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Microbial genetics

Andrew Roger
Dalhousie

The roles of gene transfer and symbiosis in the evolutionary adaptation of protists to low oxygen environments

Andrew Roger
Andrew Roger

Eukaryotic cells first arose 1.5-2 billion years ago and ultimately diversified into conspicuous complex life forms such as animals, plants, and fungi as well as the spectacularly diverse array of unicellular forms known as the protists. One of the key steps in the evolution eukaryotic cells was the origin of mitochondria – aerobic energy-generating organelles known as the ‘powerhouses’ of our cells. In this presentation, I will chronicle the evolutionary history of mitochondria from their endosymbiotic origin and role in eukaryogenesis to their diversification in function across the eukaryotic tree of life. I will focus especially on the secondary adaptation of protists to low oxygen environments, and the mechanisms by which their mitochondria have been evolutionarily ‘retailored’ to function anaerobically. Evolutionary adaptations of protists to low oxygen environments have frequently involved syntrophic interactions with prokaryotes and some of these interactions have evolved into sustained complex symbiotic associations. As an example, I will showcase a remarkable symbiosis between the recently discovered anaerobic Anaeramoebae and sulfate-reducing Desulfobacteraceae. The latter thrive within a ‘symbiosomal’ membrane network within the Anaeramoebae that positions them close to hydrogen-producing mitochondria to facilitate metabolite exchange. By combining whole genome analysis, in silico metabolic predictions, focused-ion-beam scanning electron microscopy and phylogenomic analyses, we reconstruct the metabolic interactions between host and symbiont and the structure of the symbiosome, as well as shed light on the evolutionary events that led to this unique symbiosis. I discuss the implications of these findings for our understanding of the evolutionary mechanisms generating cellular diversity within eukaryotes.