Pax gene diversity in the basal cnidarian Acropora millepora (Cnidaria, Anthozoa): Implications for the evolution of the Pax gene family

Pax genes encode a family of transcription factors, many of which play key roles in animal embryonic development but whose evolutionary relationships and ancestral functions are unclear. To address these issues, we are charac terizing the Pax gene complement of the coral Acropora millepora, an anthoz oan cnidarian. As the simplest animals at the tissue level of organization, cnidarians occupy a key position in animal evolution, and the Anthozoa are the basal class within this diverse phylum. We have identified four Pax ge nes in Acropora: two (Pax-Aam and Pax-Pam) are orthologs of genes identifie d in other cnidarians; the others (Pax-Cam and Pax-Dam) are unique to Acrop ora. Pax-Aam may be orthologous with Drosophila Pax neuro. and Pax-Pam clea rly belongs to the Pax-2/5/8 class. The Pax-Pam Paired domain binds specifi cally and preferentially to Pax-2/5/8 binding sites. The recently identifie d Acropora gene Pax-Dam belongs to the Pax-3/7 class. Clearly, substantial diversification of the Pax family occurred before the Cnidaria/higher Metaz oa split. The fourth Acropora Pax gene, Pax-Cam, may correspond to the ance stral vertebrate Pax gene and most closely resembles Pax-C. The expression pattern of Pax-Cam, in putative neurons, is consistent with an ancestral ro le of the Pax family in neural differentiation and patterning. We have dete rmined the genomic structure of each Acropora Pax gene and show that some s plice sites are shared both between the coral genes and between these and P ax genes in triploblastic metazoans. Together, these data support the monop hyly of the Pax family and indicate ancient origins of several introns.