Peculiar patterns of amino acid substitution and conservation in the fast evolving tunicate Oikopleura dioica

► Oikopleura is on average twice as fast as the already fast evolving Ciona intestinalis. ► This high amino acid evolutionary rate encompasses the whole genome. ► Genes encoding regulatory proteins are subjected to an even more extreme acceleration. ► Relaxation of functional constraints appears to be an important cause of this acceleration. ► Contrary to the general rule, cysteine is poorly conserved in Oikopleura dioica.