Isolation, expression and regulation of a zebrafish paraxis homologue

The formation of somites involves the subdivision of segmented presomitic mesoderm into segmentally arranged somite blocks. In mice and chicks, the basic-helix-loop-helix (bHLH) gene, paraxis, is involved in this process. Here, we report the isolation of a zebrafish homologue of paraxis, par1. par1 is expressed in presomitic paraxial mesoderm from late gastrula stages, and expression is maintained in ventrolateral cells after somite formation. In spt- embryos, par1 expression is both delayed and severely reduced whereas in flh- embryos, ectopic transcripts are detected in axial mesoderm. Spatial regulation of par1 expression within the somites is affected in several mutants with defects in axial midline tissues.