The BMP signaling pathway is required together with the FGF pathway for notochord induction in the ascidian embryo

The 40 notochord cells of the ascidian tadpole invariably arise from two di fferent lineages: the primary (A-line) and the secondary (B-line) lineages. It has been shown that the primary notochord cells are induced by presumpt ive endoderm blastomeres between the 24-cell and the 64-cell stage. Signali ng through the fibroblast growth factor (FGF) pathway is required for this induction. We have investigated the role of the bone morphogenetic protein (BMP) pathway in ascidian notochord formation. HrBMPb (the ascidian BMP2/4 homologue) is expressed in the anterior endoderm at the 44-cell stage befor e the completion of notochord induction. The BMP antagonist Hrchordin is ex pressed in a complementary manner in all surrounding blastomeres and appear s to be a positive target of the BMP pathway. Unexpectedly, chordin overexp ression reduced formation of both primary and secondary notochord. Converse ly, primary notochord precursors isolated prior to induction formed notocho rd in presence of BMP-4 protein. While bFGF protein had a similar activity, notochord precursors showed a different time window of competence to respo nd to BMP-4 and bFGF. Our data are consistent with bFGF acting from the 24- cell stage, while BMP-4 acts during the 44-cell stage. However, active FGF signaling was also required for induction by BMP-4. In the secondary lineag e, notochord specification also required two inducing signals: an FGF signa l from anterior and posterior endoderm from the 24-cell stage and a BMP sig nal from anterior endoderm during the 44-cell stage.