INDUCTION OF A SPECIFIC MUSCLE-CELL TYPE BY A HEDGEHOG-LIKE PROTEIN IN ZEBRAFISH

THE notochord plays a central role in vertebrate development, acting a s a signalling source that patterns the neural tube and somites(1-4). In in vitro assays, the secreted protein Sonic hedgehog mimics the ind ucing effects of notochord on both presomitic mesoderm and neural plat e explants of amniote embryos, suggesting that both patterning activit ies of the notochord may be mediated by this protein in vivo(5-8). In zebrafish, however, mutants with disrupted notochord development lack a specific muscle cell type, the muscle pioneers, although they retain tile ability to induce neural differentiation, raising the possibilit y that neural tube and somite patterning may be mediated by distinct s ignals(9,10). Here we describe a new member of the hedgehog family, ec hidna hedgehog that is expressed exclusively in the notochord and has the ability to rescue the differentiation of muscle pioneer cells in m utants with no notochord, Moreover, we show that a combination of ecto pic echidna hedgehog and sonic hedgehog expression induces supernumary muscle pioneers in wild-type embryos, suggesting that both signals ac t sequentially to pattern the developing somites.