EXPRESSION OF A TYPE-II COLLAGEN GENE IN THE ZEBRAFISH EMBRYONIC AXIS

To understand the hierarchy of developmental controls underlying axis specification in vertebrate embryos, it is helpful to identify relatio nships between regulatory molecules and the genes that give axial cell s their differentiated phenotypes. This work reports the cloning and e xpression pattern of one of these differentiation genes, a type II col lagen (col2a1) gene from the zebrafish Danio rerio. Along the embryoni c axis, col2a1 is expressed dynamically in three rows that are each a single cell wide: the notochord and the rows of cells immediately dors al and ventral to it-the floor plate of the central nervous system, an d the hypochord. In addition, col2a1 is expressed in the pharyngeal. a rches, the epithelium of the otic capsule, and in the mesenchyme of th e neurocranium. Experiments probed the expression pattern of col2a1 re lative to that of known or potential regulators of axis development, i ncluding axial, sonic hedgehog, twist, and cyclops. The results showed that the spatial and temporal pattern of col2a1 expression in axial m esoderm follows the expression of twist closer than other genes tested . In cyclops embryos, which lack an intact floor plate, col2a1 express ion was usually low, but not missing in cells in the ventral spinal co rd. Because col2a1 expression reveals abnormalities in the notochord o f cyclops(b16) embryos, and less col2a1-expressing mesenchyme accumula tes rostral to the notochord in cyclops embryos, the effects of the cy clops(b16) mutation are not confined to the central nervous system. (C ) 1995 Wiley-Liss, Inc.