| Summary: | <p>Achieving a better comprehension of the evolution of species has always been an important matter for evolutionary biologists. The deuterostome phylogeny has been described for many years, and three phyla are distinguishable: Echinodermata (including sea stars, sea urchins, etc…), Hemichordata (including acorn worms and pterobranchs), and Chordata (including urochordates, cephalochordates and extant vertebrates). Inside the Chordata phylum, the position of vertebrate species is quite unanimously accepted. Nonetheless, the position of urochordates in regard with vertebrates is still the subject of debate, and has even been suggested by some authors to be a separate phylum from cephalochordates and vertebrates. It was also the case for agnathans species –myxines and hagfish– for which phylogenetic evidence was recently given for a controversial monophyly. This raises the following question: which one of the cephalochordata or urochordata is the sister group of vertebrates and what are their relationships? In the present work, we analyzed 82 protein families presenting homologs between urochordata and other deuterostomes and focused on two points: 1) testing accurately the position of urochordata and cephalochordata phyla in regard with vertebrates as well as chordates monophyly, 2) performing an estimation of the rate of gene loss in the <italic>Ciona intestinalis</italic> genome. We showed that the urochordate phyla is the vertebrate sister group and that gene loss played a major role in structuring the urochordate genome.</p>
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