FUNCTIONAL-ANALYSIS OF AN ASCIDIAN HOMOLOG OF VERTEBRATE BMP-2 BMP-4 SUGGESTS ITS ROLE IN THE INHIBITION OF NEURAL FATE SPECIFICATION/

The ascidian tadpole larva is thought to be close to a prototype of th e ancestral chordate. The vertebrate body plan is established by a ser ies of inductive cellular interactions, whereas ascidians show a highl y determinate mode of development, Recent studies however, suggest som e roles of cell-cell interaction during ascidian embryogenesis. To elu cidate the signaling molecules responsible for the cellular interactio n, we isolated HrBMPb, an ascidian homologue of the vertebrate bone mo rphogenetic protein (BMP) gene, from Halocynthia roretzi, The amino ac id sequence of HrBMPb closely resembled those of vertebrate BMP-2 and BMP-4 and of Drosophila Decapentaplegic (DPP). In addition to the sequ ence similarity, HrBMPb overexpression induced the ventralization of X enopus embryos, suggesting functional conservation. The zygotic expres sion of HrBMPb was first detected around gastrulation. HrBMPb expressi on was maintained in some cells at the lateral edges of the neural pla te through gastrulation to neurulation, although that in the presumpti ve muscle cells was downregulated. HrBMPb was not expressed in the pre sumptive epidermis during gastrulation. When HrBMPb mRNA was injected into fertilized Halocynthia eggs, cells that normally give rise to th e neural tissue differentiated into epidermis, causing a loss of anter ior neural tissue in the larva. In addition, HrBMPb might function syn ergistically with HrBMPa, an ascidian homologue of BMPs-5 to 8. Howeve r, HrBMPb overexpression did not affect differentiation of the notocho rd and muscle cells. These results suggest that HrBMPb functions as a neural inhibitor and as an epidermal inducer but not as a ventralizing agent in ascidian development.