Early diversification of the T-box gene family in the poriferan lineage

The T-box family of transcription factors is involved in a variety of important developmental processes in all members of the Bilateria and Cnidarians. These genes play vital roles in specification of morphological structures derived from all three germ layers. T-box genes appear to be restricted to the Metazoa given that they have not been reported in plants, fungi or protists. However, T-box family members have been identified in all of the extant basal metazoans including sponges, cnidarians, ctenophores, and placozoans. Our analysis of the T-box constituency in poriferan genomes supports a recent hypothesis that the common ancestor of the Eumetazoa already had a complex complement of T-box genes. Previous studies in sponges have reported at most two T- box genes present in a given sponge genome (including isoforms of one Brachyury class gene). Our surveys of the Amphimedon queenslandica genome as well as PCR-based screening of two other demosponge genomes (Halichondria bowerbanki and Ephydatia muelleri) indicate that there are at least four different T-box family members present in some sponge genomes. Several of these gene families appear to be unique to the Porifera and a duplication event may have occurred exclusively within the poriferan lineage for one Tbx family. In situ hybridization analysis in Halichondria larvae for three of the T- box genes indicates distinct patterns of expression that may suggest possible roles in development