Happens in the best of subfamilies: establishment and repeated replacements of co‐obligate secondary endosymbionts within Lachninae aphids

Virtually all aphids maintain an obligate mutualistic symbiosis with bacteria from the Buchnera genus, which produce essential nutrients for their aphid hosts. Most aphids from the Lachninae subfamily have been consistently found to house additional endosymbionts, mainly Serratia symbiotica. This apparent dependence on secondary endosymbionts was proposed to have been triggered by the loss of the riboflavin biosynthetic capability by Buchnera in the Lachninae last common ancestor. However, an integral large‐scale analysis of secondary endosymbionts in the Lachninae is still missing, hampering the interpretation of the evolutionary and genomic analyses of these endosymbionts. Here, we analysed the endosymbionts of selected representatives from seven different Lachninae genera and nineteen species, spanning four tribes, both by FISH (exploring the symbionts’ morphology and tissue tropism) and 16S rRNA gene sequencing. We demonstrate that all analysed aphids possess dual symbiotic systems, and while most harbour S. symbiotica, some have undergone symbiont replacement by other phylogenetically‐distinct bacterial taxa. We found that these secondary associates display contrasting cell shapes and tissue tropism, and some appear to be lineage‐specific. We propose a scenario for symbiont establishment in the Lachninae, followed by changes in the symbiont's tissue tropism and symbiont replacement events, thereby highlighting the extraordinary versatility of host‐symbiont interactions.     

Stress‐induced changes in abundance differ among obligate and facultative endosymbionts of the soybean aphid

Bacterial endosymbionts can drive evolutionary novelty by conferring adaptive benefits under adverse environmental conditions. Among aphid species there is growing evidence that symbionts influence tolerance to various forms of stress. However, the extent to which stress inflicted on the aphid host has cascading effects on symbiont community dynamics remains poorly understood. Here we simultaneously quantified the effect of host‐plant induced and xenobiotic stress on soybean aphid (Aphis glycines) fitness and relative abundance of its three bacterial symbionts. Exposure to soybean defensive stress (Rag1 gene) and a neurotoxic insecticide (thiamethoxam) substantially reduced aphid composite fitness (survival × reproduction) by 74 ± 10% and 92 ± 2%, respectively, which in turn induced distinctive changes in the endosymbiont microbiota. When challenged by host‐plant defenses a 1.4‐fold reduction in abundance of the obligate symbiont Buchnera was observed across four aphid clonal lines. Among facultative symbionts of Rag1‐stressed aphids, Wolbachia abundance increased twofold and Arsenophonus decreased 1.5‐fold. A similar pattern was observed under xenobiotic stress, with Buchnera and Arsenophonus titers decreasing (1.3‐fold) and Wolbachia increasing (1.5‐fold). Furthermore, variation in aphid virulence to Rag1 was positively correlated with changes in Arsenophonus titers, but not Wolbachia or Buchnera. A single Arsenophonus multi‐locus genotype was found among aphid clonal lines, indicating strain diversity is not primarily responsible for correlated host‐symbiont stress levels. Overall, our results demonstrate the nature of aphid symbioses can significantly affect the outcome of interactions under stress and suggests general changes in the microbiome can occur across multiple stress types.     

Diversity and geographic variation of endosymbiotic bacteria in natural populations of the pea aphid (Acyrthosiphon pisum) in China

Bacterial symbionts in aphids are known to benefit the insect host and associated with aphid’s ecological adaptation. The pea aphid (Acyrthosiphon pisum), an important legume pest worldwide, carries at least eight endosymbionts, providing a model system to study insect–bacteria interactions. However, species diversity and geographic variations of endosymbionts are unknown in Chinese populations; therefore, we characterized symbiont communities and diversity of 27 pea aphid samples from 13 geographic populations of China. Via amplicon high-throughput sequencing and diagnostic PCR, we found that bacterial communities of Chinese populations were dominated by Proteobacteria and Firmicutes. Among eight known endosymbionts, five (Buchnera, Serratia, Hamiltonella, Regiella, and Rickettsia) were detected by both methods, with a specific geographical distribution. The obligate symbiont, Buchnera, was present in all aphid samples, while the four facultative symbionts showed a significant geographic variation. Each population was randomly infected with distinct endosymbionts, ranging from three to five species. Serratia and Rickettsia showed relatively higher abundance in central regions of China, Regiella was predominant in eastern and western China, whereas Hamiltonella showed an extremely low abundance and was absent in four populations. Samples grouped by altitudes showed a significant diversity difference, whereas there was no significant difference between red and green body colors. Bacterial community structures of the Chinese pea aphid populations were mainly influenced by environmental factors, other than body colors. These data can guide the development of potential biocontrol techniques against this aphid.     

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.     

Evolution and Diversity of Facultative Symbionts from the Aphid Subfamily Lachninae

Many aphids harbor a variety of endosymbiotic bacteria. The functions of these symbionts can range from an obligate nutritional role to a facultative role in protecting their hosts against environmental stresses. One such symbiont is “Candidatus Serratia symbiotica,” which is involved in defense against heat and potentially also in aphid nutrition. Lachnid aphids have been the focus of several recent studies investigating the transition of this symbiont from a facultative symbiont to an obligate symbiont. In a phylogenetic analysis of Serratia symbionts from 51 lachnid hosts, we found that diversity in symbiont morphology, distribution, and function is due to multiple independent origins of symbiosis from ancestors belonging to Serratia and possibly also to evolution within distinct symbiont clades. Our results do not support cocladogenesis of “Ca. Serratia symbiotica” with Cinara subgenus Cinara species and weigh against an obligate nutritional role. Finally, we show that species belonging to the subfamily Lachninae have a high incidence of facultative symbiont infection.Many insect species harbor heritable endosymbiotic bacteria. Among the best studied of these species are aphids. Almost all aphids are infected with the obligate nutritional symbiont Buchnera aphidicola, which is generally required for the survival of aphids and provides essential amino acids that are rare in their phloem sap diet (32). Many aphids also possess additional symbionts that may be facultative from the host''s perspective and that coexist with Buchnera (20).Three lineages of facultative symbionts that are prevalent in aphids belong to the Enterobacteriaceae. Two of these lineages (“Candidatus Hamiltonella defensa” and “Candidatus Regiella insecticola”) form well-defined clades distinct from free-living bacterial species (4, 20) and confer clear advantages to their hosts by protecting them against natural enemies. “Ca. Hamiltonella defensa” prevents wasp parasitism by arresting development of wasp larvae in pea aphids, and “Ca. Regiella insecticola” provides resistance against the fungal pathogen Pandora neoaphidis (24, 31). The third lineage, “Candidatus Serratia symbiotica,” is closely related to free-living members of the genus Serratia. This symbiont is distributed sporadically among aphid species and has been proposed to have a variety of effects on hosts. In pea aphids (Acyrthosiphon pisum; Macrosiphini), “Ca. Serratia symbiotica” ameliorates the deleterious fitness effects of heat shock by protecting symbiont-harboring bacteriocyte cells (2, 19, 29). Additionally, a strain of “Ca. Serratia symbiotica” provided some resistance to parasitoid wasp attack (24). “Ca. Serratia symbiotica” has been proposed to play a role in nutrition by producing amino acids for its aphid host and by decreasing its host''s reliance on Buchnera (10, 15, 16, 26). In contrast to most Buchnera strains, Buchnera strains from Cinara cedri (Lachnini) have lost the genes for biosynthesis of the essential amino acid tryptophan, while “Ca. Serratia symbiotica” in the same host possesses at least part of the pathway, suggesting that it has a mutualistic role in the nutrition of aphids (26).In A. pisum, “Ca. Serratia symbiotica” cells are rod-shaped bacteria that are present in the sheath cells, hemolymph, and bacteriocytes of some individuals. In contrast, in C. cedri “Ca. Serratia symbiotica” occurs in all individuals, and its cells are large, round, and pleomorphic, similar to the cells of many obligate bacterial aphid endosymbionts, including Buchnera (10, 26). Furthermore, “Ca. Serratia symbiotica” has consistently been present in other Cinara species sampled (28). Both the rod-shaped and pleomorphic forms are assigned to “Ca. Serratia symbiotica” based on phylogenetic analyses of several gene sequences, but they fall into two distinct sister clades of symbiont lineages that seem to coincide with bacterial morphology (17, 20).This diversity in “Ca. Serratia symbiotica” morphology, distribution, and functions may represent evolution of different features within lineages of a single symbiont clade. If “Ca. Serratia symbiotica” is an obligate nutritional symbiont in Cinara hosts, it is expected that Cinara-associated symbionts would form a clade in which the intraclade relationships mirror those of the hosts (cocladogenesis), as observed for Buchnera and other obligate nutritional symbionts of insects (13, 21, 38). Indeed, Lamelas et al. postulated that, based on their similar phylogenies, Serratia symbionts from aphids belonging to the subgenus Cinara have had a long-term relationship with their hosts (17).In addition to the three most common facultative symbiont types found in aphids described above, several other symbiont lineages with unknown functions have been identified by amplification of bacterial 16S rRNA gene sequences from various aphid species (10, 28, 39). Here we examine the diversity of Serratia and other facultative symbionts in aphids belonging to the subfamily Lachninae. We investigated the distribution of symbionts in aphid species and geographic locations and looked for coevolutionary patterns that may correspond to the functions of facultative symbionts within their hosts.     

Sources of variation in dietary requirements in an obligate nutritional symbiosis

The nutritional symbiosis between aphids and their obligate symbiont, Buchnera aphidicola, is often characterized as a highly functional partnership in which the symbiont provides the host with essential nutrients. Despite this, some aphid lineages exhibit dietary requirements for nutrients typically synthesized by Buchnera, suggesting that some aspect of the symbiosis is disrupted. To examine this phenomenon in the pea aphid, Acyrthosiphon pisum, populations were assayed using defined artificial diet to determine dietary requirements for essential amino acids (EAAs). Six clones exhibiting dependence on EAAs in their diet were investigated further. In one aphid clone, a mutation in a Buchnera amino acid biosynthesis gene could account for the clone''s requirement for dietary arginine. Analysis of aphid F₁ hybrids allowed separation of effects of the host and symbiont genomes, and revealed that both affect the requirement for dietary EAAs in the clones tested. Amino acid requirements were minimally affected by secondary symbiont infection. Our results indicate that variation among pea aphids in dependence on dietary amino acids can result from Buchnera mutation as well as variation in the host genotype.     

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 Sequence of the Pea Aphid Acyrthosiphon pisum

Aphids are important agricultural pests and also biological models for studies of insect-plant interactions, symbiosis, virus vectoring, and the developmental causes of extreme phenotypic plasticity. Here we present the 464 Mb draft genome assembly of the pea aphid Acyrthosiphon pisum. This first published whole genome sequence of a basal hemimetabolous insect provides an outgroup to the multiple published genomes of holometabolous insects. Pea aphids are host-plant specialists, they can reproduce both sexually and asexually, and they have coevolved with an obligate bacterial symbiont. Here we highlight findings from whole genome analysis that may be related to these unusual biological features. These findings include discovery of extensive gene duplication in more than 2000 gene families as well as loss of evolutionarily conserved genes. Gene family expansions relative to other published genomes include genes involved in chromatin modification, miRNA synthesis, and sugar transport. Gene losses include genes central to the IMD immune pathway, selenoprotein utilization, purine salvage, and the entire urea cycle. The pea aphid genome reveals that only a limited number of genes have been acquired from bacteria; thus the reduced gene count of Buchnera does not reflect gene transfer to the host genome. The inventory of metabolic genes in the pea aphid genome suggests that there is extensive metabolite exchange between the aphid and Buchnera, including sharing of amino acid biosynthesis between the aphid and Buchnera. The pea aphid genome provides a foundation for post-genomic studies of fundamental biological questions and applied agricultural problems.     

An ecological cost associated with protective symbionts of aphids

Beneficial symbioses are widespread and diverse in the functions they provide to the host ranging from nutrition to protection. However, these partnerships with symbionts can be costly for the host. Such costs, so called “direct costs”, arise from a trade‐off between allocating resources to symbiosis and other functions such as reproduction or growth. Ecological costs may also exist when symbiosis negatively affects the interactions between the host and other organisms in the environment. Although ecological costs can deeply impact the evolution of symbiosis, they have received little attention. The pea aphid Acyrthosiphon pisum benefits a strong protection against its main parasitoids from protective bacterial symbionts. The ecological cost of symbiont‐mediated resistance to parasitism in aphids was here investigated by analyzing aphid behavior in the presence of predatory ladybirds. We showed that aphids harboring protective symbionts expressed less defensive behaviors, thus suffering a higher predation than symbiont‐free aphids. Consequently, our study indicates that this underlined ecological cost may affect both the coevolutionary processes between symbiotic partners and the prevalence of such beneficial bacterial symbionts in host natural populations.     

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.     

Effects of bacterial secondary symbionts on host plant use in pea aphids

Aphids possess several facultative bacterial symbionts that have important effects on their hosts'' biology. These have been most closely studied in the pea aphid (Acyrthosiphon pisum), a species that feeds on multiple host plants. Whether secondary symbionts influence host plant utilization is unclear. We report the fitness consequences of introducing different strains of the symbiont Hamiltonella defensa into three aphid clones collected on Lathyrus pratensis that naturally lack symbionts, and of removing symbionts from 20 natural aphid–bacterial associations. Infection decreased fitness on Lathyrus but not on Vicia faba, a plant on which most pea aphids readily feed. This may explain the unusually low prevalence of symbionts in aphids collected on Lathyrus. There was no effect of presence of symbiont on performance of the aphids on the host plants of the clones from which the H. defensa strains were isolated. Removing the symbiont from natural aphid–bacterial associations led to an average approximate 20 per cent reduction in fecundity, both on the natural host plant and on V. faba, suggesting general rather than plant-species-specific effects of the symbiont. Throughout, we find significant genetic variation among aphid clones. The results provide no evidence that secondary symbionts have a major direct role in facilitating aphid utilization of particular host plant species.     

The Combined Effects of Bacterial Symbionts and Aging on Life History Traits in the Pea Aphid,Acyrthosiphon pisum

While many endosymbionts have beneficial effects on hosts under specific ecological conditions, there can also be associated costs. In order to maximize their own fitness, hosts must facilitate symbiont persistence while preventing symbiont exploitation of resources, which may require tight regulation of symbiont populations. As a host ages, the ability to invest in such mechanisms may lessen or be traded off with demands of other life history traits, such as survival and reproduction. Using the pea aphid, Acyrthosiphon pisum, we measured survival, lifetime fecundity, and immune cell counts (hemocytes, a measure of immune capacity) in the presence of facultative secondary symbionts. Additionally, we quantified the densities of the obligate primary bacterial symbiont, Buchnera aphidicola, and secondary symbionts across the host''s lifetime. We found life history costs to harboring some secondary symbiont species. Secondary symbiont populations were found to increase with host age, while Buchnera populations exhibited a more complicated pattern. Immune cell counts peaked at the midreproductive stage before declining in the oldest aphids. The combined effects of immunosenescence and symbiont population growth may have important consequences for symbiont transmission and maintenance within a host population.     

On the functional significance of symbiotic microorganisms in the Homoptera: a comparative study of Acyrthosiphon pisum and Nilaparvata lugens

All phloem‐feeding Homoptera possess symbiotic microorganisms. Although the phylogenetic position and anatomical location of the micro‐ organisms differ, the underlying theme of the symbiosis is the same; the microorganisms improve the nutritional quality of the diet through the provision of essential amino acids. The symbiosis has been well documented in aphids, but little information is available from other homopteran groups. The impact of the loss of bacterial symbionts in the pea aphid Acyrthosiphon pisum Harris and eukaryotic yeast‐like symbionts in the Asian rice brown planthopper Nilaparvata lugens Stål was examined in parallel. The weight and relative growth rate of aphids and planthoppers was significantly reduced by symbiont loss, and characteristic features of aposymbiotic pea aphids, so‐called ‘metabolic signatures’, were, for the first time, observed in aposymbiotic N. lugens. For example, the amount of protein per unit fresh weight was reduced by 26 and 10%, and the free amino acid levels increased 1.8‐ and 1.4‐fold, in aposymbiotic A. pisum and N. lugens, respectively. In addition, the concentration of the amino acid glutamine was elevated in the tissues of aposymbiotic insects. The data are discussed in the context of our current understanding of the nutritional role of the symbiosis and the mechanisms of nitrogen metabolism in the two insect species. It is concluded that the metabolic adjustments of the insects to symbiont loss are broadly equivalent.     

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.     

Evolutionary relationships of Pemphigus and allied genera (Hemiptera: Aphididae: Eriosomatinae) and their primary endosymbiont,Buchnera aphidicola

Aphids harbor primary endosymbionts, Buchnera aphidicola, in specialized cells within their body cavities. Aphids and Buchnera have strict mutualistic relationships in nutrition exchange. This ancient association has received much attention from researchers who are interested in endosymbiotic evolution. Previous studies have found parallel phylogenetic relationships between non‐galling aphids and Buchnera at lower taxonomic levels (genus, species). To understand whether relatively isolated habitats such as galls have effect on the parallel relationships between aphids and Buchnera, the present paper investigated the phylogenetic relationships of gall aphids from Pemphigus and allied genera, which induce pseudo‐galls or galls on Populus spp. (poplar) and Buchnera. The molecular phylogenies inferred from three aphid genes (COI, COII and EF‐1α) and two Buchnera genes (gnd, 16S rRNA gene) indicated significant congruence between aphids and Buchnera at generic as well as interspecific levels. Interestingly, both aphid and Buchnera phylogenies supported three main clades corresponding to the galling locations of aphids, namely leaf, the joint of leaf blade and petiole, and branch of the host plant. The results suggest phylogenetic conservatism of gall characters, which indicates gall characters are more strongly affected by aphid phylogeny, rather than host plants.     

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.     

Impact of a parasitoid on the bacterial symbiosis of its aphid host

Embryo production in aphids is absolutely dependent on the function of symbiotic bacteria, mainly Buchnera, and the growth and development of koinobiont parasitoids in aphids requires the diversion of nutrients from aphid embryo production to the parasitoid. The implication that the bacterial symbiosis may be promoted in parasitized aphids to support the growing parasitoid was explored by analysis of the number and biomass of mycetocytes, and the aphid cells bearing Buchnera, in the pea aphid Acyrthosiphon pisum Harris (Hemiptera: Aphididae) parasitized by the wasp Aphidius ervi Haliday (Hymenoptera: Braconidae). Aphids hosting a young larval parasitoid bore more mycetocytes of greater total biomass, and embryos of lower biomass than unparasitized aphids. Furthermore, one of the three aphid clones tested, which limited teratocyte growth (giant cells of parasitoid origin having a trophic role), bore smaller mycetocytes and larger embryos, than one or both of the two aphid clones with greater susceptibility to the parasitoid. These data suggest that susceptibility of the aphid‐Buchnera symbiosis to parasitoid‐mediated manipulation may, directly or indirectly, contribute to aphid susceptibility to parasitoid exploitation.     

The natural occurrence of secondary bacterial symbionts in aphids

1. Many insects host secondary bacterial symbionts that are known to have wide‐ranging effects on their hosts, from host‐plant use to resistance against natural enemies. This has been most widely studied in aphids, which have become a model system to study insect–bacteria interactions. 2. While there is an increasing understanding of the role of symbionts in aphids from controlled laboratory studies, we are only beginning to explore the impact of hosting these symbionts on eco‐evolutionary dynamics in natural systems. To date, many research groups have identified bacterial symbionts from various aphid species, providing us with a bank of literature on aphid–symbiont associations in natural populations. 3. The role of secondary symbionts in aphids is discussed, and the taxonomic and geographical distribution of symbionts among aphids are summarised, and the potential reasons for the patterns observed. The need to test for multiple symbiont species (and co‐infections) across many individuals and the whole distribution range of an aphid is highlighted, including sampling on all known host‐plant species. 4. It is further important also to consider variation within the symbiont, the aphid‐host and the surrounding community, e.g. host‐plants or the natural enemies, to understand how these have the potential to mediate aphid–symbiont interactions. 5. Finally, the knowledge gained from experimental work should now be used to understand the role of aphid secondary symbionts in field systems, to fully understand the potentially far‐reaching consequences of aphid endosymbionts on community and ecosystem processes.     

Host life stage- and temperature-dependent density of the symbiont Buchnera aphidicola in a subtropical pea aphid (Acyrthosiphon pisum) population

The function of the pea aphid's primary symbiont, Buchnera aphidicola, has been well studied. However, the factors affecting the dynamics of Buchnera density are seldom studied simultaneously. A better understanding of these factors could provide insights into its symbiosis with aphids. This study evaluated the effects of host life stage and rearing temperature on Buchnera density. We measured Buchnera density in seven life stages of pea aphids (Acyrthosiphon pisum) reared at six constant temperatures. Both host life stage and temperature significantly affected Buchnera density, which tended to decrease as aphid age and rearing temperature increased.