus zonatus (Hemiptera: Coreidae) with emphasis on the glycoside hydrolases α-amylase, α-galactosidase, and α-glucosidase

The leaffooted bug, Leptoglossus zonatus (Hemiptera: Coreidae) is an emerging pest of several crops around the World and up to now very little is known of its digestive system. In this article, glycoside hydrolase (carbohydrase) activities in the adult midgut cells and in the luminal contents of L. zonatus adult females were studied. The results showed the distribution of digestive carbohydrases in adults of this heteropteran species in the different intestinal compartments. Determination of the spatial distribution of α-glucosidase activity in L. zonatus midgut showed only one major molecular form, which was not equally distributed between soluble and membrane-bound isoforms, being more abundant as a membrane-bound enzyme. The majority of digestive carbohydrases were found in the soluble fractions. Activities against starch, maltose and the synthetic substrate NPαGlu were found to show the highest levels of activity, followed by enzymes active against galactosyl oligosaccharides. Based on ion-exchange chromatography elution profiles and banding patterns in mildly denaturing electrophoresis, both midgut α-amylases and α-galactosidases showed at least two isoforms. The data suggested that the majority of carbohydrases involved in initial digestion were present in the midgut lumen, whereas final digestion of starch and of galactosyl oligosaccharides takes place partially within the lumen and partially at the cell surface. The complex of carbohydrases here described was qualitatively appropriate for the digestion of free oligosaccharides and oligomaltodextrins released by α-amylases acting on maize seed starch granules.

A trypsin-like proteinase in the midgut of Ectomyelois ceratoniae Zeller (Lepidoptera: Pyralidae): purification, characterization, and host plant inhibitors

A trypsin-like proteinase was purified and characterized in the midgut of Ectomyelois ceratoniae. A purification process that used Sepharyl G-100 and DEAE-cellulose fast flow chromatographies revealed a proteinase with specific activity of 66.7 μmol/min/mg protein, recovery of 27.04 and purification fold of 23.35. Molecular weight of the purified protein was found to be 35.8 kDa. Optimal pH and temperature were obtained 9 and 20°C for the purified trypsin proteinase, respectively. The purified enzyme was significantly inhibited by PMSF, TLCK, and SBTI as specific inhibitors of trypsins in which TLCK showed the highest inhibitory effect. Trypsin proteinase inhibitors were extracted from four varieties of pomegranate including Brait, Torsh-Sabz, May-Khosh, and Shirin by ion exchange chromatography. It was found that fractions 17-20 of Brait; fractions 18 and 21-26 of Torsh-Sabz; fractions 1-7, 11-17, and 19-21 of May-Khosh and fraction 8 for Shirin showed presence of trypsin inhibitor in these host. Comparison of their inhibitory effects on the purified trypsin proteinase of E. ceratoniae demonstrated that fractions from May-khosh variety had the highest effect on the enzyme among other extracted fractions. Characterization of serine proteinases of insects mainly trypsins is one of the promising methods to decrease population and damages via extracting their inhibitors and providing resistant varieties.