LOSS OF CEREBUM FUNCTION VENTRALIZES THE ZEBRAFISH EMBRYO

Recent studies implicate ventrally derived signals, in addition to dor sal ones emanating from the organizer, in patterning the vertebrate ga strula, We have identified five overlapping deficiencies that uncover the zebrafish cerebum locus and dramatically alter dorsal-ventral pola rity at gastrulation. Consistent with the properties of experimentally ventralized amphibian embryos, cerebum mutants exhibit reduced neurec todermal gene expression domains and an increase in derivatives of ven tral mesoderm, Structures derived from paraxial and lateral mesoderm a lso are reduced; however, dorsal axial mesodermal derivatives, such as the hatching gland and notochord, are largely spared. The pleiotropic action of cerebrum deficiencies, and the differential response of aff ected tissues, suggest that the cerebum gene may normally function as an inhibitor of ventralizing signals, a function previously ascribed t o Noggin and Chordin in Xenopus. Analysis of the cerebum phenotype pro vides genetic evidence for the existence of ventralizing signals in th e zebrafish gastrula and for antagonists of those signals.