From: Przytycka, Teresa (NIH/NLM/NCBI) [E]
Sent: Wednesday, April 08, 2009 6:20 PM
To: 'ncbi-seminar@ncbi.nlm.nih.gov'
Subject: April 14 NCBI semimar by Natalya Yutin 

April 14, 11:00, B2 library
Speaker Natalya Yutin

Title: The Archaeal Roots of Eukaryotes

The emergence of eukaryotes is one of the central themes in evolutionary biology. It is universally accepted that mitochondria and related organelles have evolved via endosymbiosis, most likely, a single endosymbiotic event that involved an alpha-proteobacterium, the apparent ancestor of the mitochondria, and an archaeon. However, the place of the mitochondrial endosymbiosis in the course of eukaryogenesis and the nature of the host of the alpha-proteobacterial endosymbiont remain hotly debated matters. We describe a comprehensive set of 355 eukaryotic genes of apparent archaeal origin identified through ortholog detection and phylogenetic analysis. Phylogenetic hypothesis testing using constrained trees indicate that, for the majority of these genes, the preferred tree topology is one with the eukaryotic branch placed outside the extant diversity of archaea although small subsets of genes show crenarchaeal and euryarchaeal affinities. Thus, the “archaeal” genes in eukaryotes appear to descend from a distinct, ancient, and otherwise uncharacterized archaeal lineage that acquired some euryarchaeal and crenarchaeal genes via early horizontal gene transfer.
Symbiotic theories of eukaryogenesis face the potentially serious difficulty that prokaryotes have no known mechanisms for engulfing other prokaryotic cells. Thus, the origin of phagocytosis appears to be one of the key aspects of eukaryogenesis, and reconstruction of the evolution of the phagocytic function could substantially inform our thinking on the origin of the eukaryotic cell. Analyzing sets of proteins involved in phagocytosis in different eukaryotes, we suggested that the modern-type, advanced phagocytosis evolved independently and relatively late in the course of evolution of several major eukaryotic lineages. We investigated the phylogenies of two key components of phagocytosis, actin and Rac-family GTPases. The present findings suggest a hypothetical scenario of eukaryogenesis, under which, a primitive process of particle engulfment by actin-encoding archaea might antedate eukaryogenesis whereas the full-fledged phagocytosis was a late development that occurred independently in several major branches of eukaryotes.