The molecular basis of bacterial-insect symbiosis

Distinct effects of the nephridial symbionts Verminephrobacter and Candidatus Nephrothrix on reproduction and maturation of its earthworm host Eisenia andrei

Verminephrobacter, the most common specific symbionts in the nephridia (excretory organs) of lumbricid earthworms, have been shown to improve reproduction of the garden earthworm Aporrectodea tuberculata under nutrient limitation. It is unknown how general this beneficial trait is in the Verminephrobacter-earthworm symbiosis, whether other nephridial symbionts also affect host fitness and what the mechanism of the fitness increase is. Here we report beneficial effects of Verminephrobacter and Candidatus Nephrothrix on life history traits of the compost worm Eisenia andrei, which in addition to these two symbionts also hosts Agromyces-like bacteria in its mixed nephridial community: while growth was identical between control, Verminephrobacter-free and aposymbiotic worms, control worms produced significantly more cocoons and offspring than both Verminephrobacter-free and aposymbiotic worms, confirming the reproductive benefit of Verminephrobacter in a second host with different ecology and feeding behavior. Furthermore, worms with Verminephrobacter and Ca. Nephrothrix, or with only Ca. Nephrothrix present, reached sexual maturity significantly earlier than aposymbiotic worms; this is the first evidence for a beneficial role of Ca. Nephrothrix in earthworms. Riboflavin content in cocoons and whole earthworms was unaffected by the presence or absence of nephridial symbionts, suggesting that nutritional supplementation with this vitamin does not play a major role in this symbiosis.

Meta-Omics Tools in the World of Insect-Microorganism Interactions

Microorganisms are able to influence several aspects of insects’ life, and this statement is gaining increasing strength, as research demonstrates it daily. At the same time, new sequencing technologies are now available at a lower cost per base, and bioinformatic procedures are becoming more user-friendly. This is triggering a huge effort in studying the microbial diversity associated to insects, and especially to economically important insect pests. The importance of the microbiome has been widely acknowledged for a wide range of animals, and also for insects this topic is gaining considerable importance. In addition to bacterial-associates, the insect-associated fungal communities are also gaining attention, especially those including plant pathogens. The use of meta-omics tools is not restricted to the description of the microbial world, but it can be also used in bio-surveillance, food safety assessment, or even to bring novelties to the industry. This mini-review aims to give a wide overview of how meta-omics tools are fostering advances in research on insect-microorganism interactions.

Insect anal droplets contain diverse proteins related to gut homeostasis

BACKGROUND

Insects share similar fundamental molecular principles with mammals in innate immunity. For modulating normal gut microbiota, insects produce phenoloxidase (PO), which is absent in all vertebrates, and reactive nitrogen species (ROS) and antimicrobial proteins (AMPs). However, reports on insect gut phagocytosis are very few. Furthermore, most previous studies measure gene expression at the transcription level. In this study, we provided proteomic evidence on gut modulation of normal microorganisms by investigating the anal droplets from a weevil, Cryptorhynchus lapathi.

RESULTS

The results showed that the anal droplets contained diverse proteins related to physical barriers, epithelium renewal, pattern recognition, phenoloxidase activation, oxidative defense and phagocytosis, but AMPs were not detected. According to annotations, Scarb1, integrin βν, Dscam, spondin or Thbs2s might mediate phagocytosis. As a possible integrin βν pathway, βν activates Rho by an unknown mechanism, and Rho induces accumulation of mDia, which then promotes actin polymerization.

CONCLUSIONS

Our results well demonstrated that insect anal droplets can be used as materials to investigate the defense of a host to gut microorganisms and supported to the hypothesis that gut phagocytosis occurs in insects.

Bacteriocyte Reprogramming to Cope With Nutritional Stress in a Phloem Sap Feeding Hemipteran, the Pea Aphid Acyrthosiphon pisum

Nutritional symbioses play a central role in the ability of insects to thrive on unbalanced diets and in ensuring their evolutionary success. A genomic model for nutritional symbiosis comprises the hemipteran Acyrthosiphon pisum, and the gamma-3-proteobacterium, Buchnera aphidicola, with genomes encoding highly integrated metabolic pathways. A. pisum feeds exclusively on plant phloem sap, a nutritionally unbalanced diet highly variable in composition, thus raising the question of how this symbiotic system responds to nutritional stress. We addressed this by combining transcriptomic, phenotypic and life history trait analyses to determine the organismal impact of deprivation of tyrosine and phenylalanine. These two aromatic amino acids are essential for aphid development, are synthesized in a metabolic pathway for which the aphid host and the endosymbiont are interdependent, and their concentration can be highly variable in plant phloem sap. We found that this nutritional challenge does not have major phenotypic effects on the pea aphid, except for a limited weight reduction and a 2-day delay in onset of nymph laying. Transcriptomic analyses through aphid development showed a prominent response in bacteriocytes (the core symbiotic tissue which houses the symbionts), but not in gut, thus highlighting the role of bacteriocytes as major modulators of this homeostasis. This response does not involve a direct regulation of tyrosine and phenylalanine biosynthetic pathway and transporter genes. Instead, we observed an extensive transcriptional reprogramming of the bacteriocyte with a rapid down-regulation of genes encoding sugar transporters and genes required for sugar metabolism. Consistently, we observed continued overexpression of the A. pisum homolog of RRAD, a small GTPase implicated in repressing aerobic glycolysis. In addition, we found increased transcription of genes involved in proliferation, cell size control and signaling. We experimentally confirmed the significance of these gene expression changes detecting an increase in bacteriocyte number and cell size in vivo under tyrosine and phenylalanine depletion. Our results support a central role of bacteriocytes in the aphid response to amino acid deprivation: their transcriptional and cellular responses fine-tune host physiology providing the host insect with an effective way to cope with the challenges posed by the variability in composition of phloem sap.

Symbiotic cornucopia of the monophagous planthopper Ommatidiotus dissimilis (Fallén, 1806) (Hemiptera: Fulgoromorpha: Caliscelidae)

In contrast to Cicadomorpha, in which numerous symbiotic bacteria have been identified and characterized, the symbionts of fulgoromorphans are poorly known. Here, we present the results of histological, ultrastructural, and molecular analyses of the symbiotic system of the planthopper Ommatidiotus dissimilis. Amplification, cloning, and sequencing of bacterial 16S RNA genes have revealed that O. dissimilis is host to five types of bacteria. Apart from bacteria Sulcia and Vidania, which are regarded as ancestral symbionts of Fulgoromorpha, three additional types of bacteria belonging to the genera Sodalis, Wolbachia, and Rickettsia have been detected. Histological and ultrastructural investigations have shown that bacteria Sulcia, Vidania, and Sodalis house separate bacteriocytes, whereas bacteria Wolbachia and Rickettsia are dispersed within various insect tissue. Additionally, bacteria belonging to the genus Vidania occupy the bacteriome localized in the lumen of the hindgut. Both molecular and microscopic analyses have revealed that all the symbionts are transovarially transmitted between generations.

Which experimental systems should we use for human microbiome science?

Microbiome science is revealing that the phenotype and health of animals, including humans, depend on the sustained function of their resident microorganisms. In this essay, I argue for thoughtful choice of model systems for human microbiome science. A greater variety of experimental systems, including wider use of invertebrate models, would benefit biomedical research, while systems ill-suited to experimental and genetic manipulation can be used to address very limited sets of scientific questions. Microbiome science benefits from the coordinated use of multiple systems, which is facilitated by networks of researchers with expertise in different experimental systems.

Unravelling the relationship between the tsetse fly and its obligate symbiont Wigglesworthia: transcriptomic and metabolomic landscapes reveal highly integrated physiological networks

Insects with restricted diets rely on obligate microbes to fulfil nutritional requirements essential for biological function. Tsetse flies, vectors of African trypanosome parasites, feed exclusively on vertebrate blood and harbour the obligate endosymbiont Wigglesworthia glossinidia. Without Wigglesworthia, tsetse are unable to reproduce. These symbionts are sheltered within specialized cells (bacteriocytes) that form the midgut-associated bacteriome organ. To decipher the core functions of this symbiosis essential for tsetse's survival, we performed dual-RNA-seq analysis of the bacteriome, coupled with metabolomic analysis of bacteriome and haemolymph collected from normal and symbiont-cured (sterile) females. Bacteriocytes produce immune regulatory peptidoglycan recognition protein (pgrp-lb) that protects Wigglesworthia, and a multivitamin transporter (smvt) that can aid in nutrient dissemination. Wigglesworthia overexpress a molecular chaperone (GroEL) to augment their translational/transport machinery and biosynthesize an abundance of B vitamins (specifically B₁-, B₂-, B₃- and B₆-associated metabolites) to supplement the host's nutritionally deficient diet. The absence of Wigglesworthia's contributions disrupts multiple metabolic pathways impacting carbohydrate and amino acid metabolism. These disruptions affect the dependent downstream processes of nucleotide biosynthesis and metabolism and biosynthesis of S-adenosyl methionine (SAM), an essential cofactor. This holistic fundamental knowledge of the symbiotic dialogue highlights new biological targets for the development of innovative vector control methods.

Role of Endosymbionts in Insect-Parasitic Nematode Interactions

Endosymbiotic bacteria exist in many animals where they develop relationships that affect certain physiological processes in the host. Insects and their nematode parasites form great models for understanding the genetic and molecular basis of immune and parasitic processes. Both organisms contain endosymbionts that possess the ability to interfere with certain mechanisms of immune function and pathogenicity. This review summarizes recent information on the involvement of insect endosymbionts in the response to parasitic nematode infections, and the influence of nematode endosymbionts on specific aspects of the insect immune system. Analyzing this information will be particularly useful for devising endosymbiont-based strategies to intervene in insect immunity or nematode parasitism for the efficient management of noxious insects in the field.

Co-evolution of marine worms and their chemoautotrophic bacterial symbionts: unexpected host switches explained by ecological fitting?

Mutualistic associations of bacteria and invertebrates are widespread and encompass an enormous diversity on the side of both partners. The advantages gained from the symbiosis favour reciprocal adaptations that increase the stability of the association and can lead to codiversification of symbiont and host. While numerous examples of a strictly vertical transfer of the symbionts from parent to offspring among intracellular associations abound, little is known about the fidelity of the partners in extracellular associations, where symbionts colonize the surface or body cavity of their host. In this issue of Molecular Ecology, Zimmermann et al. () investigated the evolutionary history of the symbiotic association between a monophyletic clade of sulphur-oxidizing Gammaproteobacteria and two distantly related lineages of marine worms (nematodes and annelids). The study supports the surprising conclusion that partner fidelity does not necessarily increase with the intimacy of the association. Ectosymbionts on the cuticle of the nematodes seem to be cospeciating with their hosts, whereas endosymbionts housed in the body cavity of the annelids must have originated multiple times, probably by host switching, from ectosymbionts of sympatric nematodes. This excellent case study on the evolutionary history of invertebrate-microbe interactions supports the emerging concept that the co-evolutionary processes shaping such mutualistic symbioses include both codiversification and ecological fitting.

Adaptation to Chronic Nutritional Stress Leads to Reduced Dependence on Microbiota in Drosophila melanogaster

Numerous studies have shown that animal nutrition is tightly linked to gut microbiota, especially under nutritional stress. In Drosophila melanogaster, microbiota are known to promote juvenile growth, development, and survival on poor diets, mainly through enhanced digestion leading to changes in hormonal signaling. Here, we show that this reliance on microbiota is greatly reduced in replicated Drosophila populations that became genetically adapted to a poor larval diet in the course of over 170 generations of experimental evolution. Protein and polysaccharide digestion in these poor-diet-adapted populations became much less dependent on colonization with microbiota. This was accompanied by changes in expression levels of dFOXO transcription factor, a key regulator of cell growth and survival, and many of its targets. These evolutionary changes in the expression of dFOXO targets to a large degree mimic the response of the same genes to microbiota, suggesting that the evolutionary adaptation to poor diet acted on mechanisms that normally mediate the response to microbiota. Our study suggests that some metazoans have retained the evolutionary potential to adapt their physiology such that association with microbiota may become optional rather than essential.IMPORTANCE Animals depend on gut microbiota for various metabolic tasks, particularly under conditions of nutritional stress, a relationship usually regarded as an inherent aspect of animal physiology. Here, we use experimental evolution in fly populations to show that the degree of host dependence on microbiota can substantially and rapidly change as the host population evolves in response to poor diet. Our results suggest that, although microbiota may initially greatly facilitate coping with suboptimal diets, chronic nutritional stress experienced over multiple generations leads to evolutionary adaptation in physiology and gut digestive properties that reduces dependence on the microbiota for growth and survival. Thus, despite its ancient evolutionary history, the reliance of animal hosts on their microbial partners can be surprisingly flexible and may be relaxed by short-term evolution.

The bacterial virulence factor CagA induces microbial dysbiosis that contributes to excessive epithelial cell proliferation in the Drosophila gut

Gut microbiota facilitate many aspects of human health and development, but dysbiotic microbiota can promote hyperplasia and inflammation and contribute to human diseases such as cancer. Human patients infected with the gastric cancer-causing bacterium Helicobacter pylori have altered microbiota; however, whether dysbiosis contributes to disease in this case is unknown. Many H. pylori human disease phenotypes are associated with a potent virulence protein, CagA, which is translocated into host epithelial cells where it alters cell polarity and manipulates host-signaling pathways to promote disease. We hypothesized that CagA alone could contribute to H. pylori pathogenesis by inducing microbial dysbiosis that promotes disease. Here we use a transgenic Drosophila model of CagA expression to genetically disentangle the effects of the virulence protein CagA from that of H. pylori infection. We found that expression of CagA within Drosophila intestinal stem cells promotes excess cell proliferation and is sufficient to alter host microbiota. Rearing CagA transgenic flies germ-free revealed that the dysbiotic microbiota contributes to cell proliferation phenotypes and also elicits expression of innate immune components, Diptericin and Duox. Further investigations revealed interspecies interactions are required for this dysbiotic CagA-dependent microbiota to promote proliferation in CagA transgenic and healthy control Drosophila. Our model establishes that CagA can alter gut microbiota and exacerbate cell proliferation and immune phenotypes previously attributed to H. pylori infection. This work provides valuable new insights into the mechanisms by which interactions between a specific virulence factor and the resident microbiota can contribute to the development and progression of disease.

Microbial communities in the gut, termed microbiota are important for human health, and when altered can sometimes promote disease. Infections, such as with the cancer-causing bacterium Helicobacter pylori, can cause altered gut microbiota, but why these alterations occur and whether the altered communities contribute to disease remain unknown. Here, we use Drosophila expressing the H. pylori disease-causing protein CagA, to model this virulence factor’s effect on host pathology and microbiota. We found that expression of CagA in the Drosophila gut causes excessive cell proliferation and immune activation, hallmarks of H. pylori infection. Notably, these traits did not occur when flies were reared in the absence of microbes. Further examination reveals that CagA-expressing flies have an altered gut microbial community that is sufficient to promote cell proliferation even in normal flies. This proliferative activity required the presence of two interacting bacteria, illustrating a new model for disease-promoting microbiota. This work demonstrates how a bacterial protein can cause disease indirectly through altering the microbial ecology of the host, and it suggests future treatments for infections that rely on manipulating the microbiota to mitigate disease pathology.

Larval diet affects mosquito development and permissiveness to Plasmodium infection

The larval stages of malaria vector mosquitoes develop in water pools, feeding mostly on microorganisms and environmental detritus. Richness in the nutrient supply to larvae influences the development and metabolism of larvae and adults. Here, we investigated the effects of larval diet on the development, microbiota content and permissiveness to Plasmodium of Anopheles coluzzii. We tested three fish diets often used to rear mosquitoes in the laboratory, including two pelleted diets, Dr. Clarke’s Pool Pellets and Nishikoi Fish Pellets, and one flaked diet, Tetramin Fish-Flakes. Larvae grow and develop faster and produce bigger adults when feeding on both types of pellets compared with flakes. This correlates with a higher microbiota load in pellet-fed larvae, in agreement with the known positive effect of the microbiota on mosquito development. Larval diet also significantly influences the prevalence and intensity of Plasmodium berghei infection in adults, whereby Nishikoi Fish Pellets-fed larvae develop into adults that are highly permissive to parasites and survive longer after infection. This correlates with a lower amount of Enterobacteriaceae in the midgut microbiota. Together, our results shed light on the influence of larval feeding on mosquito development, microbiota and vector competence; they also provide useful data for mosquito rearing.

Phenological asynchrony between host plant and gypsy moth reduces insect gut microbiota and susceptibility to Bacillus thuringiensis

The phenological synchrony between the emergence of overwintering herbivorous insects and the budding of host plants is considered a crucial factor in the population dynamics of herbivores. However, the mechanisms driving the interactions between the host plant, herbivores, and their pathogens are often obscure. In the current study, an artificially induced phenological asynchrony was used to investigate how the asynchrony between silver birch Betula pendula and gypsy moth Lymantria dispar affects the immunity of the insect to bacteria, its susceptibility to the entomopathogenic bacteria Bacillus thuringiensis, and the diversity in its midgut microbiota. The lysozyme-like activity in both the midgut and hemolymph plasma and the nonspecific esterase activity and antimicrobial peptide gene expression in the midgut were studied in both noninfected and B. thuringiensis-infected larvae. Our results provide the first evidence that phenologically asynchronous larvae are less susceptible to B. thuringiensis infection than phenologically synchronous larvae, and our results show that these effects are related to the high basic levels and B. thuringiensis-induced levels of lysozyme-like activities. Moreover, a 16S rRNA analysis revealed that dramatic decreases in the diversity of the larval gut bacterial consortia occurred under the effect of asynchrony. Larvae infected with B. thuringiensis presented decreased microbiota diversity if the larvae were reared synchronously with the host plant but not if they were reared asynchronously. Our study demonstrates the significant effect of phenological asynchrony on innate immunity-mediated interactions between herbivores and entomopathogenic bacteria and highlights the role of nonpathogenic gut bacteria in these interactions.

Composition of the Cockroach Gut Microbiome in the Presence of Parasitic Nematodes

Cockroaches are parasitized by thelastomatid nematodes, which live in an obligate manner in their hindgut and interact with the resident microbial community. In the present study, a composition analysis was performed on the gut microbiome of Periplaneta fuliginosa and P. americana to investigate natural and artificial infection by thelastomatid nematodes. Nine libraries of the 16S rRNA gene V3-V4 region were prepared for pyrosequencing. We examined the complete gut microbiome (fore-, mid-, and hindgut) of lab-reared P. fuliginosa naturally infected with the parasitic nematode Leidynema appendiculatum and those that were nematode-free, and complemented our study by characterizing the hindgut microbial communities of lab-reared P. americana naturally infected with Hammerschmidtiella diesingi and Thelastoma bulhoesi, artificially infected with L. appendiculatum, and those that were nematode-free. Our results revealed that the fore- and midgut of naturally infected and nematode-free P. fuliginosa have close microbial communities, which is in contrast with hindgut communities; the hindgut communities of both cockroaches exhibit higher microbial diversities in the presence of their natural parasites and marked differences were observed in the abundance of the most representative taxa, namely Firmicutes, Proteobacteria, and Bacteroidetes. Our results have provided basic information and encourage further studies on multitrophic interactions in the cockroach gut as well as the thelastomatid nematodes that play a role in this environment.

Animal-microbe interactions and the evolution of nervous systems

Animals ubiquitously interact with environmental and symbiotic microbes, and the effects of these interactions on animal physiology are currently the subject of intense interest. Nevertheless, the influence of microbes on nervous system evolution has been largely ignored. We illustrate here how taking microbes into account might enrich our ideas about the evolution of nervous systems. For example, microbes are involved in animals' communicative, defensive, predatory and dispersal behaviours, and have likely influenced the evolution of chemo- and photosensory systems. In addition, we speculate that the need to regulate interactions with microbes at the epithelial surface may have contributed to the evolutionary internalization of the nervous system.

Symbiosis with Francisella tularensis provides resistance to pathogens in the silkworm

Francisella tularensis, the causative agent of tularemia, is a highly virulent facultative intracellular pathogen found in a wide range of animals, including arthropods, and environments. This bacterium has been known for over 100 years, but the lifestyle of F. tularensis in natural reservoirs remains largely unknown. Thus, we established a novel natural host model for F. tularensis using the silkworm (Bombyx mori), which is an insect model for infection by pathogens. F. tularensis established a symbiosis with silkworms, and bacteria were observed in the hemolymph. After infection with F. tularensis, the induction of melanization and nodulation, which are immune responses to bacterial infection, were inhibited in silkworms. Pre-inoculation of silkworms with F. tularensis enhanced the expression of antimicrobial peptides and resistance to infection by pathogenic bacteria. These results suggest that silkworms acquire host resistance via their symbiosis with F. tularensis, which may have important fitness benefits in natural reservoirs.

Midgut microbiota and host immunocompetence underlie Bacillus thuringiensis killing mechanism

Bacillus thuringiensis is a widely used bacterial entomopathogen producing insecticidal toxins, some of which are expressed in insect-resistant transgenic crops. Surprisingly, the killing mechanism of B. thuringiensis remains controversial. In particular, the importance of the septicemia induced by the host midgut microbiota is still debated as a result of the lack of experimental evidence obtained without drastic manipulation of the midgut and its content. Here this key issue is addressed by RNAi-mediated silencing of an immune gene in a lepidopteran host Spodoptera littoralis, leaving the midgut microbiota unaltered. The resulting cellular immunosuppression was characterized by a reduced nodulation response, which was associated with a significant enhancement of host larvae mortality triggered by B. thuringiensis and a Cry toxin. This was determined by an uncontrolled proliferation of midgut bacteria, after entering the body cavity through toxin-induced epithelial lesions. Consequently, the hemolymphatic microbiota dramatically changed upon treatment with Cry1Ca toxin, showing a remarkable predominance of Serratia and Clostridium species, which switched from asymptomatic gut symbionts to hemocoelic pathogens. These experimental results demonstrate the important contribution of host enteric flora in B. thuringiensis-killing activity and provide a sound foundation for developing new insect control strategies aimed at enhancing the impact of biocontrol agents by reducing the immunocompetence of the host.

Molecular mechanism and functional significance of acid generation in the Drosophila midgut

The gut of Drosophila melanogaster includes a proximal acidic region (~pH 2), however the genome lacks the H⁺/K⁺ ATPase characteristic of the mammalian gastric parietal cell, and the molecular mechanisms of acid generation are poorly understood. Here, we show that maintenance of the low pH of the acidic region is dependent on H⁺ V-ATPase, together with carbonic anhydrase and five further transporters or channels that mediate K⁺, Cl⁻ and HCO₃⁻ transport. Abrogation of the low pH did not influence larval survival under standard laboratory conditions, but was deleterious for insects subjected to high Na⁺ or K⁺ load. Insects with elevated pH in the acidic region displayed increased susceptibility to Pseudomonas pathogens and increased abundance of key members of the gut microbiota (Acetobacter and Lactobacillus), suggesting that the acidic region has bacteriostatic or bacteriocidal activity. Conversely, the pH of the acidic region was significantly reduced in germ-free Drosophila, indicative of a role of the gut bacteria in shaping the pH conditions of the gut. These results demonstrate that the acidic gut region protects the insect and gut microbiome from pathological disruption, and shed light on the mechanisms by which low pH can be maintained in the absence of H⁺, K⁺ ATPase.

Culex pipiens Development Is Greatly Influenced by Native Bacteria and Exogenous Yeast

Culex pipiens is the most cosmopolitan mosquito of the Pipiens Assemblage. By studying the nature of interactions between this species and microorganisms common to its breeding environment we can unravel important pitfalls encountered during development. We tested the survival rate of larval stages, pupae and adults of a Cx. pipiens colony exposed to a variety of microorganisms in laboratory conditions and assessed the transmission to offspring (F1) by those organisms that secured development up to adulthood. Three complementary experiments were designed to: 1) explore the nutritional value of yeasts and other microorganisms during Cx. pipiens development; 2) elucidate the transstadial transmission of yeast to the host offspring; and 3) to examine the relevance of all these microorganisms in female choice for oviposition-substratum. The yeast Saccharomyces cerevisiae proved to be the most nutritional diet, but despite showing the highest survival rates, vertical transmission to F1 was never confirmed. In addition, during the oviposition trials, none of the gravid females was attracted to the yeast substratum. Notably, the two native bacterial strains, Klebsiella sp. and Aeromonas sp., were the preferred oviposition media, the same two bacteria that managed to feed neonates until molting into 2^nd instar larvae. Our results not only suggest that Klebsiella sp. or Aeromonas sp. serve as attractants for oviposition habitat selection, but also nurture the most fragile instar, L1, to assure molting into a more resilient stage, L2, while yeast proves to be the most supportive diet for completing development. These experiments unearthed survival traits that might be considered in the future development of strategies of Cx. pipiens control. These studies can be extended to other members of the Pipiens Assemblage.

Influence of oxidative homeostasis on bacterial density and cost of infection in Drosophila-Wolbachia symbioses

The evolution of symbioses along the continuum between parasitism and mutualism can be influenced by the oxidative homeostasis, that is the balance between reactive oxygen species (ROS) and antioxidant molecules. Indeed, ROS can contribute to the host immune defence to regulate symbiont populations, but are also toxic. This interplay between ROS and symbiosis is notably exemplified by recent results in arthropod-Wolbachia interactions. Wolbachia are symbiotic bacteria involved in a wide range of interactions with their arthropods hosts, from facultative, parasitic associations to obligatory, mutualistic ones. In this study, we used Drosophila-Wolbachia associations to determine whether the oxidative homeostasis plays a role in explaining the differences between phenotypically distinct arthropod-Wolbachia symbioses. We used Drosophila lines with different Wolbachia infections and measured the effects of pro-oxidant (paraquat) and antioxidant (glutathione) treatments on the Wolbachia density and the host survival. We show that experimental manipulations of the oxidative homeostasis can reduce the cost of the infection through its effect on Wolbachia density. We discuss the implication of this result from an evolutionary perspective and argue that the oxidative homeostasis could underlie the evolution of tolerance and dependence on Wolbachia.

Digestion of Yeasts and Beta-1,3-Glucanases in Mosquito Larvae: Physiological and Biochemical Considerations

Aedes aegypti larvae ingest several kinds of microorganisms. In spite of studies regarding mosquito digestion, little is known about the nutritional utilization of ingested cells by larvae. We investigated the effects of using yeasts as the sole nutrient source for A. aegypti larvae. We also assessed the role of beta-1,3-glucanases in digestion of live yeast cells. Beta-1,3-glucanases are enzymes which hydrolyze the cell wall beta-1,3-glucan polyssacharide. Larvae were fed with cat food (controls), live or autoclaved Saccharomyces cerevisiae cells and larval weight, time for pupation and adult emergence, larval and pupal mortality were measured. The presence of S. cerevisiae cells inside the larval gut was demonstrated by light microscopy. Beta-1,3-glucanase was measured in dissected larval samples. Viability assays were performed with live yeast cells and larval gut homogenates, with or without addition of competing beta-1,3-glucan. A. aegypti larvae fed with yeast cells were heavier at the 4^th instar and showed complete development with normal mortality rates. Yeast cells were efficiently ingested by larvae and quickly killed (10% death in 2h, 100% in 48h). Larvae showed beta-1,3-glucanase in head, gut and rest of body. Gut beta-1,3-glucanase was not derived from ingested yeast cells. Gut and rest of body activity was not affected by the yeast diet, but head homogenates showed a lower activity in animals fed with autoclaved S. cerevisiae cells. The enzymatic lysis of live S. cerevisiae cells was demonstrated using gut homogenates, and this activity was abolished when excess beta-1,3-glucan was added to assays. These results show that live yeast cells are efficiently ingested and hydrolyzed by A. aegypti larvae, which are able to fully-develop on a diet based exclusively on these organisms. Beta-1,3-glucanase seems to be essential for yeast lytic activity of A. aegypti larvae, which possess significant amounts of these enzyme in all parts investigated.

Natural products from microbes associated with insects

Here we review discoveries of secondary metabolites from microbes associated with insects. We mainly focus on natural products, where the ecological role has been at least partially elucidated, and/or the pharmaceutical properties evaluated, and on compounds with unique structural features. We demonstrate that the exploration of specific microbial–host interactions, in combination with multidisciplinary dereplication processes, has emerged as a successful strategy to identify novel chemical entities and to shed light on the ecology and evolution of defensive associations.

The Rickettsia Endosymbiont of Ixodes pacificus Contains All the Genes of De Novo Folate Biosynthesis

Ticks and other arthropods often are hosts to nutrient providing bacterial endosymbionts, which contribute to their host’s fitness by supplying nutrients such as vitamins and amino acids. It has been detected, in our lab, that Ixodes pacificus is host to Rickettsia species phylotype G021. This endosymbiont is predominantly present, and 100% maternally transmitted in I. pacificus. To study roles of phylotype G021 in I. pacificus, bioinformatic and molecular approaches were carried out. MUMmer genome alignments of whole genome sequence of I. scapularis, a close relative to I. pacificus, against completely sequenced genomes of R. bellii OSU85-389, R. conorii, and R. felis, identified 8,190 unique sequences that are homologous to Rickettsia sequences in the NCBI Trace Archive. MetaCyc metabolic reconstructions revealed that all folate gene orthologues (folA, folC, folE, folKP, ptpS) required for de novo folate biosynthesis are present in the genome of Rickettsia buchneri in I. scapularis. To examine the metabolic capability of phylotype G021 in I. pacificus, genes of the folate biosynthesis pathway of the bacterium were PCR amplified using degenerate primers. BLAST searches identified that nucleotide sequences of the folA, folC, folE, folKP, and ptpS genes possess 98.6%, 98.8%, 98.9%, 98.5% and 99.0% identity respectively to the corresponding genes of Rickettsia buchneri. Phylogenetic tree constructions show that the folate genes of phylotype G021 and homologous genes from various Rickettsia species are monophyletic. This study has shown that all folate genes exist in the genome of Rickettsia species phylotype G021 and that this bacterium has the genetic capability for de novo folate synthesis.

A specific cathepsin-L-like protease purified from an insect midgut shows antibacterial activity against gut symbiotic bacteria

Because gut symbiotic bacteria affect host biology, host insects are expected to evolve some mechanisms for regulating symbiont population. The bean bug, Riptortus pedestris, harbors the Burkholderia genus as a gut symbiont in the midgut organ, designated as the M4 region. Recently, we demonstrated that the lysate of M4B, the region adjacent to M4, harbors potent antibacterial activity against symbiotic Burkholderia but not to cultured Burkholderia. However, the bona fide substance responsible for observed antibacterial activity was not identified in the previous study. Here, we report that cathepsin-L-like protease purified from the lysate of M4B showed strong antibacterial activity against symbiotic Burkholderia but not the cultured Burkholderia. To further confirm this activity, recombinant cathepsin-L-like protease expressed in Escherichia coli also showed antibacterial activity against symbiotic Burkholderia. These results suggest that cathepsin-L-like protease purified from the M4B region plays a critical role in controlling the population of the Burkholderia gut symbiont.

The Toll/NF-κB pathway in cuttlefish symbiotic accessory nidamental gland

The female genital apparatus of decapod cephalopods contains a symbiotic accessory nidamental gland (ANG) that harbors bacterial symbionts. Although the ANG bacterial consortium is now well described, the impact of symbiosis on Sepia officinalis innate immunity pathways remains unknown. In silico analysis of the de novo transcriptome of ANG highlighted for the first time the existence of the NF-κB pathway in S. officinalis. Several signaling components were identified, i.e. five Toll-like receptors, eight signaling cascade features, and the immune response target gene iNOS, previously described as being involved in the initiation of bacterial symbiosis in a cephalopod gland. This work provides a first key for studying bacterial symbiosis and its impact on innate immunity in S. officinalis ANG.

French invasive Asian tiger mosquito populations harbor reduced bacterial microbiota and genetic diversity compared to Vietnamese autochthonous relatives

The Asian tiger mosquito Aedes albopictus is one of the most significant pathogen vectors of the twenty-first century. Originating from Asia, it has invaded a wide range of eco-climatic regions worldwide. The insect-associated microbiota is now recognized to play a significant role in host biology. While genetic diversity bottlenecks are known to result from biological invasions, the resulting shifts in host-associated microbiota diversity has not been thoroughly investigated. To address this subject, we compared four autochthonous Ae. albopictus populations in Vietnam, the native area of Ae. albopictus, and three populations recently introduced to Metropolitan France, with the aim of documenting whether these populations display differences in host genotype and bacterial microbiota. Population-level genetic diversity (microsatellite markers and COI haplotype) and bacterial diversity (16S rDNA metabarcoding) were compared between field-caught mosquitoes. Bacterial microbiota from the whole insect bodies were largely dominated by Wolbachia pipientis. Targeted analysis of the gut microbiota revealed a greater bacterial diversity in which a fraction was common between French and Vietnamese populations. The genus Dysgonomonas was the most prevalent and abundant across all studied populations. Overall genetic diversities of both hosts and bacterial microbiota were significantly reduced in recently established populations of France compared to the autochthonous populations of Vietnam. These results open up many important avenues of investigation in order to link the process of geographical invasion to shifts in commensal and symbiotic microbiome communities, as such shifts may have dramatic impacts on the biology and/or vector competence of invading hematophagous insects.

Bacterial community composition and diversity in an ancestral ant fungus symbiosis

Fungus-farming ants (Hymenoptera: Formicidae, Attini) exhibit some of the most complex microbial symbioses because both macroscopic partners (ants and fungus) are associated with a rich community of microorganisms. The ant and fungal microbiomes are thought to serve important beneficial nutritional and defensive roles in these symbioses. While most recent research has investigated the bacterial communities in the higher attines (e.g. the leaf-cutter ant genera Atta and Acromyrmex), which are often associated with antibiotic-producing Actinobacteria, very little is known about the microbial communities in basal lineages, labeled as 'lower attines', which retain the ancestral traits of smaller and more simple societies. In this study, we used 16S amplicon pyrosequencing to characterize bacterial communities of the lower attine ant Mycocepurus smithii among seven sampling sites in central Panama. We discovered that ant and fungus garden-associated microbiota were distinct from surrounding soil, but unlike the situation in the derived fungus-gardening ants, which show distinct ant and fungal microbiomes, microbial community structure of the ants and their fungi were similar. Another surprising finding was that the abundance of actinomycete bacteria was low and instead, these symbioses were characterized by an abundance of Lactobacillus and Pantoea bacteria. Furthermore, our data indicate that Lactobacillus strains are acquired from the environment rather than acquired vertically.

French invasive Asian tiger mosquito populations harbor reduced bacterial microbiota and genetic diversity compared to Vietnamese autochthonous relatives

The Asian tiger mosquito Aedes albopictus is one of the most significant pathogen vectors of the twenty-first century. Originating from Asia, it has invaded a wide range of eco-climatic regions worldwide. The insect-associated microbiota is now recognized to play a significant role in host biology. While genetic diversity bottlenecks are known to result from biological invasions, the resulting shifts in host-associated microbiota diversity has not been thoroughly investigated. To address this subject, we compared four autochthonous Ae. albopictus populations in Vietnam, the native area of Ae. albopictus, and three populations recently introduced to Metropolitan France, with the aim of documenting whether these populations display differences in host genotype and bacterial microbiota. Population-level genetic diversity (microsatellite markers and COI haplotype) and bacterial diversity (16S rDNA metabarcoding) were compared between field-caught mosquitoes. Bacterial microbiota from the whole insect bodies were largely dominated by Wolbachia pipientis. Targeted analysis of the gut microbiota revealed a greater bacterial diversity in which a fraction was common between French and Vietnamese populations. The genus Dysgonomonas was the most prevalent and abundant across all studied populations. Overall genetic diversities of both hosts and bacterial microbiota were significantly reduced in recently established populations of France compared to the autochthonous populations of Vietnam. These results open up many important avenues of investigation in order to link the process of geographical invasion to shifts in commensal and symbiotic microbiome communities, as such shifts may have dramatic impacts on the biology and/or vector competence of invading hematophagous insects.

Microbial impacts on insect evolutionary diversification: from patterns to mechanisms

Symbiosis can favor rapid shifts in host phenotypic traits, particularly through the contribution of symbionts to the host's physiology. In addition, variations in the microbiota composition between individuals can be associated with pre-zygotic and post-zygotic barriers. All together, these phenomena may contribute to insect diversification and speciation. Recent advances have also shown that the host-microbiota molecular dialog, mediated notably by host immune and developmental pathways, is critical for the acquisition and control of the microbiota, and could also contribute to reproductive isolation. While still a controversial hypothesis, adaptation through symbiosis could thus trigger host-symbiont coevolution and accelerate differentiation.