Insect facultative symbionts: immunological interactions, transgenerational effects and their role in host defence

Facultative symbionts are found in most insects, having a range of effects on their host’s biology. They often influence how these insects respond to a range of environmental stressors, including aiding in defence against natural enemies. Aphids are a model species for studying these endosymbionts and the work in this thesis uses the pea aphid to investigate some of the open questions about host-microbe interactions within this system and beyond.

Using both experimental and genomic techniques, we investigate the interactions between the host and their facultative symbionts, the role this plays in defence and the wider implications this may have for symbiont evolution. We find that facultative symbionts mediate transgenerational effects, leading to decreased susceptibility to a natural enemy across generations, likely due to an interaction between the host immune system and the bacterial symbiont. We also find that symbiont presence alters the immune response of the aphid, though this does not appear to have large impacts on the response to an invading pathogen. Using genomic techniques, we investigate the genomes of newly sequenced isolates of the two facultative symbionts used throughout this thesis. Here, we highlight the differences across and within symbiont species, outlining the variation in genome evolution and suggesting potential mechanisms for some of the phenotypes conferred by these symbionts.

This work highlights the dynamic relationship between the pea aphid host and their bacterial endosymbionts and how ecological pressures may play a role in shaping these interactions. Furthermore, it outlines how these relationships may in turn influence symbiont evolution. The work in this thesis demonstrates the multiple levels of complexity to these host-symbiont relationships and opens up new avenues for exploration within these diverse, complex communities.