Selective and stable elimination of endosymbionts from multiple‐infected whitefly Bemisia tabaci by feeding on a cotton plant cultured in antibiotic solutions

The maternally heritable endosymbiont provides many ecosystem functions. Antibiotic elimination of a specific symbiont and establishment of experimental host lines lacking certain symbionts enable the roles of a given symbiont to be explored. The whitefly Bemisia tabaci (Gennadius) in China harbors obligate symbiont Portiera infecting each individual, as well as facultative symbionts, such as Hamiltonella, Rickettsia and Cardinium, with co‐infections occurring relatively frequently. So far no studies have evaluated the selectivity and efficacy of a specific symbiont elimination using antibiotics in whiteflies co‐infected with different symbionts. Furthermore, no success has been achieved in establishing certain symbiont‐free B. tabaci lines. In this study, we treated Hamiltonella‐infected B. tabaci line, Hamiltonella‐Rickettsia‐co‐infected line and Hamiltonella‐Cardinium co‐infected line by feeding B. tabaci adults with cotton plants cultured in water containing rifampicin, ampicillin or a mixture of them, aiming to selectively curing symbiont infections and establishing stable symbiont‐free lines. We found ampicillin selectively eliminated Cardinium without affecting Portiera, Hamiltonella and Rickettsia, although they coexisted in the same host body. Meanwhile, all of the symbionts considered in our study can be removed by rifampicin. The reduction of facultative symbionts occurred at a much quicker pace than obligate symbiont Portiera during rifampicin treatment. Also, we measured the stability of symbiont elimination in whitefly successive generations and established Rickettsia‐infected and Cardinium‐infected lines which are absent in natural populations. Our results provide new protocols for selective elimination of symbionts co‐existing in a host and establishment of different symbiont‐infected host lines.     

Cascading effects of herbivore protective symbionts on hyperparasitoids

1. Microbial symbionts can play an important role in defending their insect hosts against natural enemies. However, researchers have little idea how the presence of such protective symbionts impacts food web interactions and species diversity. 2. This study investigated the effects of a protective symbiont (Hamiltonella defensa) in pea aphids (Acyrthosiphon pisum) on hyperparasitoids, which are a trophic level above the natural enemy target of the symbiont (primary parasitoids). 3. Pea aphids, with and without their natural infections of H. defensa, were exposed first to a primary parasitoid against which the symbiont provides partial protection (either Aphidius ervi or Aphelinus abdominalis), and second to a hyperparasitoid known to attack the primary parasitoid species. 4. It was found that hyperparasitoid hatch rate was substantially affected by the presence of the symbiont. This effect appears to be entirely due to the removal of potential hosts by the action of the symbiont: there was no additional benefit or cost experienced by the hyperparasitoids in response to symbiont presence. The results were similar across the two different aphid–parasitoid–hyperparasitoid interactions we studied. 5. It is concluded that protective symbionts can have an important cascading effect on multiple trophic levels by altering the success of natural enemies, but that there is no evidence for more complex interactions. These findings demonstrate that the potential influence of protective symbionts on the wider community should be considered in future food web studies.     

Co-evolution and symbiont replacement shaped the symbiosis between adelgids (Hemiptera: Adelgidae) and their bacterial symbionts

The Adelgidae (Insecta: Hemiptera), a small group of insects, are known as severe pests on various conifers of the northern hemisphere. Despite of this, little is known about their bacteriocyte‐associated endosymbionts, which are generally important for the biology and ecology of plant sap‐sucking insects. Here, we investigated the adelgid species complexes Adelges laricis/tardus, Adelges abietis/viridis and Adelges cooleyi/coweni, identified based on their coI and ef1alpha genes. Each of these insect groups harboured two phylogenetically different bacteriocyte‐associated symbionts belonging to the Betaproteobacteria and the Gammaproteobacteria, respectively, as inferred from phylogenetic analyses of 16S rRNA gene sequences and demonstrated by fluorescence in situ hybridization. The betaproteobacterial symbionts of all three adelgid complexes (‘Candidatus Vallotia tarda’, ‘Candidatus Vallotia virida’ and ‘Candidatus Vallotia cooleyia’) share a common ancestor and show a phylogeny congruent with that of their respective hosts. Similarly, there is evidence for co‐evolution between the gammaproteobacterial symbionts (‘Candidatus Profftia tarda’, ‘Candidatus Profftia virida’) and A. laricis/tardus and A. abietis/viridis. In contrast, the gammaproteobacterial symbiont of A. cooleyi/coweni (‘Candidatus Gillettellia cooleyia’) is different from that of the other two adelgids but shows a moderate relationship to the symbiont ‘Candidatus Ecksteinia adelgidicola’ of A. nordmannianae/piceae. All symbionts were present in all adelgid populations and life stages analysed, suggesting vertical transmission from mother to offspring. In sharp contrast to their sister group, the aphids, adelgids do not consistently contain a single obligate (primary) symbiont but have acquired phylogenetically different bacterial symbionts during their evolution, which included multiple infections and symbiont replacement.     

Symbionts in excess? No correlation between symbiont density and the ability of mealybug hosts to exploit plant species or tolerate insecticide stress

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Parasitoids as drivers of symbiont diversity in an insect host

Immune systems have repeatedly diversified in response to parasite diversity. Many animals have outsourced part of their immune defence to defensive symbionts, which should be affected by similar evolutionary pressures as the host’s own immune system. Protective symbionts provide efficient and specific protection and respond to changing selection pressure by parasites. Here we use the aphid Aphis fabae, its protective symbiont Hamiltonella defensa, and its parasitoid Lysiphlebus fabarum to test whether parasite diversity can maintain diversity in protective symbionts. We exposed aphid populations with the same initial symbiont composition to parasitoid populations that differed in their diversity. As expected, single parasitoid genotypes mostly favoured a single symbiont that was most protective against that particular parasitoid, while multiple symbionts persisted in aphids exposed to more diverse parasitoid populations, which in turn affected aphid population density and rates of parasitism. Parasite diversity may be crucial to maintaining symbiont diversity in nature.     

Lichens as survivors in space and on Mars

Many lichens are able to live and photosynthesize under harsh conditions, characterized by low temperatures, aridity and high UV radiation fluxes. Some lichen species are even able to survive simulated and real space conditions. Many tests after space exposure on the satellite FOTON M3 and on the International Space Station have shown their capacity to maintain physiological and photosynthetic activity, and their capacity to germinate and grow after being exposed to space parameters. Further tests using simulated Martian atmospheres, temperatures, humidity profiles and UV radiation spectra and fluxes have shown maintenance of photosynthetic activity of Xanthoria elegans. Results from space and Mars simulation experiments on lichens such as X. elegans are valuable for determining the habitability of a planet and for the search for possible life-supporting habitats on planets like Mars.     

Change in algal symbiont communities after bleaching,not prior heat exposure,increases heat tolerance of reef corals

Mutualistic organisms can be particularly susceptible to climate change stress, as their survivorship is often limited by the most vulnerable partner. However, symbiotic plasticity can also help organisms in changing environments by expanding their realized niche space. Coral–algal (Symbiodinium spp.) symbiosis exemplifies this dichotomy: the partnership is highly susceptible to ‘bleaching’ (stress‐induced symbiosis breakdown), but stress‐tolerant symbionts can also sometimes mitigate bleaching. Here, we investigate the role of diverse and mutable symbiotic partnerships in increasing corals' ability to thrive in high temperature conditions. We conducted repeat bleaching and recovery experiments on the coral Montastraea cavernosa, and used quantitative PCR and chlorophyll fluorometry to assess the structure and function of Symbiodinium communities within coral hosts. During an initial heat exposure (32 °C for 10 days), corals hosting only stress‐sensitive symbionts (Symbiodinium C3) bleached, but recovered (at either 24 °C or 29 °C) with predominantly (>90%) stress‐tolerant symbionts (Symbiodinium D1a), which were not detected before bleaching (either due to absence or extreme low abundance). When a second heat stress (also 32 °C for 10 days) was applied 3 months later, corals that previously bleached and were now dominated by D1a Symbiodinium experienced less photodamage and symbiont loss compared to control corals that had not been previously bleached, and were therefore still dominated by Symbiodinium C3. Additional corals that were initially bleached without heat by a herbicide (DCMU, at 24 °C) also recovered predominantly with D1a symbionts, and similarly lost fewer symbionts during subsequent thermal stress. Increased thermotolerance was also not observed in C3‐dominated corals that were acclimated for 3 months to warmer temperatures (29 °C) before heat stress. These findings indicate that increased thermotolerance post‐bleaching resulted from symbiont community composition changes, not prior heat exposure. Moreover, initially undetectable D1a symbionts became dominant only after bleaching, and were critical to corals' resilience after stress and resistance to future stress.     

Phylogenetic Analysis and Fluorescence <Emphasis Type="Italic">In Situ</Emphasis> Hybridization Detection of Archaeal and Bacterial Endosymbionts in the Anaerobic Ciliate <Emphasis Type="Italic">Trimyema Compressum</Emphasis>

The anaerobic free-living ciliate, Trimyema compressum, is known to harbor both methanogenic archaeal and bacterial symbionts in the cytoplasm. To clarify their phylogenetic belongings, a full-cycle rRNA approach was applied to this symbiosis. Phylogenetic analysis showed that the methanogenic symbiont was related to Methanobrevibacter arboriphilicus, which was distantly related to symbionts found in other Trimyema species. This result suggested that Trimyema species do not require very specific methanogenic symbionts, and symbiont replacement could have occurred in the history of Trimyema species. On the other hand, the bacterial symbiont was located near the lineage of the family Syntrophomonadaceae in the phylum Firmicutes. The sequence similarity between the bacterial symbiont and the nearest species was 85%, indicating that bacterial symbionts may be specific to the Trimyema species. The elimination of bacterial symbionts from the ciliate cell by antibiotic treatment resulted in considerably decreased host growth. However, it was not restored by stigmasterol addition (<2 μg ml⁻¹), which was different from the previous report that showed that the symbiont-free strain required exogenous sterols for growth. In addition, the decline of host growth was not accompanied by host metabolism shift toward the formation of more reduced products, which suggested that the contribution of bacterial symbionts to the host ciliate was not a dispose of excessive reducing equivalent arising from the host’s fermentative metabolism as methanogenic symbionts do. This study showed that bacterial symbionts make a significant contribution to the host ciliate by an unknown function and suggested that interactions between bacterial symbionts and T. compressum are more complicated than hitherto proposed.     

Symbiont community diversity is more variable in corals that respond poorly to stress

Coral reefs are declining globally as climate change and local water quality press environmental conditions beyond the physiological tolerances of holobionts—the collective of the host and its microbial symbionts. To assess the relationship between symbiont composition and holobiont stress tolerance, community diversity metrics were quantified for dinoflagellate endosymbionts (Family: Symbiodiniaceae) from eight Acropora millepora genets that thrived under or responded poorly to various stressors. These eight selected genets represent the upper and lower tails of the response distribution of 40 coral genets that were exposed to four stress treatments (and control conditions) in a 10‐day experiment. Specifically, four ‘best performer’ coral genets were analyzed at the end of the experiment because they survived high temperature, high pCO₂, bacterial exposure, or combined stressors, whereas four ‘worst performer’ genets were characterized because they experienced substantial mortality under these stressors. At the end of the experiment, seven of eight coral genets mainly hosted Cladocopium symbionts, whereas the eighth genet was dominated by both Cladocopium and Durusdinium symbionts. Symbiodiniaceae alpha and beta diversity were higher in worst performing genets than in best performing genets. Symbiont communities in worst performers also differed more after stress exposure relative to their controls (based on normalized proportional differences in beta diversity), than did best performers. A generalized joint attribute model estimated the influence of host genet and treatment on Symbiodiniaceae community composition and identified strong associations among particular symbionts and host genet performance, as well as weaker associations with treatment. Although dominant symbiont physiology and function contribute to host performance, these findings emphasize the importance of symbiont community diversity and stochasticity as components of host performance. Our findings also suggest that symbiont community diversity metrics may function as indicators of resilience and have potential applications in diverse disciplines from climate change adaptation to agriculture and medicine.     

Ecological factors influencing the beneficial endosymbionts of the hemlock woolly adelgid (Hemiptera: Adelgidae)

Bacterial endosymbionts of sap-sucking insects provide their host with a num- ber of beneficial qualities,including the supply of nutrition,defense against parasitoids, and protection from heat stress.Damage to these bacterial associates can therefore have a negative impact on the fitness of their insect host.We evaluated observational and experi- mental factors regarding the normative hemlock woolly adelgid (Adelges tsugae Annand) (Hemiptera:Adelgidae)to help understand the roles of its three recently identified symbionts,including under heat stress conditions.The prevalence of A.tsugae's facultafive symbiont (Serratia symbiotica)was examined at different spatial scales to determine how variable infection rates are for this symbiont.There was no significant difference found in infection rates between adelgids on a tree,within a plot,or within a state.However, significantly more adelgids in Georgia (95%)had S.symbiotica compared to those in New York (68%).Microsatellite genotyping of the adelgids found that this difference was most likely not the result of a second introduction ofA.tsugae into eastern North America.Comparison orS.symbiotica proportions between first and fourth instars showed that symbiont absence did not affect the ability ofA.tsugae to survive aestivation.Evaluations of sym- biont densities within each adelgid found that when S.symbiotica was absent,the density of obligate symbionts was significantly higher.Exposure to heat stress (32.5℃)was not consistently correlated with changes in symbiont densities over a 4-d period.Overall,we have shown that symbiont prevalence and densities vary within the broad population of A.tsugae in eastern North America,with potentially significant effects upon the ecology of this important pest.     

Interspecific competition between entomopathogenic nematodes (Steinernema) is modified by their bacterial symbionts (Xenorhabdus)

Background  

Symbioses between invertebrates and prokaryotes are biological systems of particular interest in order to study the evolution of mutualism. The symbioses between the entomopathogenic nematodes Steinernema and their bacterial symbiont Xenorhabdus are very tractable model systems. Previous studies demonstrated (i) a highly specialized relationship between each strain of nematodes and its naturally associated bacterial strain and (ii) that mutualism plays a role in several important life history traits of each partner such as access to insect host resources, dispersal and protection against various biotic and abiotic factors. The goal of the present study was to address the question of the impact of Xenorhabdus symbionts on the progression and outcome of interspecific competition between individuals belonging to different Steinernema species. For this, we monitored experimental interspecific competition between (i) two nematode species: S. carpocapsae and S. scapterisci and (ii) their respective symbionts: X. nematophila and X. innexi within an experimental insect-host (Galleria mellonella). Three conditions of competition between nematodes were tested: (i) infection of insects with aposymbiotic IJs (i.e. without symbiont) of both species (ii) infection of insects with aposymbiotic IJs of both species in presence of variable proportion of their two Xenorhabdus symbionts and (iii) infection of insects with symbiotic IJs (i.e. naturally associated with their symbionts) of both species.     

A niche perspective on the range expansion of symbionts

Range expansion results from complex eco‐evolutionary processes where range dynamics and niche shifts interact in a novel physical space and/or environment, with scale playing a major role. Obligate symbionts (i.e. organisms permanently living on hosts) differ from free‐living organisms in that they depend on strong biotic interactions with their hosts which alter their niche and spatial dynamics. A symbiotic lifestyle modifies organism–environment relationships across levels of organisation, from individuals to geographical ranges. These changes influence how symbionts experience colonisation and, by extension, range expansion. Here, we investigate the potential implications of a symbiotic lifestyle on range expansion capacity. We present a unified conceptual overview on range expansion of symbionts that integrates concepts grounded in niche and metapopulation theories. Overall, we explain how niche‐driven and dispersal‐driven processes govern symbiont range dynamics through their interaction across scales, from host switching to geographical range shifts. First, we describe a background framework for range dynamics based on metapopulation concepts applied to symbiont organisation levels. Then, we integrate metapopulation processes operating in the physical space with niche dynamics grounded in the environmental arena. For this purpose, we provide a definition of the biotope (i.e. living place) specific to symbionts as a hinge concept to link the physical and environmental spaces, wherein the biotope unit is a metapopulation patch (either a host individual or a land fragment). Further, we highlight the dual nature of the symbionts' niche, which is characterised by both host traits and the external environment, and define proper conceptual variants to provide a meaningful unification of niche, biotope and symbiont organisation levels. We also explore variation across systems in the relative relevance of both external environment and host traits to the symbiont's niche and their potential implications on range expansion. We describe in detail the potential mechanisms by which hosts, through their function as biotopes, could influence how some symbionts expand their range – depending on the life history and traits of both associates. From the spatial point of view, hosts can extend symbiont dispersal range via host‐mediated dispersal, although the requirement for among‐host dispersal can challenge symbiont range expansion. From the niche point of view, homeostatic properties of host bodies may allow symbiont populations to become insensitive to off‐host environmental gradients during host‐mediated dispersal. These two potential benefits of the symbiont–host interaction can enhance symbiont range expansion capacity. On the other hand, the central role of hosts governing the symbiont niche makes symbionts strongly dependent on the availability of suitable hosts. Thus, environmental, dispersal and biotic barriers faced by suitable hosts apply also to the symbiont, unless eventual opportunities for host switching allow the symbiont to expand its repertoire of suitable hosts (thus expanding its fundamental niche). Finally, symbionts can also improve their range expansion capacity through their impacts on hosts, via protecting their affiliated hosts from environmental harshness through biotic facilitation.     

Horizontal transfer of facultative endosymbionts is limited by host relatedness

Heritable microbial symbionts can have important effects on many aspects of their hosts’ biology. Acquisition of a novel symbiont strain can provide fitness benefits to the host, with significant ecological and evolutionary consequences. We measured barriers to horizontal transmission by artificially transferring facultative symbionts from the grain aphid, Sitobion avenae, and five other aphid species into two clonal genotypes of S. avenae. We found the symbiont Hamiltonella defensa establishes infections more easily following a transfer from the same host species and that such infections are more stable. Infection success was also higher when the introduced symbiont strain was more closely related to the strain that was originally present in the host (but which had previously been removed). There were no differences among successfully established symbiont strains in their effect on aphid fecundity. Hamiltonella defensa did not confer protection against parasitoids in our S. avenae clones, although it often does in other aphid hosts. However, strains of the symbiont Regiella insecticola originating from two host species protected grain aphids against the pathogenic fungus Pandora neoaphidis. This study helps describe the extent to which facultative symbionts can act as a pool of adaptations that can be sampled by their eukaryote hosts.     

Phylogeographic patterns in the Philippine archipelago influence symbiont diversity in the bobtail squid–Vibrio mutualism

Marine microbes encounter a myriad of biotic and abiotic factors that can impact fitness by limiting their range and capacity to move between habitats. This is especially true for environmentally transmitted bacteria that cycle between their hosts and the surrounding habitat. As geologic history, biogeography, and other factors such as water temperature, salinity, and physical barriers can inhibit bacterial movement to novel environments, we chose to examine the genetic architecture of Euprymna albatrossae (Mollusca: Cephalopoda) and their Vibrio fischeri symbionts in the Philippine archipelago using a combined phylogeographic approach. Eleven separate sites in the Philippine islands were examined using haplotype estimates that were examined via nested clade analysis to determine the relationship between E. albatrossae and V. fischeri populations and their geographic location. Identical analyses of molecular variance (AMOVA) were used to estimate variation within and between populations for host and symbiont genetic data. Host animals demonstrated a significant amount of variation within island groups, while symbiont variation was found within individual populations. Nested clade phylogenetic analysis revealed that hosts and symbionts may have colonized this area at different times, with a sudden change in habitat. Additionally, host data indicate restricted gene flow, whereas symbionts show range expansion, followed by periodic restriction to genetic flow. These differences between host and symbiont networks indicate that factors “outside the squid” influence distribution of Philippine V. fischeri. Our results shed light on how geography and changing environmental factors can impact marine symbiotic associations at both local and global scales.     

The power of paired genomes

Species interactions are fundamental to ecology. Classic studies of competition, predation, parasitism and mutualism between macroscopic organisms have provided a foundation for the discipline, but many of the most important and intimate ecological interactions are microscopic in scale. These microscopic interactions include those occurring between eukaryotic hosts and their microbial symbionts. Such symbioses, ubiquitous in nature, provide experimental challenges because the partners often cannot live outside the symbiosis. With respect to the symbionts, this precludes utilizing classical microbiological and genetic techniques that require in vitro cultivation. Genomics, however, has rapidly changed the study of symbioses. In this issue of Molecular Ecology, MacDonald et al. (2011) , coupling symbiont whole‐genome sequencing, experimental studies and metabolic modelling, provide novel insights into one of the best‐studied symbioses, that between aphids and their obligate, nutrient‐provisioning, intracellular bacteria, Buchnera aphidicola ( Fig. 1 ). MacDonald and colleagues assessed variation in the ability of aphid–Buchnera pairs to thrive on artificial diets missing different amino acids. As shown previously (e.g. Wilkinson & Douglas 2003 ), aphid–Buchnera pairs can differ in their requirements for external sources of essential amino acids. Such phenotypic variation could result from differences in Buchnera’s amino acid biosynthetic capabilities or in the ability of aphids to interact with their symbionts. Whole‐genome sequencing of the Buchnera genomes from four aphid lines with alternate nutritional phenotypes revealed that the environmental nutrients required by the aphid–Buchnera pairs could not be explained by sequence variation in the symbionts. Instead, a novel metabolic modelling approach suggested that much of the variation in nutritional phenotype could be explained by host variation in the capacity to provide necessary nutrient precursors to their symbionts. MacDonald et al.’s work complements a recent study by Vogel & Moran (2011) , who through crossing experiments investigating the inheritance of a nutritional phenotype associated with a frameshift mutation in a Buchnera amino acid biosynthesis gene powerfully demonstrated that different host genotypes paired with the same symbiont genome could exhibit substantially different nutritional requirements. 2 Thus, while there is little doubt that Buchnera are evolutionarily central to the nutritional ecology of aphids, the current work by MacDonald et al. (2011) together with that of Vogel & Moran (2011) surprisingly demonstrates host dominance in defining and controlling the ecological niche of this particular symbiosis.

Figure 1 Open in figure viewer PowerPoint Pea aphids and their bacterial symbionts. (a) A pea aphid mother and her clonal offspring. (b) Flourescence In Situ Hybridization (FISH) microscopy reveals the intimate association of aphid tissues (blue) with their obligate bacterial symbiont, Buchnera aphidicola (green), and a common facultative bacterial symbiont, Hamiltonella defensa (red). Photo by T. Barribeau, FISH image provided by A. Douglas.     

Micronucleus‐Specific Bacterium Holospora elegans Irreversibly Enhances Stress Gene Expression of the Host Paramecium caudatum

ABSTRACT. The bacterium Holospora is an endonuclear symbiont of the ciliate Paramecium. Previously, we reported that paramecia bearing the macronuclear‐specific symbiont Holospora obtusa survived better than symbiont‐free paramecia, even under high temperatures unsuitable for growth. The paramecia with symbionts expressed high levels of hsp70 mRNAs even at 25 °C, a usual growth temperature. We report herein that paramecia bearing the micronuclear‐specific symbiont Holospora elegans also acquire the heat‐shock resistance. Even after the removal of the bacteria from the hosts by treatment with penicillin, the resulting aposymbiotic paramecia nevertheless maintained their heat shock‐resistant nature for over 1 yr. Like symbiotic paramecia, these aposymbiotic paramecia also expressed high levels of both hsp60 and hsp70 mRNAs even at 25 °C. Moreover, analysis by fluorescent in situ hybridization with a probe specific for Holospora 16S rRNA revealed that the 16S rRNA of H. elegans was expressed around the nucleoli of the macronucleus in the aposymbiotic cells. This result suggests the possible transfer of Holospora genomic DNA from the micronucleus into the macronucleus in symbiotic paramecia. Perhaps this exogenous DNA could trigger the aposymbiotic paramecia to induce a stress response, inducing higher expression of Hsp60 and Hsp70, and thus conferring heat‐shock resistance.     

Symbiont‐mediated phenotypic variation without co‐evolution in an insect–fungus association

Recent studies have shown that symbionts can be a source of adaptive phenotypic variation for their hosts. It is assumed that co‐evolution between hosts and symbionts underlies these ecologically significant phenotypic traits. We tested this assumption in the ectosymbiotic fungal associate of the gall midge Asteromyia carbonifera. Phylogenetic analysis placed the fungal symbiont within a monophyletic clade formed by Botryosphaeria dothidea, a typically free‐living (i.e. not associated with an insect host) plant pathogen. Symbiont isolates from four divergent midge lineages demonstrated none of the patterns common to heritable microbial symbioses, including parallel diversification with their hosts, substitution rate acceleration, or A+T nucleotide bias. Amplified fragment length polymorphism genotyping of the symbiont revealed that within‐lineage genetic diversity was not clustered along host population lines. Culture‐based experiments demonstrated that the symbiont‐mediated variation in gall phenotype is not borne out in the absence of the midge. This study shows that symbionts can be important players in phenotypic variation for their hosts, even in the absence of a co‐evolutionary association.     

Evolutionary changes in symbiont community structure in ticks

Ecological specialization to restricted diet niches is driven by obligate, and often maternally inherited, symbionts in many arthropod lineages. These heritable symbionts typically form evolutionarily stable associations with arthropods that can last for millions of years. Ticks were recently found to harbour such an obligate symbiont, Coxiella‐LE, that synthesizes B vitamins and cofactors not obtained in sufficient quantities from blood diet. In this study, the examination of 81 tick species shows that some Coxiella‐LE symbioses are evolutionarily stable with an ancient acquisition followed by codiversification as observed in ticks belonging to the Rhipicephalus genus. However, many other Coxiella‐LE symbioses are characterized by low evolutionary stability with frequent host shifts and extinction events. Further examination revealed the presence of nine other genera of maternally inherited bacteria in ticks. Although these nine symbionts were primarily thought to be facultative, their distribution among tick species rather suggests that at least four may have independently replaced Coxiella‐LE and likely represent alternative obligate symbionts. Phylogenetic evidence otherwise indicates that cocladogenesis is globally rare in these symbioses as most originate via horizontal transfer of an existing symbiont between unrelated tick species. As a result, the structure of these symbiont communities is not fixed and stable across the tick phylogeny. Most importantly, the symbiont communities commonly reach high levels of diversity with up to six unrelated maternally inherited bacteria coexisting within host species. We further conjecture that interactions among coexisting symbionts are pivotal drivers of community structure both among and within tick species.     

Bacterial symbionts as mediators of ecologically important traits of insect hosts

1. Bacterial symbionts play a prominent role in insect nutritional ecology by aiding in digestion of food or providing nutrients that are limited or lacking in the diet. Thereby, endosymbionts open niches to their insect host that would otherwise be unavailable. 2. Currently, several other ecologically relevant traits mediated by endosymbionts are being investigated, including enhanced parasite resistance, enhanced heat tolerance, and influences on insect–plant interactions such as manipulation of plant physiology to the benefit of the insect. 3. Traits mediated by endosymbionts are often identified by correlative studies where traits are found to be altered in the presence of a particular symbiont. Recent developments in genomic tools offer the opportunity for studying the impact of bacteria–insect symbioses under natural conditions in a population and community ecology context. In vivo experiments specifically testing putative functions of endosymbionts in parallel to population‐level studies on the prevalence of endosymbionts allow disentangling host versus symbiont contribution to phenotypic variability observed in individuals. Effects of symbionts on host phenotype are often large and relevant to host fitness, e.g. by significantly enhancing survival or fecundity in a context‐dependent manner. 4. Predominantly vertically transmitted endosymbionts contribute to the heritable genetic variation present in a host species. Phenotypic variation on which selection can act may be due to differences either among host genomes, symbiont genomes, or genotype × genotype interactions. Therefore the holobiont, i.e. the host including all symbionts, should be regarded as the unit of selection as the association between host and symbionts may affect the fitness of the holobiont depending on the environment.     

Bacteriome-associated endosymbionts of the green rice leafhopper <Emphasis Type="Italic">Nephotettix cincticeps</Emphasis> (Hemiptera: Cicadellidae)

The green rice leafhopper Nephotettix cincticeps (Uhler) is a commonly distributed pest of rice in East Asia. Early histological studies describe the presence of two bacteriome-associated symbionts and a rickettsial microorganism in N. cincticeps, but their microbiological affiliations have been elusive. We identified these bacterial symbionts using modern microbiological techniques. Cloning and sequencing of the 16S ribosomal RNA gene from dissected bacteriomes yielded two major and a minor bacterial sequences: a major sequence was placed in the Bacteroidetes clade of Sulcia muelleri, an ancient symbiont lineage associated with diverse hemipteran insects; another major sequence was allied to a β-proteobacterial sequence from a leafhopper Matsumuratettix hiroglyphicus; the minor sequence fell in the α-proteobacterial genus Rickettsia. In situ hybridization and transmission electron microscopy showed that the Sulcia symbiont and the β-proteobacterial symbiont are harbored within different types of bacteriocytes that constitute the outer and inner regions of the bacteriome, respectively. Oral administration of tetracycline to nymphal N. cincticeps resulted in retarded growth, high mortality rates, and failure in adult emergence, suggesting important biological roles of the symbionts for the host insect. The designation Candidatus Nasuia deltocephalinicola is proposed for the β-proteobacterial symbiont clade associated with N. cincticeps and allied leafhoppers of the subfamily Deltocephalinae.