LUCA and the Universal tree of life: breaking the phylogenetic impasse

Speaker: Professor Patrick Forterre, Pasteur Institute, France
Host: Associate Professor Xu Peng, Section for Functional Genomics

Abstract
Phylogenetic analyses of ancient evolutionary divergences are a priori difficult and risky tasks. I previously argued (with others) that phylogenetic methods cannot be used to root the tree of life or else to determine if Archaea and Bacteria are sister groups (the classical Woese’s tree) or if Eukarya are a subgroup of Archaea (the eocyte tree). Now, I think that the accumulation of complex molecular features shared by Eukarya and Archaea strongly favours the classical rooting of the universal tree between Archaea/Eukarya and Bacteria, suggesting that LUCA was much simpler than modern cells (but more complex than the Woese’s progenote). If correct, this reveals that the modern “prokaryotic” phenotype of Archaea and Bacteria resulted from convergent evolution. Recently, revisiting the phylogenies of universal proteins used to position Lokiarchaea in the tree of life, we (Violette Da Cunha, Morgan Gaia and myself) obtained results suggesting that proper phylogenetic analyses can in fact break the phylogenetic impasse in the case of the origin of Eukarya. Our analyses support the monophyly of Archaea (Woese’s tree) and place Lokiarchaea and Thorarchaea (sister groups) at the base of Euryarchaeota. Notably, we obtained evidences indicating that the branching of Eukarya within Lokiarchaea previously published was due to contamination in the original dataset. Finally, I will also discuss the position of large DNA viruses in our updated tree of life.