A two-billion-years evolutionary history of infections: host-adaptation in the early Gammaproteobacteria

The ability to feed on other organisms through phagocytosis is a crucial step in the evolution of eukaryotes, a prerequisite for food webs, multicellularity and, arguably, the acquisition of mitochondria.

It also paved the way for intracellular parasitism, as some bacteria avoided digestion by their hosts and multiplied in the nutrient-rich cytoplasm.Here, we focus on the Deep-branching Intracellular Gammaproteobacteria (DIG), a large group of related bacteria (e.g. Legionella and Francisella) with a wide variety of host-adaptation strategies.

We hypothesize that the last common ancestor of DIG, appeared during eukaryogenesis, 2 Ga ago, and was among the first bacteria to infect eukaryotes.To test this hypothesis, we will gather a solid set of DIG genomes, reconstruct their evolutionary history, assess their metabolic potential, and identify their host-adaptation genes.

We will screen genomes for horizontal gene transfers from early eukaryotes or Asgard archaea.

We will also investigate the potential for transitions from parasitic to mutualistic relationships in DIG, using comparative genomics.Through a better understanding of the relationships between the first eukaryotes and their early invaders, we will shed light on eukaryogenesis, particularly the role of phagocytosis.

By studying subsequent transitions towards mutualism in DIG, we will better understand host-adaptation processes that led, among others, to the mitochondrion.