The homeobox gene Otx of the jellyfish Podocoryne carnea: Role of a head gene in striated muscle and evolution

In many bilaterian animals members of the Otx gene family are expressed in head or brain structures. Cnidarians, however, have no clearly homologous h ead and no distinct brain; but an Otx homolog from the jellyfish Podocoryne carnea is highly conserved in sequence and domain structure. Sequence simi larities extend well beyond the homeodomain and Podocoryne Otx can be align ed over its entire length to human OTX1, OTX2, and CRX. The overall structu re of Otx is better conserved from Podocoryne to deuterostomes while protos tomes appear to be more derived. In contrast, functions seem to be conserve d from protostomes to vertebrates but not in Podocoryne or echinoderms. Pod ocoryne Otx is expressed only during medusa bud formation and becomes restr icted to the striated muscle of medusae, Cnidaria are the most basal animal s with striated muscle. Podocoryne polyps have no striated muscle and no Ot x expression; both appear only during the asexual medusa budding process. T he common ancestor of all animals that gave rise to cnidarians, protostomes , and deuterostomes already had an Otx gene more similar to today's Podocor yne and human homologs than to Drosophila otd, while the head-specific func tion appears to have evolved only later, (C) 1999 Academic Press.