Zebrafish Nodal-Related Genes Are Implicated in Axial Patterning and Establishing Left–Right Asymmetry

AbstractNodal-related 1 (ndr1)andnodal-related 2 (ndr2)genes in zebrafish encode members of the nodal subgroup of the transforming growth factor-β superfamily. We report the expression patterns and functional characteristics of these factors, implicating them in the establishment of dorsal–ventral polarity and left–right asymmetry.Ndr1is expressed maternally, andndr1andndr2are expressed during blastula stage in the blastoderm margin. During gastrulation,ndrexpression subdivides the shield into two domains: a small group of noninvoluting cells, the dorsal forerunner cells, expressndr1,whilendr2RNA is found in the hypoblast layer of the shield and later in notochord, prechordal plate, and overlying anterior neurectoderm. During somitogenesis,ndr2is expressed asymmetrically in the lateral plate as are nodal-related genes of other organisms, and in a small domain in the left diencephalon, providing the first observation of asymmetric gene expression in the embryonic forebrain. RNA injections intoXenopusanimal caps showed that Ndr1 acts as a mesoderm inducer, whereas Ndr2 is an efficient neural but very inefficient mesoderm inducer. We suggest that Ndr1 has a role in mesoderm induction, while Ndr2 is involved in subsequent specification and patterning of the nervous system and establishment of laterality