06 JANVIER 2017 - 13H

LUCA, the universal tree of life, and the origin of eukaryotes - Séminaire externe IBPS

Patrick Forterre, Institut Pasteur, Paris.

Lieu : Amphithéâtre Charpak, LPNHE | Campus Jussieu

Résumé :

The nature of LUCA, the topology of the universal tree of life, and the origin of eukaryotes are controversial issues. Recently, several authors have published molecular phylogenetic analyses suggesting that eukaryotes correspond to an archaeal branch (so we are ourselves archaea) and that early branching archaea and bacteria were nano-sized organisms. I will argue that these publications, that have received huge publicity, do not reflect correctly the history of life, but testify for the overconfidence of their authors in (i) the possibility for up to date phylogenetic methods to take into account long branch attraction artefacts, and (ii) the reliability of genome reconstruction from metagenomic data. We (Violette Da Cunha, Morgan Gaia and myself) have started to revisit all universal proteins using carefully prepared datasets for phylogenetic analyses and systematically search for informative indels (insertion/deletions) in protein alignments before trimming. Our first results with RNA polymerases suggest that Archaea and Eukarya are sister groups, as in the Woese tree, forming a clade that we propose naming Arkarya. Our preliminary indel analyses confirm that nano-size archaea and bacteria are extremely fast evolving organisms that probably branch early in universal trees because of long-branch attraction and suggest their affinity with previously described phyla. Indels also allow the detection of contaminations in metagenomic datasets. For instance, we identify eukaryotic sequences in the dataset used to locate the Lokiarchaeon in the tree of life. Our reanalysis of this dataset indicates that the Lokiarchaea are not the ancestors of Eukaryotes but early branching Euryarchaeota. I will also briefly discuss the nature of LUCA based on comparative biochemistry. It suggests that the “prokaryotic” phenotype of Archaea and Bacteria is a result of convergent evolution.

Patrick Forterre is Professor at the Institut Pasteur and at the University Paris-Saclay. During his career, he set up and supervised two research groups working on Archaea and their viruses, one in Orsay and the other at the Institut Pasteur in Paris. Patrick Forterre’s teams have been working mostly on the molecular biology and physiology of hyperthermophilic archaea. They have described several new families of enzymes involved in DNA replication and DNA topology control. Patrick Forterre has always been interested in the evolutionary implication of his work. He has proposed several provocative and controversial hypotheses on the origin of DNA and on the nature of viruses. From 2003 to 2010, he was director of the microbiology department at the Institut Pasteur and he is now member of the European Academy of Microbiology.