The Closest Relatives of Vertebrates Evolved Through Strong Genomic Turbulence

The tunicate Oikopleura dioica is a new chordate model organism with a short life cycle, high fecundity, and a key ecological role in ocean plankton. Early phylogenomic studies show that tunicates, not cephalochordates, are the closest living relatives to vertebrates. This implies that tunicate anatomical simplicity results from secondary simplification, not ancestral primitiveness. Since the split from vertebrate ancestors, the genome of Oikopleura (and related species) has considerably diverged from those of vertebrates, with an unusual level of chromosome rearrangements and the passage of numerous genes into operons. Our functional studies showed that universal pathways for DNA repair (NHEJ for double strand breaks) were lost. Species of the same group also break the dogma that splicing requires GT-AG intron ends. To explain such massive evolutionary changes which occured over hundreds of million years, it could be an idea to investigate how our such species evolve nowadays. I will show the results of current attempts to measure main genetic parameters, including mutation and recombination rates, and finally show how metagenomic studies permit to extend such studies to a worldwide dimension.