ZEBRAFISH SEGMENTATION AND PAIR-RULE PATTERNING

Segmentation in the vertebrate embryo is evident within the paraxial m esoderm in the form of somites, which are repeated structures that giv e rise io the vertebrae and muscle of the trunk and tail. In the zebra fish, our genetic screen identified two groups of mutants that affect somite formation and pattern. Mutations of one class, the fss-type mut ants, disrupt the formation of the anterior-posterior somite boundarie s during somitogenesis. However, segmentation within the paraxial meso derm is not completely eliminated in these mutants. Irregular somite b oundaries form later during embryogenesis and, strikingly, the vertebr ae are not fused. Here, we show that formation of the irregular somite boundaries in these mutants is dependent upon the activity of a secon d group of genes, the you-type genes, which include sonic you, the zeb rafish homologue of the Drosophila segment polarity gene, sonic hedgeh og. Further io characterize the defects caused by the fss-type mutatio ns, we examined their effects on the expression of herl, a zebrafish h omologue of the Drosophila pair-rule gene hairy. in wild-type embryos, herl is expressed in a dynamic, repealing pattern, remarkably similar to that of its Drosophila and Tribolium counterparts, suggesting that a pair-rule mechanism also functions in the segmentation of the verte brate paraxial mesoderm. We have found that the fss-type mutants have abnormal pair-rule patterning. Although a her I mutant could not be id entified, analysis of a double mutant that abolishes most her1 express ion suggests that a hell mutant may not display a pair-rule phenotype analogous to the hairy phenotype observed in Drosophila. Cumulatively, our data indicate that zebrafish homologues of both the Drosophila se gment polarity genes and pair-rule genes are involved in segmenting th e paraxial mesoderm. However, both the relationship between these two groups of genes with in the genetic heirarchy governing segmentation a nd the precise roles that they play during segmentation likely differ significantly between the two organisms. (C) 1998 Wiley-Liss, Inc.