Mechanisms of symbiont-conferred protection against natural enemies: an ecological and evolutionary framework

With a little help from my friends: the roles of microbial symbionts in insect populations and communities

To understand insect abundance, distribution and dynamics, we need to understand the relevant drivers of their populations and communities. While microbial symbionts are known to strongly affect many aspects of insect biology, we lack data on their effects on populations or community processes, or on insects' evolutionary responses at different timescales. How these effects change as the anthropogenic effects on ecosystems intensify is an area of intense research. Recent developments in sequencing and bioinformatics permit cost-effective microbial diversity surveys, tracking symbiont transmission, and identification of functions across insect populations and multi-species communities. In this review, we explore how different functional categories of symbionts can influence insect life-history traits, how these effects could affect insect populations and their interactions with other species, and how they may affect processes and patterns at the level of entire communities. We argue that insect-associated microbes should be considered important drivers of insect response and adaptation to environmental challenges and opportunities. We also outline the emerging approaches for surveying and characterizing insect-associated microbiota at population and community scales. This article is part of the theme issue 'Towards a toolkit for global insect biodiversity monitoring'.

Pathogen-microbiome interactions and the virulence of an entomopathogenic fungus

Bacteria shape interactions between hosts and fungal pathogens. In some cases, bacteria associated with fungi are essential for pathogen virulence. In other systems, host-associated microbiomes confer resistance against fungal pathogens. We studied an aphid-specific entomopathogenic fungus called Pandora neoaphidis in the context of both host and pathogen microbiomes. Aphids host several species of heritable bacteria, some of which confer resistance against Pandora. We first found that spores that emerged from aphids that harbored protective bacteria were less virulent against subsequent hosts and did not grow on plate media. We then used 16S amplicon sequencing to study the bacterial microbiome of fungal mycelia and spores during plate culturing and host infection. We found that the bacterial community is remarkably stable in culture despite dramatic changes in pathogen virulence. Last, we used an experimentally transformed symbiont of aphids to show that Pandora can acquire host-associated bacteria during infection. Our results uncover new roles for bacteria in the dynamics of aphid-pathogen interactions and illustrate the importance of the broader microbiological context in studies of fungal pathogenesis.

IMPORTANCE

Entomopathogenic fungi play important roles in the population dynamics of many insect species. Understanding the factors shaping entomopathogen virulence is critical for agricultural management and for the use of fungi in pest biocontrol. We show that heritable bacteria in aphids, which confer protection to their hosts against fungal entomopathogens, influence virulence against subsequent hosts. Aphids reproduce asexually and are typically surrounded by genetically identical offspring, and thus these effects likely shape the dynamics of fungal disease in aphid populations. Furthermore, fungal entomopathogens are known to rapidly lose virulence in lab culture, complicating their laboratory use. We show that this phenomenon is not driven by changes in the associated bacterial microbiome. These results contribute to our broader understanding of the aphid model system and shed light on the biology of the Entomophthorales-an important but understudied group of fungi.

Reduction of Wolbachia in Diaphorina citri (Hemiptera: Liviidae) increases phytopathogen acquisition and decreases fitness

Wolbachia pipientis is a maternally inherited intracellular bacterium that infects a wide range of arthropods. Wolbachia can have a significant impact on host biology and development, often due to its effects on reproduction. We investigated Wolbachia-mediated effects in the Asian citrus psyllid, Diaphorina citri Kuwayama, which transmits Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus greening disease. Diaphorina citri are naturally infected with Wolbachia; therefore, investigating Wolbachia-mediated effects on D. citri fitness and CLas transmission required artificial reduction of this endosymbiont with the application of doxycycline. Doxycycline treatment of psyllids reduced Wolbachia infection by approximately 60% in both male and female D. citri. Psyllids treated with doxycycline exhibited higher CLas acquisition in both adults and nymphs as compared with negative controls. In addition, doxycycline-treated psyllids exhibited decreased fitness as measured by reduced egg and nymph production as well as adult emergence as compared with control lines without the doxycycline treatment. Our results indicate that Wolbachia benefits D. citri by improving fitness and potentially competes with CLas by interfering with phytopathogen acquisition. Targeted manipulation of endosymbionts in this phytopathogen vector may yield disease management tools.

Till evolution do us part: The diversity of symbiotic associations across populations of Philaenus spittlebugs

Symbiotic bacteria have played crucial roles in the evolution of sap-feeding insects and can strongly affect host function. However, their diversity and distribution within species are not well understood; we do not know to what extent environmental factors or associations with other species may affect microbial community profiles. We addressed this question in Philaenus spittlebugs by surveying both insect and bacterial marker gene amplicons across multiple host populations. Host mitochondrial sequence data confirmed morphology-based identification of six species and revealed two divergent clades of Philaenus spumarius. All of them hosted the primary symbiont Sulcia that was almost always accompanied by Sodalis. Interestingly, populations and individuals often differed in the presence of Sodalis sequence variants, suggestive of intra-genome 16S rRNA variant polymorphism combined with rapid genome evolution and/or recent additional infections or replacements of the co-primary symbiont. The prevalence of facultative endosymbionts, including Wolbachia, Rickettsia, and Spiroplasma, varied among populations. Notably, cytochrome I oxidase (COI) amplicon data also showed that nearly a quarter of P. spumarius were infected by parasitoid flies (Verralia aucta). One of the Wolbachia operational taxonomic units (OTUs) was exclusively present in Verralia-parasitized specimens, suggestive of parasitoids as their source and highlighting the utility of host gene amplicon sequencing in microbiome studies.

Arsenophonus: A Double-Edged Sword of Aphid Defense against Parasitoids

It is widely accepted that endosymbiont interactions with their hosts have significant effects on the fitness of both pests and beneficial species. A particular type of endosymbiosis is that of beneficial associations. Facultative endosymbiotic bacteria are associated with elements that provide aphids with protection from parasitoids. Arsenophonus (Enterobacterales: Morganellaceae) is one such endosymbiont bacterium, with infections being most commonly found among the Hemiptera species. Here, black cowpea aphids (BCAs), Aphis craccivora Koch (Hemiptera: Aphididae), naturally infected with Arsenophonus, were evaluated to determine the defensive role of this bacterium in BCAs against two parasitoid wasp species, Binodoxys angelicae and Lysiphlebus fabarum (both in Braconidae: Aphidiinae). Individuals of the black cowpea aphids infected with Arsenophonus were treated with a blend of ampicillin, cefotaxime, and gentamicin (Arsenophonus-reduced infection, AR) and subsequently subjected to parasitism assays. The results showed that the presence of Arsenophonus does not prevent BCAs from being parasitized by either B. angelicae or L. fabarum. Nonetheless, in BCA colonies parasitized by B. angelicae, the endosymbiont delayed both the larval maturation period and the emergence of the adult parasitoid wasps. In brief, Arsenophonus indirectly limits the effectiveness of B. angelicae parasitism by decreasing the number of emerged adult wasps. Therefore, other members of the BCA colony can survive. Arsenophonus acts as a double-edged sword, capturing the complex dynamic between A. craccivora and its parasitoids.

Symbiotic bacteria, immune-like sentinel cells, and the response to pathogens in a social amoeba

Some endosymbionts living within a host must modulate their hosts' immune systems in order to infect and persist. We studied the effect of a bacterial endosymbiont on a facultatively multicellular social amoeba host. Aggregates of the amoeba Dictyostelium discoideum contain a subpopulation of sentinel cells that function akin to the immune systems of more conventional multicellular organisms. Sentinel cells sequester and discard toxins from D. discoideum aggregates and may play a central role in defence against pathogens. We measured the number and functionality of sentinel cells in aggregates of D. discoideum infected by bacterial endosymbionts in the genus Paraburkholderia. Infected D. discoideum produced fewer and less functional sentinel cells, suggesting that Paraburkholderia may interfere with its host's immune system. Despite impaired sentinel cells, however, infected D. discoideum were less sensitive to ethidium bromide toxicity, suggesting that Paraburkholderia may also have a protective effect on its host. By contrast, D. discoideum infected by Paraburkholderia did not show differences in their sensitivity to two non-symbiotic pathogens. Our results expand previous work on yet another aspect of the complicated relationship between D. discoideum and Paraburkholderia, which has considerable potential as a model for the study of symbiosis.

Aphid facultative symbionts confer no protection against the fungal entomopathogen Batkoa apiculata

Fungi in the family Entomophthoraceae are prevalent pathogens of aphids. Facultative symbiotic bacteria harbored by aphids, including Spiroplasma sp. and Regiella insecticola, have been shown to make their hosts more resistant to infection with the fungal pathogen Pandora neoaphidis. How far this protection extends against other species of fungi in the family Entomophthoraceae is unknown. Here we isolated a strain of the fungal pathogen Batkoa apiculata infecting a natural population of pea aphids (Acyrthosiphon pisum) and confirmed its identity by sequencing the 28S rRNA gene. We then infected a panel of aphids each harboring a different species or strain of endosymbiotic bacteria to test whether aphid symbionts protect against B. apiculata. We found no evidence of symbiont-mediated protection against this pathogen, and our data suggest that some symbionts make aphids more susceptible to infection. This finding is relevant to our understanding of this important model of host-microbe interactions, and we discuss our results in the context of aphid-microbe ecological and evolutionary dynamics.

Immune-mediated competition benefits protective microbes over pathogens in a novel host species

Microbes that protect against infection inhabit hosts across the tree of life. It is unclear whether and how the host immune system may affect the formation of new protective symbioses. We investigated the transcriptomic response of Caenorhabditis elegans following novel interactions with a protective microbe (Enterococcus faecalis) able to defend against infection by pathogenic Staphylococcus aureus. We have previously shown that E. faecalis can directly limit pathogen growth within hosts. In this study, we show that colonisation by protective E. faecalis caused the differential expression of 1,557 genes in pathogen infected hosts, including the upregulation of immune genes such as lysozymes and C-type lectins. The most significantly upregulated host lysozyme gene, lys-7, impacted the competitive abilities of E. faecalis and S. aureus when knocked out. E. faecalis has an increased ability to resist lysozyme activity compared to S. aureus, suggesting that the protective microbe could gain a competitive advantage from this host response. Our finding that protective microbes can benefit from immune-mediated competition after introduction opens up new possibilities for biocontrol design and our understanding of symbiosis evolution. Crosstalk between the host immune response and microbe-mediated protection should favour the continued investment in host immunity and avoid the potentially risky evolution of host dependence.

Dynamics of symbiotic bacterial community in whole life stage of Harmonia axyridis (Coleoptera: Coccinellidae)

INTRODUCTION

Bacteria play critical roles in the reproduction, metabolism, physiology, and detoxification of their insect hosts. The ladybird beetle (Harmonia axyridis) harbors a myriad of endosymbiotic microbes. However, to date, little is known about how the microbial composition of H. axyridis varies throughout its life cycle.

METHODS

In this study, 16S rRNA amplicon sequencing and quantitative PCR were employed to investigate the diversity and dynamics of bacterial symbionts across the egg, larval, pupae, and adults stages of H. axyridis.

RESULTS

Higher bacterial community richness and diversity were observed in eggs, followed by those in adults and pupae. The community richness index differed significantly between second-instar larvae and other developmental stages. Proteobacteria, Firmicutes, and Actinobacteria were the dominant phyla. Staphylococcus, Enterobacter, Glutamicibacter, and Acinetobacter were the dominant bacteria genera; however, their relative abundances fluctuated across host developmental stages. Interestingly, the larval stage harbored high proportions of Firmicutes, whereas the adult microbial community largely consisted of Proteobacteria.

DISCUSSION

This study is the first to determine the symbiotic bacterial composition across key life stages of H. axyridis. These outcomes can foster the development of environmental risk assessments and novel biological control strategies.

Detection and Identification of Novel Intracellular Bacteria Hosted in Strains CBS 648.67 and CFCC 80795 of Biocontrol Fungi Metarhizium

"Endosymbiosis" is a cohesive form of a symbiotic association. Endobacteria exist in many fungi and play important roles in fungal host biology. Metarhizium spp. are important entomopathogenic fungi for insect pest control. In the present study, we performed comprehensive ana-lyses of strains of Metarhizium bibionidarum and M. anisopliae using PCR, phylogenetics, and fluorescent electron microscopy to identify endobacteria within hyphae and conidia. The results of the phylogenetic ana-lysis based on 16S rRNA gene sequences indicated that these endobacteria were the most closely related to Pelomonas puraquae and affiliated with Betaproteobacteria. Ultrastructural observations indicated that endobacteria were coccoid and less than 500‍ ‍nm in diameter. The basic characteristics of endobacteria in M. bibionidarum and M. anisopliae were elucidated, and biological questions were raised regarding their biological functions in the Metarhizium hosts.

Symbiont-mediated immune priming in animals through an evolutionary lens

Protective symbionts can defend hosts from parasites through several mechanisms, from direct interference to modulating host immunity, with subsequent effects on host and parasite fitness. While research on symbiont-mediated immune priming (SMIP) has focused on ecological impacts and agriculturally important organisms, the evolutionary implications of SMIP are less clear. Here, we review recent advances made in elucidating the ecological and molecular mechanisms by which SMIP occurs. We draw on current works to discuss the potential for this phenomenon to drive host, parasite, and symbiont evolution. We also suggest approaches that can be used to address questions regarding the impact of immune priming on host-microbe dynamics and population structures. Finally, due to the transient nature of some symbionts involved in SMIP, we discuss what it means to be a protective symbiont from ecological and evolutionary perspectives and how such interactions can affect long-term persistence of the symbiosis.

A Microbial Mutualist Within Host Individuals Increases Parasite Transmission Between Host Individuals: Evidence From a Field Mesocosm Experiment

The interactions among host-associated microbes and parasites can have clear consequences for disease susceptibility and progression within host individuals. Yet, empirical evidence for how these interactions impact parasite transmission between host individuals remains scarce. We address this scarcity by using a field mesocosm experiment to investigate the interaction between a systemic fungal endophyte, Epichloë coenophiala, and a fungal parasite, Rhizoctonia solani, in leaves of a grass host, tall fescue (Lolium arundinaceum). Specifically, we investigated how this interaction impacted transmission of the parasite under field conditions in replicated experimental host populations. Epichloë-inoculated populations tended to have greater disease prevalence over time, though this difference had weak statistical support. More clearly, Epichloë-inoculated populations experienced higher peak parasite prevalences than Epichloë-free populations. Epichloë conferred a benefit in growth; Epichloë-inoculated populations had greater aboveground biomass than Epichloë-free populations. Using biomass as a proxy, host density was correlated with peak parasite prevalence, but Epichloë still increased peak parasite prevalence after controlling for the effect of biomass. Together, these results suggest that within-host microbial interactions can impact disease at the population level. Further, while Epichloë is clearly a mutualist of tall fescue, it may not be a defensive mutualist in relation to Rhizoctonia solani.

An experimental test of disease resistance function in the skin-associated bacterial communities of three tropical amphibian species

Variation in the structure of host-associated microbial communities has been correlated with the occurrence and severity of disease in diverse host taxa, suggesting a key role of the microbiome in pathogen defense. However, whether these correlations are typically a cause or consequence of pathogen exposure remains an open question, and requires experimental approaches to disentangle. In amphibians, infection by the fungal pathogen Batrachochytrium dendrobatidis (Bd) alters the skin microbial community in some host species, whereas in other species, the skin microbial community appears to mediate infection dynamics. In this study, we completed experimental Bd exposures in three species of tropical frogs (Agalychnis callidryas, Dendropsophus ebraccatus, Craugastor fitzingeri) that were sympatric with Bd at the time of the study. For all three species, we identified key taxa within the skin bacterial communities that were linked to Bd infection dynamics. We also measured higher Bd infection intensities in D. ebraccatus and C. fitzingeri that were associated with higher mortality in C. fitzingeri. Our findings indicate that microbially-mediated pathogen resistance is a complex trait that can vary within and across host species, and suggest that symbiont communities that have experienced prior selection for defensive microbes may be less likely to be disturbed by pathogen exposure.

Defensive Symbionts and the Evolution of Parasitoid Host Specialization

Insect host-parasitoid interactions abound in nature and are characterized by a high degree of host specialization. In addition to their behavioral and immune defenses, many host species rely on heritable bacterial endosymbionts for defense against parasitoids. Studies on aphids and flies show that resistance conferred by symbionts can be very strong and highly specific, possibly as a result of variation in symbiont-produced toxins. I argue that defensive symbionts are therefore an important source of diversifying selection, promoting the evolution of host specialization by parasitoids. This is likely to affect the structure of host-parasitoid food webs. I consider potential changes in terms of food web complexity, although the nature of these effects will also be influenced by whether maternally transmitted symbionts have some capacity for lateral transfer. This is discussed in the light of available evidence for horizontal transmission routes. Finally, I propose that defensive mutualisms other than microbial endosymbionts may also exert diversifying selection on insect parasitoids.

Why and how do protective symbionts impact immune priming with pathogens in invertebrates?

Growing evidence demonstrates that invertebrates display adaptive-like immune abilities, commonly known as "immune priming". Immune priming is a process by which a host improves its immune defences following an initial pathogenic exposure, leading to better protection after a subsequent infection with the same - or different - pathogens. Nevertheless, beneficial symbionts can enhance similar immune priming processes in hosts, such as when they face repeated infections with pathogens. This "symbiotic immune priming" protects the host against pathogenic viruses, bacteria, fungi, or eukaryotic parasites. In this review, we explore the extent to which protective symbionts interfere and impact immune priming against pathogens from both a mechanical (proximal) and an evolutionary (ultimate) point of view. We highlight that the immune priming of invertebrates is the cornerstone of the tripartite interaction of hosts/symbionts/pathogens. The main shared mechanism of immune priming (induced by symbionts or pathogens) is the sustained immune response at the beginning of host-microbial interactions. However, the evolutionary outcome of immune priming leads to a specific discrimination, which provides enhanced tolerance or resistance depending on the type of microbe. Based on several studies testing immune priming against pathogens in the presence or absence of protective symbionts, we observed that both types of immune priming could overlap and affect each other inside the same hosts. As protective symbionts could be an evolutionary force that influences immune priming, they may help us to better understand the heterogeneity of pathogenic immune priming across invertebrate populations and species.

The Importance of Environmentally Acquired Bacterial Symbionts for the Squash Bug (Anasa tristis), a Significant Agricultural Pest

Most insects maintain associations with microbes that shape their ecology and evolution. Such symbioses have important applied implications when the associated insects are pests or vectors of disease. The squash bug, Anasa tristis (Coreoidea: Coreidae), is a significant pest of human agriculture in its own right and also causes damage to crops due to its capacity to transmit a bacterial plant pathogen. Here, we demonstrate that complete understanding of these insects requires consideration of their association with bacterial symbionts in the family Burkholderiaceae. Isolation and sequencing of bacteria housed in the insects’ midgut crypts indicates that these bacteria are consistent and dominant members of the crypt-associated bacterial communities. These symbionts are closely related to Caballeronia spp. associated with other true bugs in the superfamilies Lygaeoidea and Coreoidea. Fitness assays with representative Burkholderiaceae strains indicate that the association can significantly increase survival and decrease development time, though strains do vary in the benefits that they confer to their hosts, with Caballeronia spp. providing the greatest benefit. Experiments designed to assess transmission mode indicate that, unlike many other beneficial insect symbionts, the bacteria are not acquired from parents before or after hatching but are instead acquired from the environment after molting to a later developmental stage. The bacteria do, however, have the capacity to escape adults to be transmitted to later generations, leaving the possibility for a combination of indirect vertical and horizontal transmission.

Competitive exclusion of phytopathogenic Serratia marcescens from squash bug vectors by the gut endosymbiont Caballeronia

Many insects harbor microbial symbiotic partners that offer protection against pathogens, parasitoids, and other natural enemies. Mounting evidence suggests that these symbiotic microbes can play key roles in determining infection outcomes in insect vectors, making them important players in the quest to develop novel vector control strategies. Using the squash bug Anasa tristis, we investigated how the presence of Caballeronia symbionts affected the persistence and intensity of phytopathogenic Serratia marcescens within the insect vector. We reared insects aposymbiotically and with different Caballeronia isolates, infected them with S. marcescens, then sampled the insects periodically to assess the intensity and persistence of pathogen infection. Squash bugs harboring Caballeronia consistently had much lower-intensity infections and cleared S. marcescens significantly faster than their aposymbiotic counterparts. These patterns held even when we reversed the timing of exposure to symbiont and pathogen. Taken together, these results indicate that Caballeronia symbionts play an essential role in S. marcescens infection outcomes in squash bugs and could be used to alter vector competence to enhance agricultural productivity in the future. Importance Insect-microbe symbioses have repeatedly been shown to profoundly impact an insect's ability to vector pathogens to other hosts. The use of symbiotic microbes to control insect vector populations is of growing interest in agricultural settings. Our study examines how symbiotic microbes affect the dynamics of a plant pathogen infection within the squash bug vector Anasa tristis-a well-documented pest of squash and other cucurbit plants and vector of Serratia marcescens, causative agent of Cucurbit Yellow Vine Disease. We provide evidence that the symbiont Caballeronia prevents successful, long-term establishment of S. marcescens in the squash bug. These findings give us insight into symbiont-pathogen dynamics within the squash bug that could ultimately determine its ability to transmit pathogens and be leveraged to interrupt disease transmission in this system.

Comparing the gut microbiome along the gastrointestinal tract of three sympatric species of wild rodents

Host-microbe interactions within the gastrointestinal tract (GIT) play a pivotal role in shaping host physiology, ecology, and life history. However, these interactions vary across gut regions due to changes in the physical environment or host immune system activity, thereby altering the microbial community. Each animal species may harbor their own unique microbial community due to host species-specific ecological traits such as dietary habits, micro-habitat preferences, and mating behavior as well as physiological traits. While the gut microbiota in wild animals has received much attention over the last decade, most studies comparing closely related species only utilized fecal or colon samples. In this study, we first compared the gut microbial community from the small intestine, cecum, colon, and rectum within three sympatric species of wild rodents (i.e. Apodemus speciosus, A. argenteus, and Myodes rufocanus). We then compared each gut region among host species to determine the effect of both gut region and host species on the gut microbiota. We found that the small intestine harbored a unique microbiome as compared to the lower GIT in all three host species, with the genus Lactobacillus in particular having higher abundance in the small intestine of all three host species. There were clear interspecific differences in the microbiome within all gut regions, although some similarity in alpha diversity and community structure within the small intestine was found. Finally, fecal samples may be appropriate for studying the lower GIT in these species, but not the small intestine.

Microbiome influence on host community dynamics: Conceptual integration of microbiome feedback with classical host-microbe theory

Microbiomes have profound effects on host fitness, yet we struggle to understand the implications for host ecology. Microbiome influence on host ecology has been investigated using two independent frameworks. Classical ecological theory powerfully represents mechanistic interactions predicting environmental dependence of microbiome effects on host ecology, but these models are notoriously difficult to evaluate empirically. Alternatively, host-microbiome feedback theory represents impacts of microbiome dynamics on host fitness as simple net effects that are easily amenable to experimental evaluation. The feedback framework enabled rapid progress in understanding microbiomes' impacts on plant ecology, and can also be applied to animal hosts. We conceptually integrate these two frameworks by deriving expressions for net feedback in terms of mechanistic model parameters. This generates a precise mapping between net feedback theory and classic population modelling, thereby merging mechanistic understanding with experimental tractability, a necessary step for building a predictive understanding of microbiome influence on host ecology.

Bacteria cultured from the gut of Meccus pallidipennis (Hemiptera: Reduviidae), a triatomine species endemic to Mexico

The study of intestinal microbiota in vector insects like triatomines is paramount in parasitology because many parasitic species inhabit the vector's gut. Although knowledge on the gut microbiota in various vectors of the parasitic flagellate Trypanosoma cruzi has grown, research efforts have focused on South American triatomines. This study reports the isolation of bacterial microbiota in the anterior and posterior gut of Meccus pallidipennis (a triatomine species endemic to Mexico) by culture, as well as its identification by phenotypic and biochemical tests and its quantification by counting colony-forming units. The study was performed on fifth-instar nymph and adult specimens of M. pallidipennis, either laboratory-bred or collected in the field and either infected or not with T. cruzi. Overall, 17 bacterial species were identified, with the genera Bacillus and Staphylococcus being the most prevalent regardless of the origin of the insects. No differences were observed in the number of bacterial species in the gut of laboratory-bred and field-collected insects, neither with respect to life stage or infection status. In general, the Shannon-Weaver diversity index was higher in non-infected insects than in infected ones. Further studies using non-culture methods are required to determine whether bacterial species diversity is modified by laboratory breeding.

Indirect interactions among co-infecting parasites and a microbial mutualist impact disease progression

Interactions among parasites and other microbes within hosts can impact disease progression, yet study of such interactions has been mostly limited to pairwise combinations of microbes. Given the diversity of microbes within hosts, indirect interactions among more than two microbial species may also impact disease. To test this hypothesis, we performed inoculation experiments that investigated interactions among two fungal parasites, Rhizoctonia solani and Colletotrichum cereale, and a systemic fungal endophyte, Epichloë coenophiala, within the grass, tall fescue (Lolium arundinaceum). Both direct and indirect interactions impacted disease progression. While the endophyte did not directly influence R. solani disease progression or C. cereale symptom development, the endophyte modified the interaction between the two parasites. The magnitude of the facilitative effect of C. cereale on the growth of R. solani tended to be greater when the endophyte was present. Moreover, this interaction modification strongly affected leaf mortality. For plants lacking the endophyte, parasite co-inoculation did not increase leaf mortality compared to single-parasite inoculations. By contrast, for endophyte-infected plants, parasite co-inoculation increased leaf mortality compared to inoculation with R. solani or C. cereale alone by 1.9 or 4.9 times, respectively. Together, these results show that disease progression can be strongly impacted by indirect interactions among microbial symbionts.

Quantitative trait locus analysis of parasitoid counteradaptation to symbiont-conferred resistance

Insect hosts and parasitoids are engaged in an intense struggle of antagonistic coevolution. Infection with heritable bacterial endosymbionts can substantially increase the resistance of aphids to parasitoid wasps, which exerts selection on parasitoids to overcome this symbiont-conferred protection (counteradaptation). Experimental evolution in the laboratory has produced counteradapted populations of the parasitoid wasp Lysiphlebus fabarum. These populations can parasitize black bean aphids (Aphis fabae) protected by the bacterial endosymbiont Hamiltonella defensa, which confers high resistance against L. fabarum. We used two experimentally evolved parasitoid populations to study the genetic architecture of the counteradaptation to symbiont-conferred resistance by QTL analysis. With simple crossing experiments, we showed that the counteradaptation is a recessive trait depending on the maternal genotype. Based on these results, we designed a customized crossing scheme to genotype a mapping population phenotyped for the ability to parasitize Hamiltonella-protected aphids. Using 1835 SNP markers obtained by ddRAD sequencing, we constructed a high-density linkage map consisting of six linkage groups (LGs) with an overall length of 828.3 cM and an average marker spacing of 0.45 cM. We identified a single QTL associated with the counteradaptation to Hamiltonella in L. fabarum on linkage group 2. Out of 120 genes located in this QTL, several genes encoding putative venoms may represent candidates for counteradaptation, as parasitoid wasps inject venoms into their hosts during oviposition.

Characterization and comparison of the bacterial microbiota of Lysiphlebia japonica parasitioid wasps and their aphid host Aphis gosypii

BACKGROUND

Endosymbiotic bacteria have been reported to mediate interactions between parasitoids and their insect hosts. How parasitic wasps influence changes in host microbial communities and the relationship between them are of great importance to the study of host-parasitoid co-evolutionary and ecological interactions. However, these interactions remain largely unreported for interactions between Aphis gossypii and Lysiphlebia japonica.

RESULTS

In this study, we characterize the bacterial microbiota of L. japonica wasps at different developmental stages and monitor changes over time in the bacterial microbiota of their parasitized and nonparasitized aphid hosts, using metagenomic analysis of 16S rDNA sequencing data. Proteobacteria, Firmicutes, and Actinobacteria were the three most abundant bacterial phyla identified in L. japonica. We found that parasitism was associated with an increased abundance of Buchnera nutritional endosymbionts, but decreased abundance of Acinetobacter, Arsenophonus, Candidatus_Hamiltonella, and Pseudomonas facultative symbionts in aphid hosts. Functional analysis of enriched pathways of parasitized aphids showed significant differences in the 'transport and metabolism of carbohydrates' and 'amino acid, lipid, and coenzyme biosynthesis' pathways. Notably, the composition of symbiotic bacteria in wasp larvae was highly similar to that of their aphid hosts, especially the high abundance of Buchnera.

CONCLUSION

The results provide a conceptual framework for L. japonica interactions with A. gossypii in which the exchange of symbiotic microbes provides a means by which microbiota can potentially serve as evolutionary drivers of complex, multilevel interactions underlying the ecology and co-evolution of these hosts and parasites. © 2021 Society of Chemical Industry.

Microbially Mediated Chemical Ecology of Animals: A Review of Its Role in Conspecific Communication, Parasitism and Predation

Microbial symbionts are nowadays considered of pivotal importance for animal life. Among the many processes where microorganisms are involved, an emerging research avenue focuses on their major role in driving the evolution of chemical communication in their hosts. Volatiles of bacterial origin may underlie chemical communication and the transfer of social information through signals, as well as inadvertent social information. We reviewed the role of microorganisms in animal communication between conspecifics, and, because the microbiome may cause beneficial as well as deleterious effects on their animal hosts, we also reviewed its role in determining the outcome of the interactions with parasites and predators. Finally, we paid special attention to the hypothetical role of predation and parasitism in driving the evolution of the animal microbiome. We highlighted the novelty of the theoretical framework derived from considering the microbiota of animals in scenarios of communication, parasitism, and predation. We aimed to encourage research in these areas, suggesting key predictions that need to be tested to better understand what is one of the main roles of bacteria in animal biology.

Interactions of host-associated multispecies bacterial communities

The oral microbiome comprises microbial communities colonizing biotic (epithelia, mucosa) and abiotic (enamel) surfaces. Different communities are associated with health (eg, immune development, pathogen resistance) and disease (eg, tooth loss and periodontal disease). Like any other host-associated microbiome, colonization and persistence of both beneficial and dysbiotic oral microbiomes are dictated by successful utilization of available nutrients and defense against host and competitor assaults. This chapter will explore these general features of microbe-host interactions through the lens of symbiotic (mutualistic and antagonistic/pathogenic) associations with nonmammalian animals. Investigations in such systems across a broad taxonomic range have revealed conserved mechanisms and processes that underlie the complex associations among microbes and between microbes and hosts.

Differential gene expression in a tripartite interaction: Drosophila, Spiroplasma and parasitic wasps

BACKGROUND

Several facultative bacterial symbionts of insects protect their hosts against natural enemies. Spiroplasma poulsonii strain sMel (hereafter Spiroplasma), a male-killing heritable symbiont of Drosophila melanogaster, confers protection against some species of parasitic wasps. Several lines of evidence suggest that Spiroplasma-encoded ribosome inactivating proteins (RIPs) are involved in the protection mechanism, but the potential contribution of the fly-encoded functions (e.g., immune response), has not been deeply explored.

METHODS

Here we used RNA-seq to evaluate the response of D. melanogaster to infection by Spiroplasma and parasitism by the Spiroplasma-susceptible wasp Leptopilina heterotoma, and the Spiroplasma-resistant wasp Ganaspis sp. In addition, we used quantitative (q)PCR to evaluate the transcript levels of the Spiroplasma-encoded Ribosomal inactivation protein (RIP) genes.

RESULTS

In the absence of Spiroplasma infection, we found evidence of Drosophila immune activation by Ganaspis sp., but not by L. heterotoma, which in turn negatively influenced functions associated with male gonad development. As expected for a symbiont that kills males, we detected extensive downregulation in the Spiroplasma-infected treatments of genes known to have male-biased expression. We detected very few genes whose expression patterns appeared to be influenced by the Spiroplasma-L. heterotoma interaction, and these genes are not known to be associated with immune response. For most of these genes, parasitism by L. heterotoma (in the absence of Spiroplasma) caused an expression change that was at least partly reversed when both L. heterotoma and Spiroplasma were present. It is unclear whether such genes are involved in the Spiroplasma-mediated mechanism that leads to wasp death and/or fly rescue. Nonetheless, the expression pattern of some of these genes, which reportedly undergo expression shifts during the larva-to-pupa transition, is suggestive of an influence of Spiroplasma on the development time of L. heterotoma-parasitized flies. One of the five RIP genes (RIP2) was consistently highly expressed independently of wasp parasitism, in two substrains of sMel. Finally, the RNAseq data revealed evidence consistent with RIP-induced damage in the ribosomal (r)RNA of the Spiroplasma-susceptible, but not the Spiroplasma-resistant, wasp. Acknowledging the caveat that we lacked adequate power to detect the majority of DE genes with fold-changes lower than 3, we conclude that immune priming is unlikely to contribute to the Spiroplasma-mediated protection against wasps, and that the mechanism by which Ganaspis sp. resists/tolerates Spiroplasma does not involve inhibition of RIP transcription.

Intraspecific variation in symbiont density in an insect-microbe symbiosis

Many insects host vertically transmitted microbes, which can confer benefits to their hosts but are costly to maintain and regulate. A key feature of these symbioses is variation: for example, symbiont density can vary among host and symbiont genotypes. However, the evolutionary forces maintaining this variation remain unclear. We studied variation in symbiont density using the pea aphid (Acyrthosiphon pisum) and the bacterium Regiella insecticola, a symbiont that can protect its host against fungal pathogens. We found that relative symbiont density varies both between two Regiella phylogenetic clades and among aphid "biotypes." Higher density symbiont infections are correlated with stronger survival costs, but variation in density has little effect on the protection Regiella provides against fungi. Instead, we found that in some aphid genotypes, a dramatic decline in symbiont density precedes the loss of a symbiont infection. Together, our data suggest that the optimal density of a symbiont infection is likely different from the perspective of aphid and microbial fitness. Regiella might prevent loss by maintaining high within-host densities, but hosts do not appear to benefit from higher symbiont numbers and may be advantaged by losing costly symbionts in certain environments. The standing variation in symbiont density observed in natural populations could therefore be maintained by antagonistic coevolutionary interactions between hosts and their symbiotic microbes.

Dysregulation of host-control causes interspecific conflict over host investment into symbiotic organs

Microbial mutualists provide substantial benefits to hosts that feed back to enhance the fitness of the associated microbes. In many systems, beneficial microbes colonize symbiotic organs, specialized host structures that house symbionts and mediate resources exchanged between parties. Mutualisms are characterized by net benefits exchanged among members of different species, however, inequalities in the magnitude of these exchanges could result in evolutionary conflict, destabilizing the mutualism. We investigated joint fitness effects of root nodule formation, the symbiotic organ of legumes that house nitrogen-fixing rhizobia in planta. We quantified host and symbiont fitness parameters dependent on the number of nodules formed using near-isogenic Lotus japonicus and Mesorhizobium loti mutants, respectively. Empirically estimated fitness functions suggest that legume and rhizobia fitness is aligned as the number of nodules formed increases from zero until the host optimum is reached, a point where aligned fitness interests shift to diverging fitness interests between host and symbiont. However, fitness conflict was only inferred when analyzing wild-type hosts along with their mutants dysregulated for control over nodule formation. These data demonstrate that to avoid conflict, hosts must tightly regulate investment into symbiotic organs maximizing their benefit to cost ratio of associating with microbes.

Thermal sensitivity of the Spiroplasma-Drosophila hydei protective symbiosis: The best of climes, the worst of climes

The outcome of natural enemy attack in insects is commonly influenced by the presence of protective symbionts in the host. The degree to which protection functions in natural populations, however, will depend on the robustness of the phenotype and symbiosis to variation in the abiotic environment. We studied the impact of a key environmental parameter-temperature-on the efficacy of the protective effect of the symbiont Spiroplasma on its host Drosophila hydei, against attack by the parasitoid wasp Leptopilina heterotoma. In addition, we investigated the thermal sensitivity of the symbiont's vertical transmission, which may be a key determinant of the ability of the symbiont to persist. We found that vertical transmission was more robust than previously considered, with Spiroplasma being maintained at 25°C, at 18°C and with 18/15°C diurnal cycles, with rates of segregational loss only increasing at 15°C. Protection against wasp attack was ablated before symbiont transmission was lost, with the symbiont failing to rescue the fly host at 18°C. We conclude that the presence of a protective symbiosis in natural populations cannot be simply inferred from the presence of a symbiont whose protective capacity has been tested under narrow controlled conditions. More broadly, we argue that the thermal environment is likely to represent an important determinant of the evolutionary ecology of defensive symbioses in natural environments, potentially driving seasonal, latitudinal and altitudinal variation in symbiont frequency.

Don't stand so close to me: Microbiota-facilitated enemy release dynamics in introduced Onthophagus taurus dung beetles

Microbial symbionts can influence their hosts in stunningly diverse ways. Emerging research suggests that an underappreciated facet of these relationships is the influence microbes can have on their host's responses to novel, or stressful, environmental conditions. We sought to address these and related questions in populations resulting from the recent introduction and subsequent rapid range expansion of Onthophagus taurus dung beetles. Specifically, we manipulated both microbial communities and rearing temperature to detect signatures of developmental and life history differentiation in response to the local thermal conditions in two populations derived from the southern most (Florida) and northern most (Michigan) extremes of the exotic Eastern U.S. range of O. taurus. We then sought to determine the contributions, if any, of host-associated microbiota to this differentiation. We found that when reared under common garden conditions individuals from Florida and Michigan populations differed significantly in developmental performance measures and life history traits, consistent with population divergence. At the same time, and contrary to our predictions, we failed to find support for the hypothesis that animals perform better if reared at temperatures that match their location of origin and that performance differences may be mediated by host-associated microbiota. Instead, we found that microbiome swapping across host populations improved developmental performance in both populations, consistent with enemy release dynamics. We discuss the implications of our results for our understanding of the rapid spread of exotic O. taurus through the Eastern United States and the significance of symbiosis in host responses to novel environmental conditions more broadly.

Pathogen resistance may be the principal evolutionary advantage provided by the microbiome

To survive, plants and animals must continually defend against pathogenic microbes that would invade and disrupt their tissues. Yet they do not attempt to extirpate all microbes. Instead, they tolerate and even encourage the growth of commensal microbes, which compete with pathogens for resources and via direct inhibition. We argue that hosts have evolved to cooperate with commensals in order to enhance the pathogen resistance this competition provides. We briefly describe competition between commensals and pathogens within the host, consider how natural selection might favour hosts that tilt this competition in favour of commensals, and describe examples of extant host traits that may serve this purpose. Finally, we consider ways that this cooperative immunity may have facilitated the adaptive evolution of non-pathogen-related host traits. On the basis of these observations, we argue that pathogen resistance vies with other commensal-provided benefits for being the principal evolutionary advantage provided by the microbiome to host lineages across the tree of life. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Evolution of animal immunity in the light of beneficial symbioses

Immune system processes serve as the backbone of animal defences against pathogens and thus have evolved under strong selection and coevolutionary dynamics. Most microorganisms that animals encounter, however, are not harmful, and many are actually beneficial. Selection should act on hosts to maintain these associations while preventing exploitation of within-host resources. Here, we consider how several key aspects of beneficial symbiotic associations may shape host immune system evolution. When host immunity is used to regulate symbiont populations, there should be selection to evolve and maintain targeted immune responses that recognize symbionts and suppress but not eliminate symbiont populations. Associating with protective symbionts could relax selection on the maintenance of redundant host-derived immune responses. Alternatively, symbionts could facilitate the evolution of host immune responses if symbiont-conferred protection allows for persistence of host populations that can then adapt. The trajectory of immune system evolution will likely differ based on the type of immunity involved, the symbiont transmission mode and the costs and benefits of immune system function. Overall, the expected influence of beneficial symbiosis on immunity evolution depends on how the host immune system interacts with symbionts, with some interactions leading to constraints while others possibly relax selection on immune system maintenance. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Pathogen resistance may be the principal evolutionary advantage provided by the microbiome

To survive, plants and animals must continually defend against pathogenic microbes that would invade and disrupt their tissues. Yet they do not attempt to extirpate all microbes. Instead, they tolerate and even encourage the growth of commensal microbes, which compete with pathogens for resources and via direct inhibition. We argue that hosts have evolved to cooperate with commensals in order to enhance the pathogen resistance this competition provides. We briefly describe competition between commensals and pathogens within the host, consider how natural selection might favour hosts that tilt this competition in favour of commensals, and describe examples of extant host traits that may serve this purpose. Finally, we consider ways that this cooperative immunity may have facilitated the adaptive evolution of non-pathogen-related host traits. On the basis of these observations, we argue that pathogen resistance vies with other commensal-provided benefits for being the principal evolutionary advantage provided by the microbiome to host lineages across the tree of life.

This article is part of the theme issue ‘The role of the microbiome in host evolution’.

Impact of Facultative Bacteria on the Metabolic Function of an Obligate Insect-Bacterial Symbiosis

Beneficial microorganisms associated with animals derive their nutritional requirements entirely from the animal host, but the impact of these microorganisms on host metabolism is largely unknown. The focus of this study was the experimentally tractable tripartite symbiosis between the pea aphid Acyrthosiphon pisum, its obligate intracellular bacterial symbiont Buchnera, and the facultative bacterium Hamiltonella which is localized primarily to the aphid hemolymph (blood). Metabolome experiments on, first, multiple aphid genotypes that naturally bear or lack Hamiltonella and, second, one aphid genotype from which Hamiltonella was experimentally eliminated revealed no significant effects of Hamiltonella on aphid metabolite profiles, indicating that Hamiltonella does not cause major reconfiguration of host metabolism. However, the titer of just one metabolite, 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR), displayed near-significant enrichment in Hamiltonella-positive aphids in both metabolome experiments. AICAR is a by-product of biosynthesis of the essential amino acid histidine in Buchnera and, hence, an index of histidine biosynthetic rates, suggesting that Buchnera-mediated histidine production is elevated in Hamiltonella-bearing aphids. Consistent with this prediction, aphids fed on [¹³C]histidine yielded a significantly elevated ¹²C/¹³C ratio of histidine in Hamiltonella-bearing aphids, indicative of increased (∼25%) histidine synthesized de novo by Buchnera However, in silico analysis predicted an increase of only 0.8% in Buchnera histidine synthesis in Hamiltonella-bearing aphids. We hypothesize that Hamiltonella imposes increased host demand for histidine, possibly for heightened immune-related functions. These results demonstrate that facultative bacteria can alter the dynamics of host metabolic interactions with co-occurring microorganisms, even when the overall metabolic homeostasis of the host is not substantially perturbed.IMPORTANCE Although microbial colonization of the internal tissues of animals generally causes septicemia and death, various animals are persistently associated with benign or beneficial microorganisms in their blood or internal organs. The metabolic consequences of these persistent associations for the animal host are largely unknown. Our research on the facultative bacterium Hamiltonella, localized primarily to the hemolymph of pea aphids, demonstrated that although Hamiltonella imposed no major reconfiguration of the aphid metabolome, it did alter the metabolic relations between the aphid and its obligate intracellular symbiont, Buchnera Specifically, Buchnera produced more histidine in Hamiltonella-positive aphids to support both Hamiltonella demand for histidine and Hamiltonella-induced increase in host demand. This study demonstrates how microorganisms associated with internal tissues of animals can influence specific aspects of metabolic interactions between the animal host and co-occurring microorganisms.

Defensive Symbiosis and the Evolution of Virulence

AbstractA microbiome rife with enemies of the host should cause selection for defensive traits in symbionts, yet such complex environments are also predicted to select for greater symbiont virulence. Why then do we so often observe defensive mutualists that protect hosts while causing little to no damage? To address this question, we build a symbiont-centered model that incorporates the evolution of two independent symbiont traits: defense and virulence. Virulence is modeled as a continuous trait spanning parasitism (positive virulence) and mutualism (negative virulence), thus accounting for the entire range of direct effects that symbionts have on host mortality. Defense is modeled as a continuous trait that ameliorates the costs to the host associated with infection by a deleterious parasite. We show that the evolution of increased defense in one symbiont may lead to the evolution of lower virulence in both symbionts and even facilitate pathogens evolving to mutualism. However, results are context dependent, and when defensive traits are costly, the evolution of greater defense may also lead to the evolution of greater virulence, breaking the common expectation that defensive symbionts are necessarily mutualists toward the host.

An integrative approach to symbiont-mediated vector control for agricultural pathogens

Vector-borne pathogens pose significant threats to agricultural productivity. Methods that exploit associations between insects and their symbiotic microbes, dubbed symbiont-mediated vector control, are emerging as viable alternatives to insecticides for the control of vector-borne agricultural plant pathogens. The development of methods for effective microbial manipulation, such as RNA interference and paratransgenesis, may facilitate symbiont-mediated vector control tactics aimed at either suppressing insect populations or at manipulating vector competence, an insect vector's ability to acquire, harbor, and transmit pathogens. As suppression strategies transition from the laboratory to the field, the need for methods to evaluate their viability and predict their outcomes is apparent. Mathematical models of symbiont impact on agricultural disease can inform the development of symbiont-mediated vector control. We propose an integrative approach, combining theoretical and empirical experiments to identify the best practices for achieving meaningful improvements to crop health and productivity.

Recurrent mutualism breakdown events in a legume rhizobia metapopulation

Bacterial mutualists generate major fitness benefits for eukaryotes, reshaping the host phenotype and its interactions with the environment. Yet, microbial mutualist populations are predicted to generate mutants that defect from providing costly services to hosts while maintaining the capacity to exploit host resources. Here, we examined the mutualist service of symbiotic nitrogen fixation in a metapopulation of root-nodulating Bradyrhizobium spp. that associate with the native legume Acmispon strigosus. We quantified mutualism traits of 85 Bradyrhizobium isolates gathered from a 700 km transect in California spanning 10 sampled A. strigosus populations. We clonally inoculated each Bradyrhizobium isolate onto A. strigosus hosts and quantified nodulation capacity and net effects of infection, including host growth and isotopic nitrogen concentration. Six Bradyrhizobium isolates from five populations were categorized as ineffective because they formed nodules but did not enhance host growth via nitrogen fixation. Six additional isolates from three populations failed to form root nodules. Phylogenetic reconstruction inferred two types of mutualism breakdown, including three to four independent losses of effectiveness and five losses of nodulation capacity on A. strigosus. The evolutionary and genomic drivers of these mutualism breakdown events remain poorly understood.

The Developmental Stage Symbionts of the Pea Aphid-Feeding Chrysoperla sinica (Tjeder)

Chrysoperla sinica (Tjeder) is widely recognized as an important holometabolous natural enemy of various insect pests in different cropping systems and as a non-target surrogate in environmental risk assessment of Bt rice (i.e., genetically modified rice to express a toxin gene from Bacillus thuringiensis). Like other complex organisms, abundant microbes live inside C. sinica; however, to date, microbiome composition and diversity of the whole life cycle in C. sinica has not yet been well characterized. In the current study, we analyze the composition and biodiversity of microbiota across the whole life cycle of C. sinica by using high-throughput Illumina sequencing of the 16S ribosomal RNA gene. Collectively, Proteobacteria and Firmicutes dominated the microenvironment at all stages, but their relative abundances fluctuated by host developmental stage. Interestingly, eggs, neonates, and adults shared similar microbes, including an abundance of Rickettsia and Wolbachia. After larva feeding, Staphylococcus, Enterobacteriaceae, and Serratia were enriched in larvae and pupa, suggesting that food may serve as a major factor contributing to altered microbial community divergence at different developmental stages. Our findings demonstrated that C. sinica harbor a variety of bacteria, and that dynamic changes in community composition and relative abundances of members of its microbiome occur during different life cycle stages. Evaluating the role of these bacterial symbionts in this natural enemy may assist in developing environmental risk assessments and novel biological control strategies.

Not all animals need a microbiome

It is often taken for granted that all animals host and depend upon a microbiome, yet this has only been shown for a small proportion of species. We propose that animals span a continuum of reliance on microbial symbionts. At one end are the famously symbiont-dependent species such as aphids, humans, corals and cows, in which microbes are abundant and important to host fitness. In the middle are species that may tolerate some microbial colonization but are only minimally or facultatively dependent. At the other end are species that lack beneficial symbionts altogether. While their existence may seem improbable, animals are capable of limiting microbial growth in and on their bodies, and a microbially independent lifestyle may be favored by selection under some circumstances. There is already evidence for several ‘microbiome-free’ lineages that represent distantly related branches in the animal phylogeny. We discuss why these animals have received such little attention, highlighting the potential for contaminants, transients, and parasites to masquerade as beneficial symbionts. We also suggest ways to explore microbiomes that address the limitations of DNA sequencing. We call for further research on microbiome-free taxa to provide a more complete understanding of the ecology and evolution of macrobe-microbe interactions.

Comparative genomics and transcriptomics of host-pathogen interactions in insects: evolutionary insights and future directions

Classical evolutionary studies of protein-coding genes have established that genes in the canonical immune system are often among the most rapidly evolving within and between species. As more genomes and transcriptomes across insects are sequenced, it is becoming clear that duplications and losses of immune genes are also a likely consequence of host-pathogen interactions. Furthermore, particular species respond to diverse pathogenic challenges with a wide range of challenge-specific responses that are still poorly understood. Transcriptional studies, using RNA-seq to characterize the infection-regulated transcriptome of diverse insects, are crucial for additional progress in understanding the ecology and evolution of the full complexity of the host response.

Interactions Between Facultative Symbionts Hamiltonella and Cardinium in Bemisia tabaci (Hemiptera: Aleyrodoidea): Cooperation or Conflict?

Maternally-inherited facultative symbionts are widespread in most insect species, and it is common that several symbionts coexist in the same host individual. Hence, the symbionts may compete or share for the limited resources and space in the host. The whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodoidea), harbors a diverse array of facultative symbionts, among which Hamiltonella sp. and Cardinium sp. are abundant species. Hamiltonella alone increases host fitness, while Cardinium alone confers lower fitness. Locking those different partners together creates ideal situations for the evolution of interactions between symbionts. In this study, we compared the fitness effects of whiteflies infected with only Hamiltonella to Hamiltonella-Cardnium co-infected whiteflies and measured the density of Hamiltonella and Cardinium during host aging, aiming to explore Hamiltonella-Cardinium interactions in B. tabaci. Our results illustrated that Hamiltonella-Cardinium coinfection induced lower fecundity, egg hatchability and number of female offspring, leading to a male-biased sex ratio in offspring, while there is no evidence for reproductive incompatibility between the infections. We also found an antagonistic interaction between Hamiltonella and Cardinium given that the density of the latter increased across time and led to a decrease of Hamiltonella density, which may be the underlying causes of the fitness cost in double-infected B. tabaci. Exploring the ecological consequences of co-infections of these different symbionts helps us to understand the nature of host-symbiont interactions in this species and potential for evolutionary conflict.

Readapting to DCV Infection without Wolbachia: Frequency Changes of Drosophila Antiviral Alleles Can Replace Endosymbiont Protection

There is now ample evidence that endosymbionts can contribute to host adaptation to environmental challenges. However, how endosymbiont presence affects the adaptive trajectory and outcome of the host is yet largely unexplored. In Drosophila, Wolbachia confers protection to RNA virus infection, an effect that differs between Wolbachia strains and can be targeted by selection. Adaptation to RNA virus infections is mediated by both Wolbachia and the host, raising the question of whether adaptive genetic changes in the host vary with the presence/absence of the endosymbiont. Here, we address this question using a polymorphic D. melanogaster population previously adapted to DCV infection for 35 generations in the presence of Wolbachia, from which we removed the endosymbiont and followed survival over the subsequent 20 generations of infection. After an initial severe drop, survival frequencies upon DCV selection increased significantly, as seen before in the presence of Wolbachia. Whole-genome sequencing, revealed that the major genes involved in the first selection experiment, pastrel and Ubc-E2H, continued to be selected in Wolbachia-free D. melanogaster, with the frequencies of protective alleles being closer to fixation in the absence of Wolbachia. Our results suggest that heterogeneity in Wolbachia infection status may be sufficient to maintain polymorphisms even in the absence of costs.

Bacterial symbionts in human blood-feeding arthropods: Patterns, general mechanisms and effects of global ecological changes

Due to their high impact on public health, human blood-feeding arthropods are one of the most relevant animal groups. Bacterial symbionts have been long known to play a role in the metabolism, and reproduction of these arthropod vectors. Nowadays, we have a more complete picture of their functions, acknowledging the wide influence of bacterial symbionts on processes ranging from the immune response of the arthropod host to the possible establishment of pathogens and parasites. One or two primary symbiont species have been found to co-evolve along with their host in each taxon (being ticks an exception), leading to various kinds of symbiosis, mostly mutualistic in nature. Moreover, several secondary symbiont species are shared by all arthropod groups. With respect to gut microbiota, several bacterial symbionts genera are hosted in common, indicating that these bacterial groups are prone to invade several hematophagous arthropod species feeding on humans. The main mechanisms underlying bacterium-arthropod symbiosis are discussed, highlighting that even primary symbionts elicit an immune response from the host. Bacterial groups in the gut microbiota play a key role in immune homeostasis, and in some cases symbiont bacteria could be competing directly or indirectly with pathogens and parasites. Finally, the effects climate change, great human migrations, and the increasingly frequent interactions of wild and domestic animal species are analyzed, along with their implications on microbiota alteration and their possible impacts on public health and the control of pathogens and parasites harbored in arthropod vectors of human parasites and pathogens.

Symbiont-Mediated Host-Parasite Dynamics in a Fungus-Gardening Ant

Group-living can promote the evolution of adaptive strategies to prevent and control disease. Fungus-gardening ants must cope with two sets of pathogens, those that afflict the ants themselves and those of their symbiotic fungal gardens. While much research has demonstrated the impact of specialized fungal pathogens that infect ant fungus gardens, most of these studies focused on the so-called higher attine ants, which are thought to coevolve diffusely with two clades of leucocoprinaceous fungi. Relatively few studies have addressed disease ecology of lower Attini, which are thought to occasionally recruit (domesticate) novel leucocoprinaceous fungi from free-living populations; coevolution between lower-attine ants and their fungi is therefore likely weaker (or even absent) than in the higher Attini, which generally have many derived modifications. Toward understanding the disease ecology of lower-attine ants, this study (a) describes the diversity in the microfungal genus Escovopsis that naturally infect fungus gardens of the lower-attine ant Mycocepurus smithii and (b) experimentally determines the relative contributions of Escovopsis strain (a possible garden disease), M. smithii ant genotype, and fungal cultivar lineage to disease susceptibility and colony fitness. In controlled in-vivo infection laboratory experiments, we demonstrate that the susceptibility to Escovopsis infection was an outcome of ant-cultivar-Escovopsis interaction, rather than solely due to ant genotype or fungal cultivar lineage. The role of complex ant-cultivar-Escovopsis interactions suggests that switching M. smithii farmers onto novel fungus types might be a strategy to generate novel ant-fungus combinations resistant to most, but perhaps not all, Escovopsis strains circulating in a local population of this and other lower-attine ants.

The cotton stainer's gut microbiota suppresses infection of a cotransmitted trypanosomatid parasite

The evolutionary and ecological success of many insects is attributed to mutualistic partnerships with bacteria that confer hosts with novel traits including food digestion, nutrient supplementation, detoxification of harmful compounds and defence against natural enemies. Dysdercus fasciatus firebugs (Hemiptera: Pyrrhocoridae), commonly known as cotton stainers, possess a simple but distinctive gut bacterial community including B vitamin-supplementing Coriobacteriaceae symbionts. In addition, their guts are often infested with the intestinal trypanosomatid parasite Leptomonas pyrrhocoris (Kinetoplastida: Trypanosomatidae). In this study, using experimental bioassays and fluorescence in situ hybridization (FISH), we report on the protective role of the D. fasciatus gut bacteria against L. pyrrhocoris. We artificially infected 2nd instars of dysbiotic and symbiotic insects with a parasite culture and measured parasite titres, developmental time and survival rates. Our results show that L. pyrrhocoris infection increases developmental time and slightly modifies the quantitative composition of the gut microbiota. More importantly, we found significantly higher parasite titres and a tendency towards lower survival rates in parasite-infected dysbiotic insects compared to symbiotic controls, indicating that the gut bacteria successfully interfere with the establishment or proliferation of L. pyrrhocoris. The colonization of symbiotic bacteria on the peritrophic matrix along the gut wall, as revealed by FISH, likely acts as a barrier blocking parasite attachment or entry into the hemolymph. Our findings show that in addition to being nutritionally important, D. fasciatus' gut bacteria complement the host's immune system in preventing parasite invasions and that a stable gut microbial community is integral for the host's health.

The role of defensive symbionts in host-parasite coevolution

Understanding the coevolution of hosts and parasites is a long-standing goal of evolutionary biology. There is a well-developed theoretical framework to describe the evolution of host-parasite interactions under the assumption of direct, two-species interactions, which can result in arms race dynamics or sustained genotype fluctuations driven by negative frequency dependence (Red Queen dynamics). However, many hosts rely on symbionts for defence against parasites. Whilst the ubiquity of defensive symbionts and their potential importance for disease control are increasingly recognized, there is still a gap in our understanding of how symbionts mediate or possibly take part in host-parasite coevolution. Herein we address this question by synthesizing information already available from theoretical and empirical studies. First, we briefly introduce current hypotheses on how defensive mutualisms evolved from more parasitic relationships and highlight exciting new experimental evidence showing that this can occur very rapidly. We go on to show that defensive symbionts influence virtually all important determinants of coevolutionary dynamics, namely the variation in host resistance available to selection by parasites, the specificity of host resistance, and the trade-off structure between host resistance and other components of fitness. In light of these findings, we turn to the limited theory and experiments available for such three-species interactions to assess the role of defensive symbionts in host-parasite coevolution. Specifically, we discuss under which conditions the defensive symbiont may take over from the host the reciprocal adaptation with parasites and undergo its own selection dynamics, thereby altering or relaxing selection on the hosts' own immune defences. Finally, we address potential effects of defensive symbionts on the evolution of parasite virulence. This is an important problem for which there is no single, clear-cut prediction. The selection on parasite virulence resulting from the presence of defensive symbionts in their hosts will depend on the underlying mechanism of defence. We identify the evolutionary predictions for different functional categories of symbiont-conferred resistance and we evaluate the empirical literature for supporting evidence. We end this review with outstanding questions and promising avenues for future research to improve our understanding of symbiont-mediated coevolution between hosts and parasites.