An IMD-like pathway mediates the intestinal immunity to modulate the homeostasis of gut microbiota in Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae)

Gut bacterium Burkholderia cepacia (BsNLG8) and immune gene Defensin A contribute to the resistance against Nicotine-induced stress in Nilaparvata lugens (Stål)

Nicotine, a naturally occurring alkaloid found in tobacco, is a potent neurotoxin extensively used to control Nilaparvata lugens (Stål), a destructive insect pest of rice crops. The insect gut harbors a wide array of resident microorganisms that profoundly influence several biological processes, including host immunity. Maintaining an optimal gut microbiota and immune homeostasis requires a complex network of reciprocal regulatory interactions. However, the underlying molecular mechanisms driving these symbiotic exchanges, particularly between specific gut microbe and immunity, remain largely unknown in insects. Our previous investigations identified and isolated a nicotine-degrading Burkholderia cepacia strain (BsNLG8) with antifungal properties. Building on those findings, we found that nicotine intake significantly increased the abundance of a symbiotic bacteria BsNLG8, induced a stronger bacteriostatic effect in hemolymph, and enhanced the nicotine tolerance of N. lugens. Additionally, nicotine-induced antimicrobial peptides (AMPs) exhibited significant antibacterial effects against Staphylococcus aureus. We adopted RNA-seq to explore the underlying immunological mechanisms in nicotine-stressed N. lugens. Bioinformatic analyses identified numerous differentially expressed immune genes, including recognition/immune activation (GRPs and Toll) and AMPs (i.e., Defensin, Lugensin, lysozyme). Temporal expression profiling (12, 24, and 48 hours) of immune genes revealed pattern recognition proteins and immune effectors as primary responders to nicotine-induced stress. Defensin A, a broad-spectrum immunomodulatory cationic peptide, exhibited significantly high expression. RNA interference-mediated silencing of Defensin A reduced the survival, enhanced nicotine sensitivity of N. lugens to nicotine, and decreased the abundance of BsNLG8. The reintroduction of BsNLG8 improved the expression of immune genes, aiding nicotine resistance of N. lugens. Our findings indicate a potential reciprocal immunomodulatory interaction between Defensin A and BsNLG8 under nicotine stress. Moreover, this study offers novel and valuable insights for future research into enhancing nicotine-based pest management programs and developing alternative biocontrol methods involving the implication of insect symbionts.

Relish involved in immunity and larval survival in the willow leaf beetle Plagiodera versicolora

BACKGROUND

Insects mainly rely on innate immunity against pathogen infection. Plagiodera versicolora (Coleoptera: Chrysomelidae), is a worldwide leaf-eating forest pest in salicaceous trees. However, the mechanisms behind the immunodeficiency pathway (IMD) remain poorly understood.

RESULTS

In this study, we obtained a Relish gene from transcriptome analysis. Tissue and instar expression profiles were subsequently obtained using quantitative real-time polymerase chain reaction analysis. The results showed that Relish has high expression levels in eggs, larvae and adults, and especially in fat bodies. Transcripts of the tested antimicrobial peptides (AMPs), defensin1, defensin2 and attacin2 were downregulated by dsRelish. Knockdown of Relish led to greater mortality in larvae after Staphylococcus aureus infection. In addition, we performed bacterial 16S ribosomal RNA-based high-throughput sequencing. The results showed that the relative abundance of some gut bacteria was significantly altered after dsRelish ingestion.

CONCLUSION

This study provides a greater understanding of the IMD signaling pathway, facilitating functional studies of Relish in P. versicolora. Moreover, a genetic pest management technique might be developed using Relish as a lethal gene to control the pest P. versicolora. © 2024 Society of Chemical Industry.

Host plant recognition by two odorant-binding proteins in Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

BACKGROUND

Rhynchophorus ferrugineus, the red palm weevil (RPW), is a key pest that attacks many economically important palm species and that has evolved a sensitive and specific olfactory system to seek palm hosts. Odorant-binding proteins (OBPs) not only play crucial roles in its olfactory perception process but are also important molecular targets for the development of new approaches for pest management.

RESULTS

Analysis of the tissue expression profiles of RferOBP8 and RferOBP11 revealed that these two Rhynchophorus ferrugineus odorant binding proteins (RferOBPs) exhibited high expression in the antennae and showed sexual dimorphism. We analyzed the volatiles of seven host plants by gas chromatography-mass spectrometry and screened 13 potential ligands by molecular docking. The binding affinity of two recombinant OBPs to aggregation pheromones and 13 palm odorants was tested by fluorescence competitive binding assays. The results revealed that eight tested palm volatiles and ferrugineol have high binding affinities with RferOBP8 or RferOBP11. Behavioral trials showed that these eight odor compounds could elicit an attraction response in adult RPW. RNA interference analysis indicated that the reduction in the expression levels of the two RferOBPs led to a decrease in behavioral responses to these volatiles.

CONCLUSION

These results suggest that RferOBP8 and RferOBP11 are involved in mediating the responses of RPW to palm volatiles and to aggregation pheromones and may play important roles in RPW host-seeking. This study also provides a theoretical foundation for the promising application of novel molecular targets in the development of new behavioral interference strategies for RPW management in the future. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

A peptidoglycan recognition protein regulates the immune response of Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae) during exposure to pathogenic Gram-positive bacteria and fungi

Red palm weevil (RPW), Rhynchophorus ferrugineus Olivier, is a tremendously destructive insect pest of palm trees worldwide. Although some biological agents have been used to fight against RPW larvae, the control efficiency is still dissatisfactory. This study aimed to determine the role of a peptidoglycan recognition protein (PGRP), RfPGRP-S3, in RPW immunity. RfPGRP-S3 is a secreted protein with a DF (Asp⁸⁵-Phe⁸⁶) motif, implying that it can discriminate Gram-positive bacteria. The abundance of RfPGRP-S3 transcripts in the hemolymph was significantly higher than that in other tissues. The expression of RfPGRP-S3 can be markedly induced by challenge with Staphylococcus aureus and Beauveria bassiana. After RfPGRP-S3 was silenced, the ability of individuals to clear the pathogenic bacteria in the body cavity and gut was significantly compromised. Furthermore, silencing RfPGRP-S3 dramatically impaired the survival rate of RPW larvae upon challenge with S. aureus. RT‒qPCR revealed that the expression levels of RfDefensin in the fat body and gut were decreased by RfPGRP-S3 silencing. Taken together, these results demonstrated that RfPGRP-S3 acts as a circulating receptor to promote the expression of the antimicrobial peptide gene upon the discrimination of pathogenic microbes.

The Intestinal Immune Defense System in Insects

Over a long period of evolution, insects have developed unique intestinal defenses against invasion by foreign microorganisms, including physical defenses and immune responses. The physical defenses of the insect gut consist mainly of the peritrophic matrix (PM) and mucus layer, which are the first barriers to pathogens. Gut microbes also prevent the colonization of pathogens. Importantly, the immune-deficiency (Imd) pathways produce antimicrobial peptides to eliminate pathogens; mechanisms related to reactive oxygen species are another important pathway for insect intestinal immunity. The janus kinase/STAT signaling pathway is involved in intestinal immunity by producing bactericidal substances and regulating tissue repair. Melanization can produce many bactericidal active substances into the intestine; meanwhile, there are multiple responses in the intestine to fight against viral and parasitic infections. Furthermore, intestinal stem cells (ISCs) are also indispensable in intestinal immunity. Only the coordinated combination of the intestinal immune defense system and intestinal tissue renewal can effectively defend against pathogenic microorganisms.

The IMD and Toll canonical immune pathways of Triatoma pallidipennis are preferentially activated by Gram-negative and Gram-positive bacteria, respectively, but cross-activation also occurs

BACKGROUND

Antimicrobial peptides (AMPs) participate in the humoral immune response of insects eliminating invasive microorganisms. The immune deficiency pathway (IMD) and Toll are the main pathways by which the synthesis of these molecules is regulated in response to Gram-negative (IMD pathway) or Gram-positive (Toll pathway) bacteria. Various pattern-recognition receptors (PRRs) participate in the recognition of microorganisms, such as pgrp-lc and toll, which trigger signaling cascades and activate NF-κB family transcription factors, such as relish, that translocate to the cell nucleus, mainly in the fat body, inducing AMP gene transcription.

METHODS

T. pallidipennis inhibited in Tppgrp-lc, Tptoll, and Tprelish were challenged with E. coli and M. luteus to analyze the expression of AMPs transcripts in the fat body and to execute survival assays.

RESULTS

In this work we investigated the participation of the pgrp-lc and toll receptor genes and the relish transcription factor (designated as Tppgrp-lc, Tptoll, and Tprelish), in the transcriptional regulation of defensin B, prolixicin, and lysozyme B in Triatoma pallidipennis, one of the main vectors of Chagas disease. AMP transcript abundance was higher in the fat body of blood-fed than non-fed bugs. Challenge with Escherichia coli or Micrococcus luteus induced differential increases in AMP transcripts. Additionally, silencing of Tppgrp-lc, Tptoll, and Tprelish resulted in reduced AMP transcription and survival of bugs after a bacterial challenge.

CONCLUSIONS

Our findings demonstrated that the IMD and Toll pathways in T. pallidipennis preferentially respond to Gram-negative and Gram-positive bacteria, respectively, by increasing the expression of AMP transcripts, but cross-induction also occurs.

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.

Topical Fungal Infection Induces Shifts in the Gut Microbiota Structure of Brown Planthopper, Nilaparvata lugens (Homoptera: Delphacidae)

Simple Summary

Fungal entomopathogens are important natural enemies of insect pests and widely applied for biocontrol. Gut microbiota play important roles in mediating insect physiology and behavior. There is growing evidence that alteration of gut microbial communities due to pathological and environmental exposure can have detrimental impacts on host health and pathogen resistance. Here, we investigated the effects of topical infection with Metarhizium anisopliae fungus on the gut microbial community structure of the brown planthopper (Nilaparvata lugens, BPH), a destructive insect pest of rice. Our results demonstrated dramatic changes of gut bacterial community structure after topical fungal infection in BPH, as indicated by a significant increase in bacterial load, a significant decrease in bacterial community evenness and significant shifts in dominant bacterial abundance at the taxonomic level below the class. The dysbiosis of the gut bacteria might partly be due to the suppression of gut immunity caused by topical fungal infection. Our results highlighted the importance of the gut microbial community in fungal pathogenesis in insects.

Abstract

The brown planthopper (Nilaparvata lugens, BPH) is a destructive insect pest posing a serious threat to rice production. The fungal entomopathogen Metarhizium anisopliae is a promising alternative that can be used for BPH biocontrol. Recent studies have highlighted the significant involvement of gut microbiota in the insect–fungus interactions. In the presented study, we investigated the effects of topical fungal infection on the gut microbial community structure in BPH. Our results revealed that topical infection with M. anisopliae increased the bacterial load and altered the bacterial community structure in the gut of BPH. The relative abundances of the dominant gut bacteria at the order, family and genus level were significantly different between fungus-infected and uninfected groups. At the genus level, the uninfected BPH harbored high proportions of Pantoea and Enterobacter in the gut, whereas the fungus-infected BPH gut was absolutely dominated by Acinetobacter. Moreover, topical fungal infection significantly inhibited the expressions of immune-related genes encoding anti-microbial protein and dual oxidase that were involved in the maintenance of gut microbiota homeostasis, indicating that gut bacteria imbalance might be attributed in part to the suppression of gut immunity caused by fungal pathogen. Our results highlighted the importance of the gut microbial community during interactions between fungal pathogens and insect hosts.

Current understanding and perspectives on the potential mechanisms of immune priming in beetles

Beetles are the most diverse group of insects in Insecta which can be found in almost every habitat and environment on Earth. The possessing of the rapid and effective immune defenses is one of the important factors for their success. It is generally recognized that beetles only rely on the non-specific innate immune defense, without immunological memory, to fight against pathogens. However, there was cumulative evidence for the innate immune memory in invertebrates, including beetles, over the last decades, implying that insect innate immunity is more complex and has more features than previously thought. In beetles, it has been well documented that the specific or nonspecific enhanced immunocompetence can persist throughout development within generations and can even be transferred to the descendents in the next generation. Although insect immune priming might be shaped by epigenetic modifications and transferring effectors, mRNA and microbial signals, the solid experimental evidence to support the causal relationship between any of them and immune priming is still scarce. The combined usage of 'omics' approaches and CRISPR/Cas9 in the appropriate insect models with well-known genetic background, Tribolium castaneum and Tenebrio molitor, will help us to decipher the molecular mechanisms by which immune priming occurs in beetles in depth.

Increasing Anthonomus grandis susceptibility to Metarhizium anisopliae through RNAi-induced AgraRelish knockdown: a perspective to combine biocontrol and biotechnology

BACKGROUND

The hemolymph and insect gut together have an essential role in the immune defense against microorganisms, including the production of antimicrobial peptides (AMP). AMPs are mainly induced by two specific signaling pathways, Toll and immune deficiency (IMD). Here, we characterize the expression profile of four genes from both pathways and describe the importance of AgraRelish in the immune defense of Anthonomus grandis against the entomopathogenic fungus Metarhizium anisopliae by RNA interference (RNAi).

RESULTS

To characterize the pathway that is activated early during the A. grandis-M. anisopliae interaction, we assessed the expression profiles of AgraMyD88 and AgraDorsal (Toll pathway), AgraIMD and AgraRelish (IMD pathway), and several AMP genes. Interestingly, we found that IMD pathway genes are upregulated early, and Toll pathway genes are upregulated just 3 days after inoculation (DAI). Furthermore, nine AMPs were upregulated 24 h after fungus inoculation, including attacins, cecropins, coleoptericins, and defensins. AgraRelish knockdown resulted in a reduction in median lethal time (LT₅₀ ) for M. anisopliae-treated insects of around 2 days compared to control treatments. In addition, AgraRelish remained knocked down at 3 DAI. Finally, we identified that AgraRelish knockdown increased fungal loads at 2 DAI compared to control treatments, possibly indicating a faster infection.

CONCLUSIONS

Our data indicate the influence of the IMD pathway on the antifungal response in A. grandis. Combining biocontrol and RNAi could significantly improve cotton boll weevil management. Hence, AgraRelish is a potential target for the development of biotechnological tools aimed at improving the efficacy of M. anisopliae against A. grandis.

The Effect of Gut Bacteria on the Physiology of Red Palm Weevil, Rhynchophorus ferrugineus Olivier and Their Potential for the Control of This Pest

Red Palm Weevil (RPW), Rhynchophorus ferrugineus Olivier, is a notorious pest, which infests palm trees and has caused great economic losses worldwide. At present, insecticide applications are still the main way to control this pest. However, pesticide resistance has been detected in the field populations of RPW. Thus, future management strategies based on the novel association biological control need be developed. Recent studies have shown that the intestinal tract of RPW is often colonized by multiple microbial species as mammals and model insects, and gut bacteria have been found to promote the growth, development and immune activity of RPW larvae by modulating nutrient metabolism. Furthermore, two peptidoglycan recognition proteins (PGRPs), PGRP-LB and PGRP-S1, can act as the negative regulators to modulate the intestinal immunity to maintain the homeostasis of gut bacteria in RPW larvae. Here, we summarized the current knowledge on the gut bacterial composition of RPW and their impact on the physiological traits of RPW larvae. In contrast with metazoans, it is much easier to make genetic engineered microbes to produce some active molecules against pests. From this perspective, because of the profound effects of gut bacteria on host phenotypes, it is promising to dissect the molecular mechanisms behind their effect on host physiology and facilitate the development of microbial resource-based management methods for pest control.

Regulatory mechanisms of microbial homeostasis in insect gut

Insects live in incredibly complex environments. The intestinal epithelium of insects is in constant contact with microorganisms, some of which are beneficial and some harmful to the host. Insect gut health and function are maintained through multidimensional mechanisms that can proficiently remove foreign pathogenic microorganisms while effectively maintaining local symbiotic microbial homeostasis. The basic immune mechanisms of the insect gut, such as the dual oxidase-reactive oxygen species (Duox-ROS) system and the immune deficiency (Imd)-signaling pathway, are involved in the maintenance of microbial homeostasis. This paper reviews the role of physical defenses, the Duox-ROS and Imd signaling pathways, the Janus kinase/signal transducers and activators of transcription signaling pathway, and intestinal symbiotic flora in the homeostatic maintenance of the insect gut microbiome.

Formicine ants swallow their highly acidic poison for gut microbial selection and control

Animals continuously encounter microorganisms that are essential for health or cause disease. They are thus challenged to control harmful microbes while allowing the acquisition of beneficial microbes. This challenge is likely especially important for social insects with respect to microbes in food, as they often store food and exchange food among colony members. Here we show that formicine ants actively swallow their antimicrobial, highly acidic poison gland secretion. The ensuing acidic environment in the stomach, the crop, can limit the establishment of pathogenic and opportunistic microbes ingested with food and improve the survival of ants when faced with pathogen contaminated food. At the same time, crop acidity selectively allows acquisition and colonization by Acetobacteraceae, known bacterial gut associates of formicine ants. This suggests that swallowing of the poison in formicine ants acts as a microbial filter and that antimicrobials have a potentially widespread but so far underappreciated dual role in host-microbe interactions.

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.

Complex Relationships at the Intersection of Insect Gut Microbiomes and Plant Defenses

Insect herbivores have ubiquitous associations with microorganisms that have major effects on how host insects may interact in their environment. Recently, increased attention has been given to how insect gut microbiomes mediate interactions with plants. In this paper, I discuss the ecology and physiology of gut bacteria associated with insect herbivores and how they may shape interactions between insects and their various host plants. I first establish how microbial associations vary between insects with different feeding styles, and how the insect host physiology and ecology can shape stable or transient relationships with gut bacteria. Then, I describe how these relationships factor in with plant nutrition and plant defenses. Within this framework, I suggest that many of the interactions between plants, insects, and the gut microbiome are context-dependent and shaped by the type of defense and the isolates present in the environment. Relationships between insects and plants are not pairwise, but instead highly multipartite, and the interweaving of complex microbial interactions is needed to fully explore the context-dependent aspects of the gut microbiome in many of these systems. I conclude the review by suggesting studies that would help reduce the unsureness of microbial interactions with less-defined herbivore systems and identify how each could provide a path to more robust roles and traits.

Spätzle Homolog-Mediated Toll-Like Pathway Regulates Innate Immune Responses to Maintain the Homeostasis of Gut Microbiota in the Red Palm Weevil, Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae)

Spätzle (Spz) is a dimeric ligand that responds to the Gram-positive bacterial or fungal infection by binding Toll receptors to induce the secretion of antimicrobial peptides. However, whether the Toll-like signaling pathway mediates the innate immunity of Rhynchophorus ferrugineus to modulate the homeostasis of gut microbiota has not been determined. In this study, we found that a Spz homolog, RfSpätzle, is a secretory protein comprising a signal peptide and a conservative Spz domain. RT-qPCR analysis revealed that RfSpätzle was significantly induced to be expressed in the fat body and gut by the systemic and oral infection with pathogenic microbes. The expression levels of two antimicrobial peptide genes, RfColeoptericin and RfCecropin, were downregulated significantly by RfSpätzle knockdown, indicating that their secretion is under the regulation of the RfSpätzle-mediated signaling pathway. After being challenged by pathogenic microbes, the cumulative mortality rate of RfSpätzle-silenced individuals was drastically increased as compared to that of the controls. Further analysis indicated that these larvae possessed the diminished antibacterial activity. Moreover, RfSpätzle knockdown altered the relative abundance of gut bacteria at the phylum and family levels. Taken together, these findings suggest that RfSpätzle is involved in RPW immunity to confer protection and maintain the homeostasis of gut microbiota by mediating the production of antimicrobial peptides.

Two relish isoforms produced by alternative splicing participate in the regulation of antimicrobial peptides expression in Procambarus clarkii intestine

Nuclear factor κB (NF-κB) plays a key role in the innate immunity of invertebrates. Relish belongs to the NF-κB family. In insects, alternative splicing induces the sequence diversity of the Relish gene. However, information on the roles of various relish isoforms in crustacean innate immune response is limited. Here, two alternatively spliced Relish isoforms (designated as SPcRelish and LPcRelish) were identified from freshwater crayfish (Procambarus clarkii), and functional analysis was performed. The Relish gene has 25 exons and 24 introns. The long isoform LPcRelish is fully spliced, whereas the short isoform SPcRelish is alternatively spliced and contains exon 1-9 and a retention of intron 9. LPcRelish contains the Rel homology domain (RHD), the ig-like, plexins, transcription factors (IPT), and ankyrin-repeat (ANK) inhibitory domain. However, SPcRelish contains only the RHD and IPT domain, and does not have an ANK domain. The transcripts of SPcRelish and LPcRelish can be regulated by Vibrio parahaemolyticus. The intestinal immunological barrier and bacterial balance in the intestine play crucial roles in host health. In this study, we analyzed the connection between Relish isoforms and the transcripts of antimicrobial peptides (AMPs) in intestine. The transcripts of all the tested AMPs, except ALF-41125, were upregulated by V. parahaemolyticus. The knock down of the SPcRelish gene resulted in a significant decrease in the expression levels of ALF-7032, ALF-13162, and Crustin-42012 during V. parahaemolyticus invasion. The expression levels of four AMP genes (ALF-41125, ALF-42430, Crustin-41354, and Crustin-42993) were obviously increased in V. parahaemolyticus-challenged SPcRelish-silenced crayfish. ALF-7032, ALF-9228, ALF-13162, ALF-42430, Crustin-41354, Crustin-42012, and Crustin-42993 were evidently downregulated in V. parahaemolyticus-infected LPcRelish-silenced crayfish. Overall, generating the two Relish isoforms by alternative splicing may be an important mechanism of the host immune system to promote molecular diversity, which results in the functional diversity of the relish transcription factor.

Neuropeptides and G-Protein Coupled Receptors (GPCRs) in the Red Palm Weevil Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae)

The red palm weevil Rhynchophorus ferrugineus is a devastating, invasive pest that causes serious damages to palm trees, and its invasiveness depends on its strong ability of physiological and behavioral adaptability. Neuropeptides and their receptors regulate physiology and behavior of insects, but these protein partners have not been identified from many insects. Here, we systematically identified neuropeptide precursors and the corresponding receptors in the red palm weevil, and analyzed their tissue expression patterns under control conditions and after pathogen infection. A total of 43 putative neuropeptide precursors were identified, including an extra myosuppressin peptide was identified with amino acid substitutions at two conserved sites. Forty-four putative neuropeptide receptors belonging to three classes were also identified, in which neuropeptide F receptors and insulin receptors were expanded compared to those in other insects. Based on qRT-PCR analyses, genes coding for several neuropeptide precursors and receptors were highly expressed in tissues other than the nervous system, suggesting that these neuropeptides and receptors play other roles in addition to neuro-reception. Some of the neuropeptides and receptors, like the tachykinin-related peptide and receptor, were significantly induced by pathogen infection, especially sensitive to Bacillus thuringiensis and Metarhizium anisopliae. Systemic identification and initial characterization of neuropeptides and their receptors in the red palm weevil provide a framework for further studies to reveal the functions of these ligand- and receptor-couples in regulating physiology, behavior, and immunity in this important insect pest species.

Preparation of Red Palm Weevil Rhynchophorus Ferrugineus (Olivier) (Coleoptera: Dryophthoridae) Germ-free Larvae for Host-gut Microbes Interaction Studies

Red palm weevil (RPW), Rhynchophorus ferrugineus Olivier, is a devastating pest of palm trees worldwide. RPW gut is colonized by diverse bacterial species which profoundly influence host development and nutritional metabolism. However, the molecular mechanisms behind the interactions between RPW and its gut microbiota remain mostly unknown. Antibiotics are usually employed to remove gut bacteria to investigate the impact of gut bacteria on insect fitness. However, administration of antibiotics cannot thoroughly remove gut bacteria for most insect species. Therefore, establishing germfree (GF) organisms is a powerful way to reveal the mutual interactions between gut bacteria and their insect hosts. Here, we describe a protocol to generate and maintain RPW GF larvae, being completely devoid of gut bacteria in laboratory. RPW GF larvae were established from the dechorionated fresh eggs which were reared on the sterilized artificial food under axenic conditions. The establishment of GF larvae set a solid foundation to deeply elucidate the molecular mechanisms behind the interactions between RPW and its gut microbiota.

Intestinal Microbiota Confer Protection by Priming the Immune System of Red Palm Weevil Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae)

The immune system of animals, including insects, is the vital factor to maintain the symbiotic interactions between animals and their associated microbes. However, the effects of gut microbiota on insect immunity remain mostly elusive. Red palm weevil (RPW), Rhynchophorus ferrugineus Olivier, is a destructive pest of palm trees worldwide, which has forged alliances with its gut microbiota. Here, we found that the aposymbiotic insects succumbed at a significantly faster rate than conventionally reared (CR) ones upon bacterial infection. Physiological assays confirmed that CR insects had stronger antimicrobial activity and higher phenoloxidase activity in contrast to germfree (GF) ones, indicating that the systemic immune responses of GF individuals were compromised markedly. Interestingly, under the bacterial challenge conditions, the reassociation of gut microbiota with GF insects could enhance their survival rate by rescuing their immunocompetence. Furthermore, comparative transcriptome analysis uncovered that 35 immune-related genes, including pathogen recognition receptors, effectors and immune signaling pathway, were significantly downregulated in GF insects as compared to CR ones. Collectively, our findings corrobate that intestinal commensal bacteria have profound immunostimulatory effects on RPW larvae. Therefore, knowledge on the effects of gut microbiota on RPW immune defenses may contribute to of set up efficient control strategies of this pest.

The Promoting Effect of Gut Microbiota on Growth and Development of Red Palm Weevil, Rhynchophorus ferrugineus (Olivier) (Coleoptera: Dryophthoridae) by Modulating Its Nutritional Metabolism

Red palm weevil (RPW), Rhynchophorus ferrugineus Olivier, is a destructive pest for palm trees worldwide. Recent studies have shown that RPW gut is colonized by microbes and alterations in gut microbiota can significantly modify its hemolymph nutrition content. However, the exact effects of gut microbiota on RPW phenotype and the underlying mechanisms remain elusive. Here germ-free (GF) RPW larvae were generated from dechorionated eggs which were reared on sterilized artificial food under axenic conditions. Compared with controls, the larval development of GF RPW individuals was markedly depressed and their body mass was reduced as well. Furthermore, the content of hemolymph protein, glucose and triglyceride were dropped significantly in GF RPW larvae. Interestingly, introducing gut microbiota into GF individuals could significantly increase the levels of the three nutrition indices. Additionally, it has also been demonstrated that RPW larvae monoassociated with Lactococcus lactis exhibited the same level of protein content with the CR (conventionally reared) insects while feeding Enterobacter cloacae to GF larvae increased their hemolymph triglyceride and glucose content markedly. Consequently, our findings suggest that gut microbiota profoundly affect the development of this pest by regulating its nutrition metabolism and different gut bacterial species show distinct impact on host physiology. Taken together, the establishment of GF and gnotobiotic RPW larvae will advance the elucidation of molecular mechanisms behind the interactions between RPW and its gut microbiota.