ZEBRAFISH NODAL-RELATED GENES ARE IMPLICATED IN AXIAL PATTERNING AND ESTABLISHING LEFT-RIGHT ASYMMETRY

Nodal-related 1 (ndr1) and nodal-related 2 (ndr2) genes in zebrafish e ncode members of the nodal subgroup of the transforming growth factor- beta superfamily. We report the expression patterns and functional cha racteristics of these factors, implicating them in the establishment o f dorsal-ventral polarity and left-right asymmetry. Ndr1 is expressed maternally, and ndr2 and ndr2 are expressed during blastula stage in t he blastoderm margin. During gastrulation, ndr expression subdivides t he shield into two domains: a small group of noninvoluting cells, the dorsal forerunner cells, express ndr1, while ndr2 RNA is found in the hypoblast layer of the shield and later in notochord, prechordal plate , and overlying anterior neurectoderm. During somitogenesis, ndr2 is e xpressed asymmetrically in the lateral plate as are nodal-related gene s of other organisms, and in a small domain in the left diencephalon, providing the first observation of asymmetric gene expression in the e mbryonic forebrain. RNA injections into Xenopus animal caps showed tha t Ndr1 acts as a mesoderm inducer, whereas Ndr2 is an efficient neural but very inefficient mesoderm inducer. We suggest that Ndr1 has a rol e in mesoderm induction, while Ndr2 is involved in subsequent specific ation and patterning of the nervous system and establishment of latera lity. (C) 1998 Academic Press.