Host adaptation and genome dynamics during experimental evolution of chlamydial symbionts

Paul Herrera
2020 unpublished
The Chlamydiae are an ancient group of obligate intracellular bacteria that reside within eukaryotic cells and display a unique biphasic developmental cycle. They are of global significance because of their success as major human pathogens as well as causing numerous diseases in a variety of animals. But over the past few decades we have discovered an enormous diversity of Chlamydiae that are associated with an impressive range of eukaryotic hosts. The majority of these so-called environmental
more » ... hlamydiae are considered to be symbionts of unicellular eukaryotes—free-living amoebae or other protists—that are found ubiquitously in the environment. The main aim of my research was to improve our understanding of the evolution of these unique host-dependent bacteria living in amoebae. I combined experimental evolution approaches, infectivity assays, genome re-sequencing, and global gene expression analysis to achieve this goal. The first study involved the experimental manipulation of the transmission of symbionts among host generations to favour horizontal or vertical transmission using Parachlamydia acanthamoebae. We revealed that increased parasitism in the horizontally transmitted symbionts was driven by a dramatic shift in the frequency of standing genetic variants coupled with an extensive modification of gene expression. In particular, we observed an upregulation of genes affecting functions crucial at the infectious stage of the symbiont and required for extracellular survival. We then used Protochlamydia amoebophila for the rest of our work as a model to gain initial insight into the process of temperature adaptation in these symbionts, a crucial first step in the evolution of the major chlamydial pathogens. We showed evidence for positive selection at the elevated temperature and observed an attenuation in symbiont infectivity, very likely so as to reduce host cell burden. In addition, shallow expression dynamics throughout the infection cycle may have contributed to this reduced infectivity. Taken together, [...]
doi:10.25365/thesis.64641 fatcat:kadehmq2dng2hlrlphyevwu5je