Nodal-related signals establish mesendodermal fate and trunk neural identity in zebrafish

The vertebrate body plan arises during gastrulation, when morphogenetic mov ements form the ectoderm, mesoderm, and endoderm. In zebrafish, mesoderm an d endoderm derive from the marginal region of the late blastula, and cells located nearer the animal pole form the ectoderm [1]. Analysis in mouse, Xe nopus, and zebrafish has demonstrated that Nodal-related proteins, a subcla ss of the TGF-beta superfamily, are essential for mesendoderm development [ 2], but previous mutational studies have not established whether Nodal-rela ted signals control fate specification, morphogenetic movements, or surviva l of mesendodermal precursors. Here, we report that Nodal-related signals a re required to allocate marginal cells to mesendodermal fates in the zebraf ish embryo. In double mutants for the zebrafish nodal related genes squint (sqt) and cyclops (cyc) [3-5], dorsal marginal cells adopt neural fates, wh ereas in wild type embryos, cells at this position form endoderm and axial mesoderm. Involution movements characteristic of developing mesendoderm are also blocked in the absence of Nodal signaling. Because it has been propos ed [6] that inhibition of Nodal related signals promotes the development of anterior neural fates, we also examined anteroposterior organization of th e neural tube in sqt;cyc mutants. Anterior trunk spinal cord is absent in s qt;cyc mutants, despite the presence of more anterior and posterior neural fates. These results demonstrate that nodal-related genes are required for the allocation of dorsal marginal cells to mesendodermal fates and for ante roposterior patterning of the neural tube.