| Summary: | <div style="margin-top:10px; clear:both;"><ol><li>Insects are often infected with facultative endosymbiotic bacteria, which can have a range of important ecological effects. The grain aphid, <em>Sitobion avenae</em>, harbours diverse facultative symbionts, which suggests their importance in grain aphid biology.</li><li>This thesis attempts to explain the ecological roles of the facultative endosymbionts in <em>S. avenae</em>. It also examines the question of whether the horizontal transmission of symbionts between aphid clones and species can be important for shaping the ecology and evolution of multi-species aphid communities. Novel techniques developed for research with the grain aphid study system are presented.</li><li>Grain aphid clones vary in their tolerance to low temperatures, but this trait is not affected by their facultative endosymbionts.</li><li>Strains of a symbiont <em>Hamiltonella defensa</em> do not protect grain aphids from hymenopterous parasitoids, regardless of the host genotype. However, experienced parasitoid females preferentially oviposit in aphids which do not harbour symbionts.</li><li>Comparison of the fitness consequences of infection with the same <em>Hamiltonella</em> strains in their original and in novel grain aphid host clones reveal no consistent differences.</li><li>Symbiont strains establish easily following artificial transfer between clones of the grain aphid, but the symbionts transferred from other aphid species form less stable infections. <em>Hamiltonella</em> strains do not affect the fecundity of their grain aphid host clones regardless of their host species of origin, but also do not generally confer protection against parasitoids.</li><li>There are no clear patterns in the distribution of parasitoid-resistant phenotypes across phylogenetic trees of <em>Hamiltonella</em> and its bacteriophage APSE.</li><li>Strains of four unrelated species of endosymbionts, <em>Rickettsia</em>, <em>Spiroplasma</em>, <em>Rickettsiella</em> and <em>Regiella</em>, confer the same pathogen-resistant phenotype to a single pea aphid clone. The same symbiont strains can confer resistance to clones of two different aphid species. Some strains in multiple infections may compensate for the costs of infections with other symbionts.</li><li>The importance of these results for understanding the ecological and evolutionary role of facultative endosymbionts in aphids and other insects are discussed, and directions for further research are proposed.</li></ol></div>
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