Strain diversity and evolution in endosymbionts of Bathymodiolus mussels

Accumulating evidence shows that closely related strains of the same bacterial species can differ strongly in their functions. However, in intimate symbioses between bacteria and animal hosts, the impact of strain-level diversity is largely unexplored. Evolutionary theory predicts that mutualistic symbioses, should be destabilized by genetic diversity among closely-related symbiont strains. Yet, to date it is unknown if this prediction holds true in environmental symbioses. In this thesis I therefore aimed to deepen our current understanding of mutualistic symbioses, and the potential role that symbiont strain diversity and evolution may play in it. To do so, I performed high-resolution genomic analyses of the endosymbionts of Bathymodiolus mussels. My findings challenge and extent current evolutionary theories and point out the value of in-depth analysis of environmental bacterial communities to deepen our understanding of evolution, microbial interaction and symbiosis.