VALENTINO - A ZEBRAFISH GENE REQUIRED FOR NORMAL HINDBRAIN SEGMENTATION

Mutational analysis can serve both to identify new genes essential for patterning embryonic development and to determine their functions. He re we describe the identification and phenotypic characterization of a lleles of valentino, which we recovered in a genetic screen that sough t to identify mutations in the zebrafish that disrupt region-specific gene expression patterns in the embryonic brain. valentino is required for normal hindbrain segmentation and the hindbrain of valentino muta nt embryos is shortened by the length of one rhombomere. We demonstrat e that valentine is required cell-autonomously in the development of r hombomeres 5 and 6, and propose that valentine functions in the subdiv ision and expansion of a common precursor region in the presumptive hi ndbrain into the definitive rhombomeres 5 and 6. These results provide genetic evidence for a two-segment periodicity in the hindbrain and s uggest that this periodicity arises sequentially, through the specific ation and later subdivision of a two-rhombomere unit, or 'protosegment '.