Differential responses of the antennal proteome of male and female migratory locusts to infection by a fungal pathogen

Odorant binding protein 18 increases the pathogen resistance of the imported willow leaf beetle, Plagiodera versicolora

Background

Insect odorant-binding proteins (OBPs) are a class of small molecular weight soluble proteins. In the past few years, OBPs had been found to work as carriers of ligands and play a crucial role in olfaction and various other physiological processes, like immunity. A subset of insect OBPs had been found to be expressed differently and play a function in immunity of fungal infection. However, there are few studies on the role of OBPs in immunity of bacterial infection.

Methods

To identify the immune-related OBPs of Plagiodera versicolora after infected by Pseudomonas aeruginosa, we determined the mortality of P. versicolora to P. aeruginosa and selected the time point of 50% mortality of larvae to collect samples for RNA-seq. RNAi technology was used to investigate the function of immune-related OBPs after P. aeruginosa infection.

Results

RNA-seq data shows that PverOBP18 gene significantly up-regulated by 1.8-fold and further RT-qPCR affirmed its expression. Developmental expression profile showed that the expression of PverOBP18 was highest in the pupae, followed by the female adults, and lower in the 1st-3rd larvae and male adults with lowest in eggs. Tissue expression profiling showed that PverOBP18 was dominantly expressed in the epidermis. RNAi knockdown of PverOBP18 significantly reduced the expression of bacterial recognition receptor gene PGRP and antibacterial peptide gene Attacin and reduced the resistance of P. versicolora to P. aeruginosa infection.

Conclusion

Our results indicated that PverOBP18 gene increased the pathogen resistance of P. versicolora by cooperating with the immune genes and provided valuable insights into using OBPs as targets to design novel strategies for management of P. versicolora.

Cross-talk between immunity and behavior: insights from entomopathogenic fungi and their insect hosts

Insects are one of the most successful animals in nature, and entomopathogenic fungi play a significant role in the natural epizootic control of insect populations in many ecosystems. The interaction between insects and entomopathogenic fungi has continuously coevolved over hundreds of millions of years. Many components of the insect innate immune responses against fungal infection are conserved across phyla. Additionally, behavioral responses, which include avoidance, grooming, and/or modulation of body temperature, have been recognized as important mechanisms for opposing fungal pathogens. In an effort to investigate possible cross-talk and mediating mechanisms between these fundamental biological processes, recent studies have integrated and/or explored immune and behavioral responses. Current information indicates that during discrete stages of fungal infection, several insect behavioral and immune responses are altered simultaneously, suggesting important connections between the two systems. This review synthesizes recent advances in our understanding of the physiological and molecular aspects influencing cross-talk between behavioral and innate immune antifungal reactions, including chemical perception and olfactory pathways.

An insect chemosensory protein facilitates locust avoidance to fungal pathogens via recognition of fungal volatiles

Locusts (Locusta migratoria) are one of the most destructive insect pests worldwide. Entomopathogenic fungi can infect and kill locusts, with Metarhizium acridum having evolved as a specialized acridid pathogen. However, locusts have evolved countermeasures to limit or avoid microbial pathogens, although the underlying molecular mechanisms behind these defenses remain obscure. Here, we demonstrate that L. migratoria exhibit avoidance behaviors towards M. acridum contaminated food via recognition of fungal volatiles, with locust perception of the volatile mediated by the LmigCSP60 chemosensory protein. RNAi-knockdown of LmigCSP60 lowered locust M. acridum avoidance behavior and increased infection and mortality. The fungal volatile, 2-phenylethanol (PEA), was identified to participate in locust behavioral avoidance. RNAi-knockdown of LmigCSP60 reduced antennal electrophysiological responses to PEA and impaired locust avoidance to the compound. Purified LmigCSP60 was able to bind a set of fungal volatiles including PEA. Furthermore, reduction of PEA emission by M. acridum via construction of a targeted gene knockout mutant of the alcohol dehydrogenase gene (ΔMaAdh strain) that contributes to PEA production reduced locust avoidance behavior towards the pathogen. These findings identify an olfactory circuit used by locusts to detect and avoid potential microbial pathogens before they are capable of initiating infection and highlight behavioral and olfactory adaptations affecting the co-evolution of host-pathogen interactions.

Influences of Microbial Symbionts on Chemoreception of Their Insect Hosts

Chemical communication is widespread among insects and exploited to adjust their behavior, such as food and habitat seeking and preferences, recruitment, defense, and mate attraction. Recently, many studies have revealed that microbial symbionts could regulate host chemical communication by affecting the synthesis and perception of insect semiochemicals. In this paper, we review recent studies of the influence of microbial symbionts on insect chemoreception. Microbial symbionts may influence insect sensitivity to semiochemicals by regulating the synthesis of odorant-binding proteins or chemosensory proteins and olfactory or gustatory receptors and regulating host neurotransmission, thereby adjusting insect behavior. The manipulation of insect chemosensory behavior by microbial symbionts is conducive to their proliferation and dispersal and provides the impetus for insects to change their feeding habits and aggregation and dispersal behavior, which contributes to population differentiation in insects. Future research is necessary to reveal the material and information exchange between both partners to improve our comprehension of the evolution of chemoreception in insects. Manipulating insect chemoreception physiology by inoculating them with microbes could be utilized as a potential approach to managing insect populations.

Predatory stink bug, Eocanthecona furcellata (Wolff) responses to oral exposure route of λ-cyhalothrin via sex-specific modulation manner

The toxic effects of insecticides on predatory arthropods have closely related to their exposure routes. However, little is known about the effects of insecticide on reproductive parameters when the route of exposure occurs at a trophic level via prey intake. We therefore conducted current studies assessing whether Eocanthecona furcellata adults would be affected by feeding with λ-cyhalothrin-contaminated prey. Reproductive parameters, i.e. prolonged premating and preoviposition durations, reduced number of egg batches and egg amount, disturbed ovarian development, and suppressed expression of reproductive related genes were observed in E. furcellata females by feeding with treated prey. Moreover, reduced survival rate and altered carbohydrate metabolism parameters were detected in male bugs. Biochemical parameters, including MDA content, the activities of three antioxidant enzymes and three detoxification enzymes exhibited sex-specific responses after oral-exposure to λ-cyhalothrin in E. furcellata. The results indicate that the insecticide affects the fitness and leads to impairing reproductive potential via sex-specific modulation manner in predator insects. Taken together, our results provide a comprehensive assessment about detrimental impacts of λ-cyhalothrin-exposure on predators via prey intake, as well as a solid basis for further research to protect the predators from hazardous impacts of insecticides.

Sensing of a spore surface protein by a Drosophila chemosensory protein induces behavioral defense against fungal parasitic infections

In addition to innate immunity in a physiological context, insects have evolved behavioral defenses against parasite attacks. Here, we report that Drosophila can sense the CFEM (common in fungal extracellular membrane) protein Mcdc9, which acts as a negative virulence factor of the entomopathogenic fungus Metarhizium robertsii. The individual deletions of 18 CFEM genes in Metarhizium followed by fly infection identified three null mutants that could kill the flies more quickly than the wild-type strain, among which Mcdc9 can coat fungal spores and interact with the fly chemosensory protein CheA75a. The deletion of Mcdc9 in the fungus or the knockdown of CheA75a in flies had a similar effect, in which a greater number of fungal spores were left on flies than on the respective controls after topical infection. Thus, similar to the accelerated death of the wild-type flies treated with ΔMcdc9, the CheA75a^RNAi flies succumbed more quickly than the control insects topically challenged with the wild-type strain. The CheA75a gene is highly transcribed in fly legs and wings, and positive electrophysiological responses were evidenced in tarsal sensilla after stimulation with the Mcdc9 protein. The results imply that this CFEM protein could be sensed as a contact elicitor inducing the hygienic behavior of flies against fungal parasitic infection, which reveals a previously unsuspected mechanism of fungus-insect interactions.

An odorant binding protein is involved in counteracting detection-avoidance and Toll-pathway innate immunity

INTRODUCTION

Odorant-binding proteins (OBPs) are a class of small molecular weight soluble proteins that exist as expanded gene families in all insects, acting as ligand carriers mediating olfaction and other physiological processes. During fungal infection, a subset of insect OBPs were shown to be differentially expressed.

OBJECTIVES

We tested whether the altered expression of insect OBPs during pathogenic infection plays a role in behavioral or immune interactions between insect hosts and their pathogens.

METHODS

A wide range of techniques including RNAi-directed knockdown, heterologous protein expression, electrophysiological/behavioral analyses, transcriptomics, gut microbiome analyses, coupled with tandem mass spectrometry ion monitoring, were used to characterize the function of a locust OBP in host behavioral and immune responses.

RESULTS

The entomopathogenic fungus Metarhizium anisopliae produces the volatile compound phenylethyl alcohol (PEA) that causes behavioral avoidance in locusts. This is mediated by the locust odorant binding protein 11 (LmOBP11). Expression of LmOBP11 is induced by M. anisopliae infection and PEA treatment. LmOBP11 participates in insect detection of the fungal-produced PEA and avoidance of PEA-contaminated food, but the upregulation of LmOBP11 upon M. anisopliae infection negatively affects the insect immune responses to ultimately benefit successful mycosis by the pathogen. RNAi knockdown of LmOBP11 increases the production of antimicrobial peptides and enhances locust resistance to M. anisopliae infection, while reducing host antennal electrophysiological responses to PEA and locust avoidance of PEA treated food. Also, transcriptomic and gut microbiome analyses reveal microbiome dysbiosis and changes in host genes involved in behavior and immunity. These results are consistent with the elevated expression of LmOBP11 leading to enhanced volatile detection and suppression of immune responses.

CONCLUSION

These findings suggest a crosstalk between olfaction and immunity, indicating manipulation of host OBPs as a novel target exploited by fungal pathogens to alter immune activation and thus promote the successful infection of the host.