Insect facultative endosymbionts: phenotypic effects and competitive interactions

Facultative endosymbionts are ubiquitous in insect populations, and can affect a wide range of ecological and life-history traits. Symbiont infections, therefore, have the potential to affect insect responses to natural enemies and climate warming. Aphids are a model for endosymbiont research, and the work in this thesis uses pea aphids to explore symbiont effects and interactions. We manipulate symbiont infections in clonal genotypes of aphids to investigate the phenotypic effects of microbial infection and expose the insects to a range of stress and fecundity tests. We find that a single species of symbiont (known as X-type) can provide multiple ecological benefits to a host, but that there is a fitness cost to infection. We also discover that symbiont-mediated protection to heat is caused by two species of facultative symbiont protecting the obligate symbiont. Although many symbiont communities involve multiple species, much previous research has focused on individual infections. We create superinfections of symbionts to explore interactions between the microbes, and how these may affect host effects. We find that infections of two closely related symbionts can lead to loss of superinfections and that the microbes have different responses to competition. Our work suggests a dynamic, diverse and complex pool of symbiont effects and interactions, and that the symbiont-mediated effects can depend strongly on host and symbiont genotype. As a result, benefits caused by facultative symbionts may vary depending on host population, and determine how vulnerable insect communities are to disturbance and natural enemies. Loss of symbiont infection can also correspond to trait loss in aphid populations. Our work highlights that aphids are an ideal system for studying insect symbiosis and a simple model for more complex free-living or symbiotic microbial communities