Genomic basis of endosymbiont-conferred protection against an insect parasitoid

A dual insect symbiont and plant pathogen improves insect host fitness under arginine limitation

Some facultative bacterial symbionts are known to benefit insects, but nutritional advantages are rare among these non-obligate symbionts. Here, we demonstrate that the facultative symbiont Candidatus Liberibacter psyllaurous enhances the fitness of its psyllid insect host, Bactericera cockerelli, by providing nutritional benefits. L. psyllaurous, an unculturable pathogen of solanaceous crops, also establishes a close relationship with its insect vector, B. cockerelli, increasing in titer during insect development, vertically transmitting through eggs, and colonizing various tissues, including the bacteriome, which houses the obligate nutritional symbiont, Carsonella. Carsonella supplies essential amino acids to its insect host but has gaps in some of its essential amino acid pathways that the psyllid complements with its own genes, many of which have been acquired through horizontal gene transfer (HGT) from bacteria. Our findings reveal that L. psyllaurous increases psyllid fitness on plants by reducing developmental time and increasing adult weight. In addition, through metagenomic sequencing, we reveal that L. psyllaurous maintains complete pathways for synthesizing the essential amino acids arginine, lysine, and threonine, unlike the psyllid's other resident microbiota, Carsonella, and two co-occurring Wolbachia strains. RNA sequencing reveals the downregulation of a HGT collaborative psyllid gene (ASL), which indicates a reduced demand for arginine supplied by Carsonella when the psyllid is infected with L. psyllaurous. Notably, artificial diet assays show that L. psyllaurous enhances psyllid fitness on an arginine-deplete diet. These results corroborate the role of L. psyllaurous as a beneficial insect symbiont, contributing to the nutrition of its insect host.IMPORTANCEUnlike obligate symbionts that are permanently associated with their hosts, facultative symbionts rarely show direct nutritional contributions, especially under nutrient-limited conditions. This study demonstrates, for the first time, that Candidatus Liberibacter psyllaurous, a facultative symbiont and a plant pathogen, enhances the fitness of its Bactericera cockerelli host by supplying an essential nutrient arginine that is lacking in the plant sap diet. Our findings reveal how facultative symbionts can play a vital role in helping their insect hosts adapt to nutrient-limited environments. This work provides new insights into the dynamic interactions between insect hosts, their symbiotic microbes, and their shared ecological niches, broadening our understanding of symbiosis and its role in shaping adaptation and survival.

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.

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.

Cascading effects of prey identity on gene expression in a kleptoplastidic ciliate

Kleptoplastidic, or chloroplast stealing, lineages transiently retain functional photosynthetic machinery from algal prey. This machinery, and its photosynthetic outputs, must be integrated into the host's metabolism, but the details of this integration are poorly understood. Here, we study this metabolic integration in the ciliate Mesodinium chamaeleon, a coastal marine species capable of retaining chloroplasts from at least six distinct genera of cryptophyte algae. To assess the effects of feeding history on ciliate physiology and gene expression, we acclimated M. chamaeleon to four different types of prey and contrasted well-fed and starved treatments. Consistent with previous physiological work on the ciliate, we found that starved ciliates had lower chlorophyll content, photosynthetic rates, and growth rates than their well-fed counterparts. However, ciliate gene expression mirrored prey phylogenetic relationships rather than physiological status, suggesting that, even as M. chamaeleon cells were starved of prey, their overarching regulatory systems remained tuned to the prey type to which they had been acclimated. Collectively, our results indicate a surprising degree of prey-specific host transcriptional adjustments, implying varied integration of prey metabolic potential into many aspects of ciliate physiology.

Influence of Bacterial Secondary Symbionts in Sitobion avenae on Its Survival Fitness against Entomopathogenic Fungi, Beauveria bassiana and Metarhizium brunneum

The research was focused on the ability of wheat aphids Sitobion avenae, harboring bacterial secondary symbionts (BSS) Hamiltonella defensa or Regiella insecticola, to withstand exposure to fungal isolates of Beauveria bassiana and Metarhizium brunneum. In comparison to aphids lacking bacterial secondary symbionts, BSS considerably increased the lifespan of wheat aphids exposed to B. bassiana strains (Bb1022, EABb04/01-Tip) and M. brunneum strains (ART 2825 and BIPESCO 5) and also reduced the aphids' mortality. The wheat aphid clones lacking bacterial secondary symbionts were shown to be particularly vulnerable to M. brunneum strain BIPESCO 5. As opposed to wheat aphids carrying bacterial symbionts, fungal pathogens infected the wheat aphids lacking H. defensa and R. insecticola more quickly. When treated with fungal pathogens, bacterial endosymbionts had a favorable effect on the fecundity of their host aphids compared to the aphids lacking these symbionts, but there was no change in fungal sporulation on the deceased aphids. By defending their insect hosts against natural enemies, BSS increase the population of their host society and may have a significant impact on the development of their hosts.

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.

Hosting certain facultative symbionts modulates the phenoloxidase activity and immune response of the pea aphid Acyrthosiphon pisum

The pea aphid Acyrthosiphon pisum hosts different facultative symbionts (FS) which provide it with various benefits, such as tolerance to heat or protection against natural enemies (e.g., fungi, parasitoid wasps). Here, we investigated whether and how the presence of certain FS could affect phenoloxidase (PO) activity, a key component of insect innate immunity, under normal and stressed conditions. For this, we used clones of A. pisum of different genetic backgrounds (LL01, YR2 and T3-8V1) lacking FS or harboring one or two (Regiella insecticola, Hamiltonella defensa, Serratia symbiotica + Rickettsiella viridis). Gene expression and proteomics analyses of the aphid hemolymph indicated that the two A. pisum POs, PPO1 and PPO2, are expressed and translated into proteins. The level of PPO genes expression as well as the amount of PPO proteins and phenoloxidase activity in the hemolymph depended on both the aphid genotype and FS species. In particular, H. defensa and R. insecticola, but not S. symbiotica + R. viridis, caused a sharp decrease in PO activity by interfering with both transcription and translation. The microinjection of different types of stressors (yeast, Escherichia coli, latex beads) in the YR2 lines hosting different symbionts affected the survival rate of aphids and, in most cases, also decreased the expression of PPO genes after 24 h. The amount and activity of PPO proteins varied according to the type of FS and stressor, without clear corresponding changes in gene expression. These data demonstrate that the presence of certain FS influences an important component of pea aphid immunity.

Influence of "protective" symbionts throughout the different steps of an aphid-parasitoid interaction

Microbial associates are widespread in insects, some conferring a protection to their hosts against natural enemies like parasitoids. These protective symbionts may affect the infection success of the parasitoid by modifying behavioral defenses of their hosts, the development success of the parasitoid by conferring a resistance against it or by altering life-history traits of the emerging parasitoids. Here, we assessed the effects of different protective bacterial symbionts on the entire sequence of the host-parasitoid interaction (i.e., from parasitoid attack to offspring emergence) between the pea aphid, Acyrthosiphon pisum, and its main parasitoid, Aphidius ervi and their impacts on the life-history traits of the emerging parasitoids. To test whether symbiont-mediated phenotypes were general or specific to particular aphid–symbiont associations, we considered several aphid lineages, each harboring a different strain of either Hamiltonella defensa or Regiella insecticola, two protective symbionts commonly found in aphids. We found that symbiont species and strains had a weak effect on the ability of aphids to defend themselves against the parasitic wasps during the attack and a strong effect on aphid resistance against parasitoid development. While parasitism resistance was mainly determined by symbionts, their effects on host defensive behaviors varied largely from one aphid–symbiont association to another. Also, the symbiotic status of the aphid individuals had no impact on the attack rate of the parasitic wasps, the parasitoid emergence rate from parasitized aphids nor the life-history traits of the emerging parasitoids. Overall, no correlations between symbiont effects on the different stages of the host–parasitoid interaction was observed, suggesting no trade-offs or positive associations between symbiont-mediated phenotypes. Our study highlights the need to consider various sequences of the host-parasitoid interaction to better assess the outcomes of protective symbioses and understand the ecological and evolutionary dynamics of insect–symbiont associations.

Effect of the Genotypic Variation of an Aphid Host on the Endosymbiont Associations in Natural Host Populations

Simple Summary

The host–endosymbiont complex could be a key determinant in spread and maintenance of the infection polymorphism of endosymbionts. Variation among host–endosymbiont complexes can contribute to genetic variation of a host species and then provide the necessary material for the operating coevolutionary dynamics. We studied the seasonal dynamic of facultative endosymbiont infections among different host clones of the grain aphid Sitobion avenae and whether their presence affects the total hymenopteran parasitism of aphid hosts at the field level. We observed that aphid infections in the field with endosymbionts increase over time, by favoring particular aphid clones closely associated with endosymbionts, but without an effect of endosymbionts on parasitism rate in the host populations. Our results highlight the importance of host–endosymbiont couples in shaping the prevalence and distributions of symbionts throughout nature and the success of their hosts as pests.

Abstract

Understanding the role of facultative endosymbionts on the host’s ecology has been the main aim of the research in symbiont–host systems. However, current research on host–endosymbiont dynamics has failed to examine the genetic background of the hosts and its effect on host–endosymbiont associations in real populations. We have addressed the seasonal dynamic of facultative endosymbiont infections among different host clones of the grain aphid Sitobion avenae, on two cereal crops (wheat and oat) and whether their presence affects the total hymenopteran parasitism of aphid hosts at the field level. We present evidence of rapid seasonal shifts in the endosymbiont frequency, suggesting a positive selection of endosymbionts at the host-level (aphids) through an agricultural growing season, by two mechanisms; (1) an increase of aphid infections with endosymbionts over time, and (2) the seasonal replacement of host clones within natural populations by increasing the prevalence of aphid clones closely associated to endosymbionts. Our results highlight how genotypic variation of hosts can affect the endosymbiont prevalence in the field, being an important factor for understanding the magnitude and direction of the adaptive and/or maladaptive responses of hosts to the environment.

Diversity of bacteria associated with Hormaphidinae aphids (Hemiptera: Aphididae)

Bacteria are ubiquitous inhabitants of animals. Hormaphidinae is a particular aphid group exhibiting very diverse life history traits. However, the microbiota in this group is poorly known. In the present study, using high-throughput sequencing of bacterial 16S ribosomal RNA gene amplicons, we surveyed the bacterial flora in hormaphidine aphids and explored whether the aphid tribe, host plant and geographical distribution are associated with the distribution of secondary symbionts. The most dominant bacteria detected in hormaphidine species are heritable symbionts. As expected, the primary endosymbiont Buchnera aphidicola is the most abundant symbiont across all species and has cospeciated with its host aphids. Six secondary symbionts were detected in Hormaphidinae. Arsenophonus is widespread in Hormaphidinae species, suggesting the possibility of ancient acquisition of this symbiont. Ordination analyses and statistical tests show that the symbiont composition does not seem to relate to any of the aphid tribes, host plants or geographical distributions, which indicate that horizontal transfers might occur for these symbionts in Hormaphidinae. Correlation analysis exhibits negative interference between Buchnera and coexisting secondary symbionts, while the interactions between different secondary symbionts are complicated. These findings display a comprehensive picture of the microbiota in Hormaphidinae and may be helpful in understanding the symbiont diversity within a group of aphids.

Genome Analysis of "Candidatus Regiella insecticola" Strain TUt, Facultative Bacterial Symbiont of the Pea Aphid Acyrthosiphon pisum

The genome of “Candidatus Regiella insecticola” strain TUt, a facultative bacterial symbiont of the pea aphid Acyrthosiphon pisum, was analyzed. We determined a 2.5-Mb draft genome consisting of 14 contigs; this will contribute to the understanding of the symbiont, which underpins various ecologically adaptive traits of the host insect.

The genome of “Candidatus Regiella insecticola” strain TUt, a facultative bacterial symbiont of the pea aphid Acyrthosiphon pisum, was analyzed. We determined a 2.5-Mb draft genome consisting of 14 contigs; this will contribute to the understanding of the symbiont, which underpins various ecologically adaptive traits of the host insect.

Dynamics of Insect-Microbiome Interaction Influence Host and Microbial Symbiont

Insects share an intimate relationship with their gut microflora and this symbiotic association has developed into an essential evolutionary outcome intended for their survival through extreme environmental conditions. While it has been clearly established that insects, with very few exceptions, associate with several microbes during their life cycle, information regarding several aspects of these associations is yet to be fully unraveled. Acquisition of bacteria by insects marks the onset of microbial symbiosis, which is followed by the adaptation of these bacterial species to the gut environment for prolonged sustenance and successful transmission across generations. Although several insect-microbiome associations have been reported and each with their distinctive features, diversifications and specializations, it is still unclear as to what led to these diversifications. Recent studies have indicated the involvement of various evolutionary processes operating within an insect body that govern the transition of a free-living microbe to an obligate or facultative symbiont and eventually leading to the establishment and diversification of these symbiotic relationships. Data from various studies, summarized in this review, indicate that the symbiotic partners, i.e., the bacteria and the insect undergo several genetic, biochemical and physiological changes that have profound influence on their life cycle and biology. An interesting outcome of the insect-microbe interaction is the compliance of the microbial partner to its eventual genome reduction. Endosymbionts possess a smaller genome as compared to their free-living forms, and thus raising the question what is leading to reductive evolution in the microbial partner. This review attempts to highlight the fate of microbes within an insect body and its implications for both the bacteria and its insect host. While discussion on each specific association would be too voluminous and outside the scope of this review, we present an overview of some recent studies that contribute to a better understanding of the evolutionary trajectory and dynamics of the insect-microbe association and speculate that, in the future, a better understanding of the nature of this interaction could pave the path to a sustainable and environmentally safe way for controlling economically important pests of crop plants.

Host Plants Influence the Symbiont Diversity of Eriosomatinae (Hemiptera: Aphididae)

Eriosomatinae is a particular aphid group with typically heteroecious holocyclic life cycle, exhibiting strong primary host plant specialization and inducing galls on primary host plants. Aphids are frequently associated with bacterial symbionts, which can play fundamental roles in the ecology and evolution of their host aphids. However, the bacterial communities in Eriosomatinae are poorly known. In the present study, using high-throughput sequencing of the bacterial 16S ribosomal RNA gene, we surveyed the bacterial flora of eriosomatines and explored the associations between symbiont diversity and aphid relatedness, aphid host plant and geographical distribution. The microbiota of Eriosomatinae is dominated by the heritable primary endosymbiont Buchnera and several facultative symbionts. The primary endosymbiont Buchnera is expectedly the most abundant symbiont across all species. Six facultative symbionts were identified. Regiella was the most commonly identified facultative symbiont, and multiple infections of facultative symbionts were detected in the majority of the samples. Ordination analyses and statistical tests show that the symbiont community of aphids feeding on plants from the family Ulmaceae were distinguishable from aphids feeding on other host plants. Species in Eriosomatinae feeding on different plants are likely to carry different symbiont compositions. The symbiont distributions seem to be not related to taxonomic distance and geographical distance. Our findings suggest that host plants can affect symbiont maintenance, and will improve our understanding of the interactions between aphids, their symbionts and ecological conditions.

Cultivation-Assisted Genome of Candidatus Fukatsuia symbiotica; the Enigmatic "X-Type" Symbiont of Aphids

Heritable symbionts are common in terrestrial arthropods and often provide beneficial services to hosts. Unlike obligate, nutritional symbionts that largely persist under strict host control within specialized host cells, heritable facultative symbionts exhibit large variation in within-host lifestyles and services rendered with many retaining the capacity to transition among roles. One enigmatic symbiont, Candidatus Fukatsuia symbiotica, frequently infects aphids with reported roles ranging from pathogen, defensive symbiont, mutualism exploiter, and nutritional co-obligate symbiont. Here, we used an in vitro culture-assisted protocol to sequence the genome of a facultative strain of Fukatsuia from pea aphids (Acyrthosiphon pisum). Phylogenetic and genomic comparisons indicate that Fukatsuia is an aerobic heterotroph, which together with Regiella insecticola and Hamiltonella defensa form a clade of heritable facultative symbionts within the Yersiniaceae (Enterobacteriales). These three heritable facultative symbionts largely share overlapping inventories of genes associated with housekeeping functions, metabolism, and nutrient acquisition, while varying in complements of mobile DNA. One unusual feature of Fukatsuia is its strong tendency to occur as a coinfection with H. defensa. However, the overall similarity of gene inventories among aphid heritable facultative symbionts suggests that metabolic complementarity is not the basis for coinfection, unless playing out on a H. defensa strain-specific basis. We also compared the pea aphid Fukatsuia with a strain from the aphid Cinara confinis (Lachninae) where it is reported to have transitioned to co-obligate status to support decaying Buchnera function. Overall, the two genomes are very similar with no clear genomic signatures consistent with such a transition, which suggests co-obligate status in C. confinis was a recent event.

More Is Not Always Better: Coinfections with Defensive Symbionts Generate Highly Variable Outcomes

Animal-associated microbes are highly variable, contributing to a diverse set of symbiont-mediated phenotypes. Given that host and symbiont genotypes, and their interactions, can impact symbiont-based phenotypes across environments, there is potential for extensive variation in fitness outcomes. Pea aphids, Acyrthosiphon pisum, host a diverse assemblage of heritable facultative symbionts (HFS) with characterized roles in host defense. Protective phenotypes have been largely studied as single infections, but pea aphids often carry multiple HFS species, and particular combinations may be enriched or depleted compared to expectations based on chance. Here, we examined the consequences of single infection versus coinfection with two common HFS exhibiting variable enrichment, the antiparasitoid Hamiltonella defensa and the antipathogen Regiella insecticola, across three host genotypes and environments. As expected, single infections with either H. defensa or R. insecticola raised defenses against their respective targets. Single infections with protective H. defensa lowered aphid fitness in the absence of enemy challenge, while R. insecticola was comparatively benign. However, as a coinfection, R. insecticola ameliorated H. defensa infection costs. Coinfected aphids continued to receive antiparasitoid protection from H. defensa, but protection was weakened by R. insecticola in two clones. Notably, H. defensa eliminated survival benefits conferred after pathogen exposure by coinfecting R. insecticola Since pathogen sporulation was suppressed by R. insecticola in coinfected aphids, the poor performance likely stemmed from H. defensa-imposed costs rather than weakened defenses. Our results reveal a complex set of coinfection outcomes which may partially explain natural infection patterns and suggest that symbiont-based phenotypes may not be easily predicted based solely on infection status.IMPORTANCE The hyperdiverse arthropods often harbor maternally transmitted bacteria that protect against natural enemies. In many species, low-diversity communities of heritable symbionts are common, providing opportunities for cooperation and conflict among symbionts, which can impact the defensive services rendered. Using the pea aphid, a model for defensive symbiosis, we show that coinfections with two common defensive symbionts, the antipathogen Regiella and the antiparasite Hamiltonella, produce outcomes that are highly variable compared to single infections, which consistently protect against designated enemies. Compared to single infections, coinfections often reduced defensive services during enemy challenge yet improved aphid fitness in the absence of enemies. Thus, infection with multiple symbionts does not necessarily create generalist aphids with "Swiss army knife" defenses against numerous enemies. Instead, particular combinations of symbionts may be favored for a variety of reasons, including their abilities to lessen the costs of other defensive symbionts when enemies are not present.

The price of protection: a defensive endosymbiont impairs nymph growth in the bird cherry-oat aphid, Rhopalosiphum padi

Bacterial endosymbionts have enabled aphids to adapt to a range of stressors, but their effects in many aphid species remain to be established. The bird cherry-oat aphid, Rhopalosiphum padi (Linnaeus), is an important pest of cereals worldwide and has been reported to form symbiotic associations with Serratia symbiotica and Sitobion miscanthi L-type symbiont endobacteria, although the resulting aphid phenotype has not been described. This study presents the first report of R. padi infection with the facultative bacterial endosymbiont Hamiltonella defensa. Individuals of R. padi were sampled from populations in Eastern Scotland, UK, and shown to represent seven R. padi genotypes based on the size of polymorphic microsatellite markers; two of these genotypes harbored H. defensa. In parasitism assays, survival of H. defensa-infected nymphs following attack by the parasitoid wasp Aphidius colemani (Viereck) was 5 fold higher than for uninfected nymphs. Aphid genotype was a major determinant of aphid performance on two Hordeum species, a modern cultivar of barley H. vulgare and a wild relative H. spontaneum, although aphids infected with H. defensa showed 16% lower nymph mass gain on the partially resistant wild relative compared with uninfected individuals. These findings suggest that deploying resistance traits in barley will favor the fittest R. padi genotypes, but symbiont-infected individuals will be favored when parasitoids are abundant, although these aphids will not achieve optimal performance on a poor quality host plant.

Microbial Symbionts of Parasitoids

Parasitoids depend on other insects for the development of their offspring. Their eggs are laid in or on a host insect that is consumed during juvenile development. Parasitoids harbor a diversity of microbial symbionts including viruses, bacteria, and fungi. In contrast to symbionts of herbivorous and hematophagous insects, parasitoid symbionts do not provide nutrients. Instead, they are involved in parasitoid reproduction, suppression of host immune responses, and manipulation of the behavior of herbivorous hosts. Moreover, recent research has shown that parasitoid symbionts such as polydnaviruses may also influence plant-mediated interactions among members of plant-associated communities at different trophic levels, such as herbivores, parasitoids, and hyperparasitoids. This implies that these symbionts have a much more extended phenotype than previously thought. This review focuses on the effects of parasitoid symbionts on direct and indirect species interactions and the consequences for community ecology.

A fitness cost resulting from Hamiltonella defensa infection is associated with altered probing and feeding behaviour in Rhopalosiphum padi

Many herbivorous arthropods, including aphids, frequently associate with facultative endosymbiotic bacteria, which influence arthropod physiology and fitness. In aphids, endosymbionts can increase resistance against natural enemies, enhance aphid virulence and alter aphid fitness. Here, we used the electrical penetration graph technique to uncover physiological processes at the insect-plant interface affected by endosymbiont infection. We monitored the feeding and probing behaviour of four independent clonal lines of the cereal-feeding aphid Rhopalosiphum padi derived from the same multilocus genotype containing differential infection (+/-) with a common facultative endosymbiont, Hamiltonella defensa Aphid feeding was examined on a partially resistant wild relative of barley known to impair aphid fitness and a susceptible commercial barley cultivar. Compared with uninfected aphids, endosymbiont-infected aphids on both plant species exhibited a twofold increase in the number of plant cell punctures, a 50% reduction in the duration of each cellular puncture and a twofold higher probability of achieving sustained phloem ingestion. Feeding behaviour was also altered by host plant identity: endosymbiont-infected aphids spent less time probing plant tissue, required twice as many probes to reach the phloem and showed a 44% reduction in phloem ingestion when feeding on the wild barley relative compared with the susceptible commercial cultivar. Reduced feeding success could explain the 22% reduction in growth of H. defensa-infected aphids measured on the wild barley relative. This study provides the first demonstration of mechanisms at the aphid-plant interface contributing to physiological effects of endosymbiont infection on aphid fitness, through altered feeding processes on different quality host plants.

Context-dependent interactions of insects and defensive symbionts: insights from a novel system in siricid woodwasps

Many insect species derive fitness benefits from associations with defensive microbial symbionts that confer protection against pathogens and parasites. These relationships are varied and diverse, but a number of studies highlight important trends. The effects of defensive symbionts can be context-dependent and influenced by variable selection imposed by the organism against which the symbiont protects. Additionally, genetic variation in both hosts and symbionts can greatly influence the outcome of these interactions. Here, we describe interactions between siricid woodwasps, their fungal symbionts and parasitic nematodes and show how defense by symbionts in this system is also context-dependent. The species or strain of the white rot fungus used as a symbiont by Sirex can influence parasitism of these hosts by Deladenus nematodes.

The hemolymph microbiome of insects

Hemolymph has long been recognized as a key mediator of nutritional and immunological homeostasis in insects, with the tacit understanding that hemolymph is a hostile environment for microorganisms, and microbiologically sterile in healthy insects. Recent research is overturning the conventional wisdom, and there is now overwhelming evidence that various non-pathogenic microorganisms can stably or transiently inhabit hemolymph in a diversity of insects. Most is known about Spiroplasma, especially in Drosophila species, and secondary symbionts of the Enterobacteriaceae, notably Hamiltonella defensa, in aphids. These bacteria require many nutrients, representing a likely drain on host nutritional resources, and they persist in the hemolymph by a combination of evasion and tolerance of insect immune effectors. These traits can be costly to the insect host. For some hemolymph microorganisms, these costs are balanced by other traits beneficial to the insect, notably protection against natural enemies mediated by specific toxins or competition for key nutrients. Three key priorities for future research are: to investigate the prevalence and taxonomic diversity of hemolymph microorganisms in insects; to establish the role of host nutritional and immune factors as determinants of the abundance and proliferation rates of hemolymph microorganisms; and to integrate the developing understanding of these microorganisms and their impacts (both costs and benefits) on insect nutrition and immune function into the wider study of insect physiology.

Evolutionary costs and benefits of infection with diverse strains of Spiroplasma in pea aphids

The heritable endosymbiont Spiroplasma infects many insects and has repeatedly evolved the ability to protect its hosts against different parasites. Defenses do not come for free to the host, and theory predicts that more costly symbionts need to provide stronger benefits to persist in host populations. We investigated the costs and benefits of Spiroplasma infections in pea aphids (Acyrthosiphon pisum), testing 12 bacterial strains from three different clades. Virtually all strains decreased aphid lifespan and reproduction, but only two had a (weak) protective effect against the parasitoid Aphidius ervi, an important natural enemy of pea aphids. Spiroplasma-induced fitness costs were variable, with strains from the most slowly evolving clade reaching higher titers and curtailing aphid lifespan more strongly than other strains. Some Spiroplasma strains shared their host with a second endosymbiont, Regiella insecticola. Although the result of an unfortunate handling error, these co-infections proved instructive, because they showed that the cost of infection with Spiroplasma may be attenuated in the presence of Regiella. These results suggest that mechanisms other than protection against A. ervi maintain pea aphid infections with diverse strains of Spiroplasma, and that studying them in isolation will not provide a complete picture of their effects on host fitness.

Variations on a protective theme: Hamiltonella defensa infections in aphids variably impact parasitoid success

Protective mutualisms are common in nature and include insect infections with cryptic symbionts that defend against pathogens and parasites. An archetypal defensive symbiont, Hamiltonella defensa protects aphids against parasitoids by disabling wasp development. Successful defense requires H. defensa infection with bacteriophages (APSEs), which play other key roles in mutualism maintenance. Genomes of H. defensa strains are highly similar in gene inventories, varying primarily in mobile element content. Protective phenotypes are highly variable across aphid models depending on H. defensa/APSE, aphid and wasp genotypes. Infection frequencies of H. defensa are highly dynamic in field populations, influenced by a variety of selective and non-selective factors confounding biological control implications. Overall, H. defensa infections likely represent a global aphid protection network with effects radiating outward from focal interactions.

The influence of symbiotic bacteria on reproductive strategies and wing polyphenism in pea aphids responding to stress

Environmental stressors can be key drivers of phenotypes, including reproductive strategies and morphological traits. The response to stress may be altered by the presence of microbial associates. For example, in aphids, facultative (secondary) bacterial symbionts can provide protection against natural enemies and stress induced by elevated temperatures. Furthermore, aphids exhibit phenotypic plasticity, producing winged (rather than wingless) progeny that may be better able to escape danger, and the combination of these factors improves the response to stress. How symbionts and phenotypic plasticity, both of which shape aphids' stress response, influence one another, and together influence host fitness, remains unclear. In this study, we investigate how environmental stressors drive shifts in fecundity and winged/wingless offspring production, and how secondary symbionts influence the process. We induced production of winged offspring through distinct environmental stressors, including exposure to aphid alarm pheromone and crowding, and, in one experiment, we assessed whether the aphid response is influenced by host plant. In the winged morph, energy needed for wing maintenance may lead to trade-offs with other traits, such as reproduction or symbiont maintenance. Potential trade-offs between symbiont maintenance and fitness have been proposed but have not been tested. Thus, beyond studying the production of offspring of alternative morphs, we also explore the influence of symbionts across wing/wingless polyphenism as well as symbiont interaction with cross-generational impacts of environmental stress on reproductive output. All environmental stressors resulted in increased production of winged offspring and shifts in fecundity rates. Additionally, in some cases, aphid host-by-symbiont interactions influenced fecundity. Stress on first-generation aphids had cross-generational impacts on second-generation adults, and the impact on fecundity was further influenced by the presence of secondary symbionts and presence/absence of wings. Our study suggests a complex interaction between beneficial symbionts and environmental stressors. Winged aphids have the advantage of being able to migrate out of danger with more ease, but energy needed for wing production and maintenance may come with reproductive costs for their mothers and for themselves, where in certain cases, these costs are altered by secondary symbionts.

Parasitoid envenomation alters the Galleria mellonella midgut microbiota and immunity, thereby promoting fungal infection

Gut bacteria influence the development of different pathologies caused by bacteria, fungi and parasitoids in insects. Wax moth larvae became more susceptible to fungal infections after envenomation by the ectoparasitoid Habrobracon hebetor. In addition, spontaneous bacterioses occurred more often in envenomated larvae. We analyzed alterations in the midgut microbiota and immunity of the wax moth in response to H. hebetor envenomation and topical fungal infection (Beauveria bassiana) alone or in combination using 16S rRNA sequencing, an analysis of cultivable bacteria and a qPCR analysis of immunity- and stress-related genes. Envenomation led to a predominance shift from enterococci to enterobacteria, an increase in CFUs and the upregulation of AMPs in wax moth midguts. Furthermore, mycosis nonsignificantly increased the abundance of enterobacteria and the expression of AMPs in the midgut. Combined treatment led to a significant increase in the abundance of Serratia and a greater upregulation of gloverin. The oral administration of predominant bacteria (Enterococcus faecalis, Enterobacter sp. and Serratia marcescens) to wax moth larvae synergistically increased fungal susceptibility. Thus, the activation of midgut immunity might prevent the bacterial decomposition of envenomated larvae, thus permitting the development of fungal infections. Moreover, changes in the midgut bacterial community may promote fungal killing.

A large-scale field study of bacterial communities in cereal aphid populations across Morocco

Insects are frequently associated with bacteria that can have significant ecological and evolutionary impacts on their hosts. To date, few studies have examined the influence of environmental factors to microbiome composition of aphids. The current work assessed the diversity of bacterial communities of five cereal aphid species (Sitobion avenae, Rhopalosiphum padi, R. maidis, Sipha maydis and Diuraphis noxia) collected across Morocco, covering a wide range of environmental conditions. We aimed to test whether symbiont combinations are host or environment specific. Deep 16S rRNA sequencing enabled us to identify 17 bacterial operational taxonomic units (OTUs). The obligate symbiont Buchnera aphidicola was represented by five OTUs with multiple haplotypes in many single samples. Facultative endosymbionts were presented by a high prevalence of Regiella insecticola and Serratia symbiotica in S. avenae and Si. maydis, respectively. In addition to these symbiotic partners, Pseudomonas, Acinetobacter, Pantoea, Erwinia and Staphyloccocus were also identified in aphids, suggesting that the aphid microbiome is not limited to the presence of endosymbiotic bacteria. Beside a significant association between host species and bacterial communities, an inverse correlation was also found between altitude and α-diversity. Overall, our results support that symbiont combinations are mainly host specific.

The draft genome of strain cCpun from biting midges confirms insect Cardinium are not a monophyletic group and reveals a novel gene family expansion in a symbiont

BACKGROUND

It is estimated that 13% of arthropod species carry the heritable symbiont Cardinium hertigii. 16S rRNA and gyrB sequence divides this species into at least four groups (A-D), with the A group infecting a range of arthropods, the B group infecting nematode worms, the C group infecting Culicoides biting midges, and the D group associated with the marine copepod Nitocra spinipes. To date, genome sequence has only been available for strains from groups A and B, impeding general understanding of the evolutionary history of the radiation. We present a draft genome sequence for a C group Cardinium, motivated both by the paucity of genomic information outside of the A and B group, and the importance of Culicoides biting midge hosts as arbovirus vectors.

METHODS

We reconstructed the genome of cCpun, a Cardinium strain from group C that naturally infects Culicoides punctatus, through Illumina sequencing of infected host specimens.

RESULTS

The draft genome presented has high completeness, with BUSCO scores comparable to closed group A Cardinium genomes. Phylogenomic analysis based on concatenated single copy core proteins do not support Cardinium from arthropod hosts as a monophyletic group, with nematode Cardinium strains nested within the two groups infecting arthropod hosts. Analysis of the genome of cCpun revealed expansion of a variety of gene families classically considered important in symbiosis (e.g., ankyrin domain containing genes), and one set-characterized by DUF1703 domains-not previously associated with symbiotic lifestyle. This protein group encodes putative secreted nucleases, and the cCpun genome carried at least 25 widely divergent paralogs, 24 of which shared a common ancestor in the C group. The genome revealed no evidence in support of B vitamin provisioning to its haematophagous host, and indeed suggests Cardinium may be a net importer of biotin.

DISCUSSION

These data indicate strains of Cardinium within nematodes cluster within Cardinium strains found in insects. The draft genome of cCpun further produces new hypotheses as to the interaction of the symbiont with the midge host, in particular the biological role of DUF1703 nuclease proteins that are predicted as being secreted by cCpun. In contrast, the coding content of this genome provides no support for a role for the symbiont in provisioning the host with B vitamins.

Multi-scale characterization of symbiont diversity in the pea aphid complex through metagenomic approaches

BACKGROUND

Most metazoans are involved in durable relationships with microbes which can take several forms, from mutualism to parasitism. The advances of NGS technologies and bioinformatics tools have opened opportunities to shed light on the diversity of microbial communities and to give some insights into the functions they perform in a broad array of hosts. The pea aphid is a model system for the study of insect-bacteria symbiosis. It is organized in a complex of biotypes, each adapted to specific host plants. It harbors both an obligatory symbiont supplying key nutrients and several facultative symbionts bringing additional functions to the host, such as protection against biotic and abiotic stresses. However, little is known on how the symbiont genomic diversity is structured at different scales: across host biotypes, among individuals of the same biotype, or within individual aphids, which limits our understanding on how these multi-partner symbioses evolve and interact.

RESULTS

We present a framework well adapted to the study of genomic diversity and evolutionary dynamics of the pea aphid holobiont from metagenomic read sets, based on mapping to reference genomes and whole genome variant calling. Our results revealed that the pea aphid microbiota is dominated by a few heritable bacterial symbionts reported in earlier works, with no discovery of new microbial associates. However, we detected a large and heterogeneous genotypic diversity associated with the different symbionts of the pea aphid. Partitioning analysis showed that this fine resolution diversity is distributed across the three considered scales. Phylogenetic analyses highlighted frequent horizontal transfers of facultative symbionts between host lineages, indicative of flexible associations between the pea aphid and its microbiota. However, the evolutionary dynamics of symbiotic associations strongly varied depending on the symbiont, reflecting different histories and possible constraints. In addition, at the intra-host scale, we showed that different symbiont strains may coexist inside the same aphid host.

CONCLUSIONS

We present a methodological framework for the detailed analysis of NGS data from microbial communities of moderate complexity and gave major insights into the extent of diversity in pea aphid-symbiont associations and the range of evolutionary trajectories they could take.

Facultative bacterial endosymbionts shape parasitoid food webs in natural host populations: A correlative analysis

Facultative bacterial endosymbionts can protect their aphid hosts from natural enemies such as hymenopteran parasitoids. As such, they have the capability to modulate interactions between aphids, parasitoids and hyperparasitoids. However, the magnitude of these effects in natural aphid populations and their associated parasitoid communities is currently unknown. Moreover, environmental factors such as plant fertilization and landscape complexity are known to affect aphid–parasitoid interactions but it remains unclear how such environmental factors affect the interplay between aphids, parasitoids and endosymbionts.

Here, we tested whether facultative endosymbionts confer protection to parasitoids in natural populations of the English grain aphid, Sitobion avenae, and if this is affected by plant fertilization and landscape complexity. Furthermore, we examined whether the effects of facultative endosymbionts can cascade up to the hyperparasitoid level and increase primary‐hyperparasitoid food web specialization.

Living aphids and mummies were collected in fertilized and unfertilized plots within 13 wheat fields in Central Germany. We assessed the occurrence of primary parasitoid, hyperparasitoid and endosymbiont species in aphids and mummies using a newly established molecular approach.

Facultative endosymbiont infection rates were high across fields (~80%), independent of whether aphids were parasitized or unparasitized. Aphid mummies exhibited a significantly lower share of facultative endosymbiont infection (~38%). These findings suggest that facultative endosymbionts do not affect parasitoid oviposition behaviour, but decrease parasitoid survival in the host. Facultative endosymbiont infection rates were lower in mummies collected from fertilized compared to unfertilized plants, indicating that plant fertilization boosts the facultative endosymbiont protective effect. Furthermore, we found strong evidence for species‐specific and negative cascading effects of facultative endosymbionts on primary and hyperparasitoids, respectively. Facultative endosymbionts impacted parasitoid assemblages and increased the specialization of primary‐hyperparasitoid food webs: these effects were independent from and much stronger than other environmental factors.

The current findings strongly suggest that facultative endosymbionts act as a driving force in aphid–parasitoid–hyperparasitoid networks: they shape insect community composition at different trophic levels and modulate, directly and indirectly, the interactions between aphids, parasitoids and their environment.

Convergent Evolution in Intracellular Elements: Plasmids as Model Endosymbionts

Endosymbionts are organisms that live inside the cells of other species. This lifestyle is ubiquitous across the tree of life and is featured by unicellular eukaryotes, prokaryotes, and by extrachromosomal genetic elements such as plasmids. Given that all of these elements dwell in the cytoplasm of their host cell, they should be subject to similar selection pressures. Here we show that strikingly similar features have evolved in both bacterial endosymbionts and plasmids. Since host and endosymbiont are often metabolically tightly intertwined, they are difficult to disentangle experimentally. We propose that using plasmids as tractable model systems can help to solve this problem, thus allowing fundamental questions to be experimentally addressed about the ecology and evolution of endosymbiotic interactions.

Culture-Facilitated Comparative Genomics of the Facultative Symbiont Hamiltonella defensa

Many insects host facultative, bacterial symbionts that confer conditional fitness benefits to their hosts. Hamiltonella defensa is a common facultative symbiont of aphids that provides protection against parasitoid wasps. Protection levels vary among strains of H. defensa that are also differentially infected by bacteriophages named APSEs. However, little is known about trait variation among strains because only one isolate has been fully sequenced. Generating complete genomes for facultative symbionts is hindered by relatively large genome sizes but low abundances in hosts like aphids that are very small. Here, we took advantage of methods for culturing H. defensa outside of aphids to generate complete genomes and transcriptome data for four strains of H. defensa from the pea aphid Acyrthosiphon pisum. Chosen strains also spanned the breadth of the H. defensa phylogeny and differed in strength of protection conferred against parasitoids. Results indicated that strains shared most genes with roles in nutrient acquisition, metabolism, and essential housekeeping functions. In contrast, the inventory of mobile genetic elements varied substantially, which generated strain specific differences in gene content and genome architecture. In some cases, specific traits correlated with differences in protection against parasitoids, but in others high variation between strains obscured identification of traits with likely roles in defense. Transcriptome data generated continuous distributions to genome assemblies with some genes that were highly expressed and others that were not. Single molecule real-time sequencing further identified differences in DNA methylation patterns and restriction modification systems that provide defense against phage infection.

An effective molecular approach for assessing cereal aphid-parasitoid-endosymbiont networks

Molecular approaches are increasingly being used to analyse host-parasitoid food webs as they overcome several hurdles inherent to conventional approaches. However, such studies have focused primarily on the detection and identification of aphids and their aphidiid primary parasitoids, largely ignoring primary parasitoid-hyperparasitoid interactions or limiting these to a few common species within a small geographical area. Furthermore, the detection of bacterial secondary endosymbionts has not been considered in such assays despite the fact that endosymbionts may alter aphid-parasitoid interactions, as they can confer protection against parasitoids. Here we present a novel two-step multiplex PCR (MP-PCR) protocol to assess cereal aphid-primary parasitoid-hyperparasitoid-endosymbiont interactions. The first step of the assay allows detection of parasitoid DNA at a general level (24 primary and 16 hyperparasitoid species) as well as the species-specific detection of endosymbionts (3 species) and cereal aphids (3 species). The second step of the MP-PCR assay targets seven primary and six hyperparasitoid species that commonly occur in Central Europe. Additional parasitoid species not covered by the second-step of the assay can be identified via sequencing 16S rRNA amplicons generated in the first step of the assay. The approach presented here provides an efficient, highly sensitive, and cost-effective (~consumable costs of 1.3 € per sample) tool for assessing cereal aphid-parasitoid-endosymbiont interactions.

Toward a better understanding of the mechanisms of symbiosis: a comprehensive proteome map of a nascent insect symbiont

Symbiotic bacteria are common in insects and can affect various aspects of their hosts’ biology. Although the effects of insect symbionts have been clarified for various insect symbiosis models, due to the difficulty of cultivating them in vitro, there is still limited knowledge available on the molecular features that drive symbiosis. Serratia symbiotica is one of the most common symbionts found in aphids. The recent findings of free-living strains that are considered as nascent partners of aphids provide the opportunity to examine the molecular mechanisms that a symbiont can deploy at the early stages of the symbiosis (i.e., symbiotic factors). In this work, a proteomic approach was used to establish a comprehensive proteome map of the free-living S. symbiotica strain CWBI-2.3^T. Most of the 720 proteins identified are related to housekeeping or primary metabolism. Of these, 76 were identified as candidate proteins possibly promoting host colonization. Our results provide strong evidence that S. symbiotica CWBI-2.3^T is well-armed for invading insect host tissues, and suggest that certain molecular features usually harbored by pathogenic bacteria are no longer present. This comprehensive proteome map provides a series of candidate genes for further studies to understand the molecular cross-talk between insects and symbiotic bacteria.

Genotype specificity among hosts, pathogens, and beneficial microbes influences the strength of symbiont-mediated protection

The microbial symbionts of eukaryotes influence disease resistance in many host-parasite systems. Symbionts show substantial variation in both genotype and phenotype, but it is unclear how natural selection maintains this variation. It is also unknown whether variable symbiont genotypes show specificity with the genotypes of hosts or parasites in natural populations. Genotype by genotype interactions are a necessary condition for coevolution between interacting species. Uncovering the patterns of genetic specificity among hosts, symbionts, and parasites is therefore critical for determining the role that symbionts play in host-parasite coevolution. Here, we show that the strength of protection conferred against a fungal pathogen by a vertically transmitted symbiont of an aphid is influenced by both host-symbiont and symbiont-pathogen genotype by genotype interactions. Further, we show that certain symbiont phylogenetic clades have evolved to provide stronger protection against particular pathogen genotypes. However, we found no evidence of reciprocal adaptation of co-occurring host and symbiont lineages. Our results suggest that genetic variation among symbiont strains may be maintained by antagonistic coevolution with their host and/or their host's parasites.

Drosophila Adaptation to Viral Infection through Defensive Symbiont Evolution

Microbial symbionts can modulate host interactions with biotic and abiotic factors. Such interactions may affect the evolutionary trajectories of both host and symbiont. Wolbachia protects Drosophila melanogaster against several viral infections and the strength of the protection varies between variants of this endosymbiont. Since Wolbachia is maternally transmitted, its fitness depends on the fitness of its host. Therefore, Wolbachia populations may be under selection when Drosophila is subjected to viral infection. Here we show that in D. melanogaster populations selected for increased survival upon infection with Drosophila C virus there is a strong selection coefficient for specific Wolbachia variants, leading to their fixation. Flies carrying these selected Wolbachia variants have higher survival and fertility upon viral infection when compared to flies with the other variants. These findings demonstrate how the interaction of a host with pathogens shapes the genetic composition of symbiont populations. Furthermore, host adaptation can result from the evolution of its symbionts, with host and symbiont functioning as a single evolutionary unit.

Animals live in close association with microbial partners that can shape many aspects of their lives. For instance, several insects carry bacteria that defend them against parasites and infectious diseases. The intracellular bacterium Wolbachia protects the fruit fly Drosophila melanogaster against viral infection. Natural populations of Drosophila carry different variants of Wolbachia, which differ from one another in the strength of this protection. Here we show that a population of Drosophila infected with viruses during several generations adapts to this challenge through turnover in Wolbachia composition. The Wolbachia variants that give higher protection to viruses, by increasing fly survival and fecundity upon infection, are strongly selected. This work demonstrates that the interaction of an animal with a pathogen can shape its associated microbial populations. We show that adaptation to pathogens can be achieved not only through selection of resistance on the host proper but also through the evolutionary shaping of its microbial community.

Snapshots of a shrinking partner: Genome reduction in Serratia symbiotica

Genome reduction is pervasive among maternally-inherited endosymbiotic organisms, from bacteriocyte- to gut-associated ones. This genome erosion is a step-wise process in which once free-living organisms evolve to become obligate associates, thereby losing non-essential or redundant genes/functions. Serratia symbiotica (Gammaproteobacteria), a secondary endosymbiont present in many aphids (Hemiptera: Aphididae), displays various characteristics that make it a good model organism for studying genome reduction. While some strains are of facultative nature, others have established co-obligate associations with their respective aphid host and its primary endosymbiont (Buchnera). Furthermore, the different strains hold genomes of contrasting sizes and features, and have strikingly disparate cell shapes, sizes, and tissue tropism. Finally, genomes from closely related free-living Serratia marcescens are also available. In this study, we describe in detail the genome reduction process (from free-living to reduced obligate endosymbiont) undergone by S. symbiotica, and relate it to the stages of integration to the symbiotic system the different strains find themselves in. We establish that the genome reduction patterns observed in S. symbiotica follow those from other dwindling genomes, thus proving to be a good model for the study of the genome reduction process within a single bacterial taxon evolving in a similar biological niche (aphid-Buchnera).

Endosymbiotic bacteria in honey bees: Arsenophonus spp. are not transmitted transovarially

Intracellular endosymbiotic bacteria are common and can play a crucial role for insect pathology. Therefore, such bacteria could be a potential key to our understanding of major losses of Western honey bees (Apis mellifera) colonies. However, the transmission and potential effects of endosymbiotic bacteria in A. mellifera and other Apis spp. are poorly understood. Here, we explore the prevalence and transmission of the genera Arsenophonus, Wolbachia, Spiroplasma and Rickettsia in Apis spp. Colonies of A. mellifera (N = 33, with 20 eggs from worker brood cells and 100 adult workers each) as well as mated honey bee queens of A. cerana, A. dorsata and A. florea (N = 12 each) were screened using PCR. While Wolbachia, Spiroplasma and Rickettsia were not detected, Arsenophonus spp. were found in 24.2% of A. mellifera colonies and respective queens as well as in queens of A. dorsata (8.3%) and A. florea (8.3%), but not in A. cerana. The absence of Arsenophonus spp. from reproductive organs of A. mellifera queens and surface-sterilized eggs does not support transovarial vertical transmission. Instead, horizontal transmission is most likely.

Arsenophonus endosymbiotic bacteria are not transmitted transovarially in honey bees.

Evidence for specificity in symbiont-conferred protection against parasitoids

Many insects harbour facultative symbiotic bacteria, some of which have been shown to provide resistance against natural enemies. One of the best-known protective symbionts is Hamiltonella defensa, which in pea aphid (Acyrthosiphon pisum) confers resistance against attack by parasitoid wasps in the genus Aphidius (Braconidae).We asked (i) whether this symbiont also confers protection against a phylogenetically distant group of parasitoids (Aphelinidae) and (ii) whether there are consistent differences in the effects of bacteria found in pea aphid biotypes adapted to different host plants. We found that some H. defensa strains do provide protection against an aphelinid parasitoid Aphelinus abdominalis. Hamiltonella defensa from the Lotus biotype provided high resistance to A. abdominalis and moderate to low resistance to Aphidius ervi, while the reverse was seen from Medicago biotype isolates. Aphids from Ononis showed no evidence of symbiont-mediated protection against either wasp species and were relatively vulnerable to both. Our results may reflect the different selection pressures exerted by the parasitoid community on aphids feeding on different host plants, and could help explain the maintenance of genetic diversity in bacterial symbionts.

Gut Microbiotas and Host Evolution: Scaling Up Symbiosis

Our understanding of species evolution is undergoing restructuring. It is well accepted that host-symbiont coevolution is responsible for fundamental aspects of biology. However, the emerging importance of plant- and animal-associated microbiotas to their hosts suggests a scale of coevolutionary interactions many-fold greater than previously considered. This review builds on current understanding of symbionts and their contributions to host evolution to evaluate recent data demonstrating similar contributions of gut microbiotas. It further considers a multilayered model for microbiota to account for emerging themes in host-microbiota interactions. Drawing on the structure of bacterial genomes, this model distinguishes between a host-adapted core microbiota, and a flexible, environmentally modulated microbial pool, differing in constraints on their maintenance and in their contributions to host adaptation.

Condition-dependent alteration of cellular immunity by secondary symbionts in the pea aphid, Acyrthosiphon pisum

Endosymbionts can fundamentally alter host physiology. Whether such changes are beneficial or detrimental to one or both partners may depend on the dynamics of the symbiotic relationship. Here we investigate the relationship between facultative symbionts and host immune responses. The pea aphid, Acyrthosiphon pisum, maintains an obligate primary symbiont, but may also harbour one or more facultative, secondary symbionts. Given their more transient nature and relatively recent adoption of a symbiotic lifestyle compared to primary symbionts, secondary symbionts may present a challenge for the host immune system. We assessed the response of several key components of the cellular immune system (phenoloxidase activity, encapsulation, immune cell counts) in the presence of alternative secondary symbionts, investigating the role of host and secondary symbiont genotype in specific responses. There was no effect of secondary symbiont presence on the phenoloxidase response, but we found variation in the encapsulation response and in immune cell counts based largely on the secondary symbiont. Host genotype was less influential in determining immunity outcomes. Our results highlight the importance of secondary symbionts in shaping host immunity. Understanding the complex physiological responses that can be propagated by host-symbiont associations has important consequences for host ecology, including symbiont and pathogen transmission dynamics.

The Endosymbiont Arsenophonus Provides a General Benefit to Soybean Aphid (Hemiptera: Aphididae) Regardless of Host Plant Resistance (Rag)

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), invokes substantial chemical treatment and economic cost in North America. Resistant soybean genotypes hold promise as a low-impact control methodology, but soybean aphid "biotypes" capable of development on resistant soy cast doubt on the durability of soy resistance. We hypothesized that variation in soybean aphid ability to colonize resistant soy is partially attributable to a bacterial symbiont of soybean aphid, Arsenophonus. We used microinjection to manipulate Arsenophonus infection in both virulent and avirulent aphid biotypes, resulting in five pairs of infected versus uninfected isolines. These isolines were subjected to various population growth rate assays on resistant Rag versus susceptible soybean. We found that aphid virulence on Rag soybean was not dependent on Arsenophonus: virulent aphid biotypes performed well on Rag soybean, and avirulent aphid biotypes performed poorly on Rag soybean, regardless of whether Arsenophonus was present or not. However, we did find that Arsenophonus-infected clones on average performed significantly better than their paired uninfected isolines. This pattern was not consistently evident on every date for every clone, either in the population assays nor when we compared lifetime fecundity of individual aphids in a separate experiment. Nevertheless, this overall benefit for infected aphids may be sufficient to explain the high frequency of Arsenophonus infection in soybean aphids.

Phylogenetic distribution of symbiotic bacteria from Panamanian amphibians that inhibit growth of the lethal fungal pathogen Batrachochytrium dendrobatidis

The introduction of next-generation sequencing has allowed for greater understanding of community composition of symbiotic microbial communities. However, determining the function of individual members of these microbial communities still largely relies on culture-based methods. Here, we present results on the phylogenetic distribution of a defensive functional trait of cultured symbiotic bacteria associated with amphibians. Amphibians are host to a diverse community of cutaneous bacteria and some of these bacteria protect their host from the lethal fungal pathogen Batrachochytrium dendrobatidis (Bd) by secreting antifungal metabolites. We cultured over 450 bacterial isolates from the skins of Panamanian amphibian species and tested their interactions with Bd using an in vitro challenge assay. For a subset of isolates, we also completed coculture experiments and found that culturing isolates with Bd had no effect on inhibitory properties of the bacteria, but it significantly decreased metabolite secretion. In challenge assays, approximately 75% of the bacterial isolates inhibited Bd to some extent and these inhibitory isolates were widely distributed among all bacterial phyla. Although there was no clear phylogenetic signal of inhibition, three genera, Stenotrophomonas, Aeromonas and Pseudomonas, had a high proportion of inhibitory isolates (100%, 77% and 73%, respectively). Overall, our results demonstrate that antifungal properties are phylogenetically widespread in symbiotic microbial communities of Panamanian amphibians and that some functional redundancy for fungal inhibition occurs in these communities. We hope that these findings contribute to the discovery and development of probiotics for amphibians that can mitigate the threat of chytridiomycosis.

Two host clades, two bacterial arsenals: evolution through gene losses in facultative endosymbionts

Bacterial endosymbiosis is an important evolutionary process in insects, which can harbor both obligate and facultative symbionts. The evolution of these symbionts is driven by evolutionary convergence, and they exhibit among the tiniest genomes in prokaryotes. The large host spectrum of facultative symbionts and the high diversity of strategies they use to infect new hosts probably impact the evolution of their genome and explain why they undergo less severe genomic erosion than obligate symbionts. Candidatus Hamiltonella defensa is suitable for the investigation of the genomic evolution of facultative symbionts because the bacteria are engaged in specific relationships in two clades of insects. In aphids, H. defensa is found in several species with an intermediate prevalence and confers protection against parasitoids. In whiteflies, H. defensa is almost fixed in some species of Bemisia tabaci, which suggests an important role of and a transition toward obligate symbiosis. In this study, comparisons of the genome of H. defensa present in two B. tabaci species (Middle East Asia Minor 1 and Mediterranean) and in the aphid Acyrthosiphon pisum revealed that they belong to two distinct clades and underwent specific gene losses. In aphids, it contains highly virulent factors that could allow protection and horizontal transfers. In whiteflies, the genome lost these factors and seems to have a limited ability to acquire genes. However it contains genes that could be involved in the production of essential nutrients, which is consistent with a primordial role for this symbiont. In conclusion, although both lineages of H. defensa have mutualistic interactions with their hosts, their genomes follow distinct evolutionary trajectories that reflect their phenotype and could have important consequences on their evolvability.