The caspase family in urochordates: distinct evolutionary fates in ascidians and larvaceans

BACKGROUND INFORMATION: Caspases are cysteine proteases that mediate apoptosis (programmed cell death) initiation and execution. Apoptosis is a conserved mechanism shared by all metazoans, although its physiological function and complexity show considerable taxon-dependent variations. To gain insight into the caspase repertoire of putative ancestors to vertebrates, we performed exhaustive genomic searches in urochordates, a sister taxon to vertebrates in which ascidians and appendicularians display chordate characters at early stages of their development. RESULTS: We identified the complete caspase families of two ascidians (Ciona intestinalis and C. savignyi) and one larvacean (Oikopleura dioica). We found in ascidian species an extremely high number of caspase genes (17 for C. intestinalis and 22 for C. savignyi), deriving from five founder gene orthologues to human pro-inflammatory, initiator and executioner caspases. Although considered to be sibling species, C. intestinalis and C. savignyi only share 11 orthologues, most of the additional genes resulting from recent mass duplications. A sharply contrasted picture was found in O. dioica, which displayed only three caspase genes deriving from a single founder gene distantly related to caspase 3/7. The difference between ascidian and larvacean caspase repertoires is discussed in the light of their developmental patterns and life cycles. CONCLUSIONS: The identification of caspase members in two ascidian species delineates five founder genes that bridge the gap between vertebrates and Ecdysozoa (arthropods and nematodes). Given the amazing diversity among urochordates, determination and comparison of the caspase repertoires in species from orders additional to Enterogona (ascidians) and Oikopleuridae might be highly informative on the evolution of caspase-dependent physiological processes.