AN ASCIDIAN HOMOLOG OF VERTEBRATE BMPS-5-8 IS EXPRESSED IN THE MIDLINE OF THE ANTERIOR NEUROECTODERM AND IN THE MIDLINE OF THE VENTRAL EPIDERMIS OF THE EMBRYO

The ascidian tadpole larva is thought to be the prototype for the ance stral chordate. Although ascidians show a highly determinate mode of d evelopment, recent studies suggest significant roles of cell-cell inte raction during embryogenesis. To elucidate the signaling molecules res ponsible for the cellular interaction, we investigated an ascidian hom ologue of the transforming growth factor beta (TGF-beta) superfamily. HrBMPa is an ascidian member of the 60A subclass of the BMP subfamily Molecular phylogenetic analysis suggested that HrBMPa branched prior t o further divergence of vertebrate BMPs-5-8. The zygotic expression of HrBMPa was initiated around gastrulation. HrBMPa transcripts were fir st evident in precursor cells of the spinal cord, notochord, epidermis and nervous system, although signals in the first two regions quickly disappeared. In neurulae and early tailbud embryos, transcripts were evident in the adhesive organ, midline of the anterior dorsal neuroect oderm and midline of both ventral and dorsal ectoderm, suggesting that HrBMPa plays a major role in neuroectodermal cell differentiation dur ing embryogenesis. This HrBMPa expression profile resembled that of Xe nopus BMP-7, implying a primordial function of BMP-7 among vertebrate BMPs-5-8.