Fig. 2.

Phylogeny of the species whose sequences are studied in this review. The main groups displayed are the chordates, which comprise all species considered except Drosophila and Caenorhabditis, and display a notochord, at least at some point during their embryonic development. Vertebrates are species displaying vertebra and comprise all considered chordate species, with the exception of Ciona and Amphioxus. Finally, the tetrapods comprise all our vertebrate species, with the exception of fishes, either cartilage fishes (elephant shark, Callorhinchus Milii), bony fishes (zebrafish, Danio rerio), coelacants, or lungfishes