A zebrafish Six4 homologue with early expression in head mesoderm

Similar to the Drosophila homeobox gene sine oculis, several of the vertebrate homologues (Six genes) are expressed during eye formation and differentiation. In addition, most of these vertebrate genes show expression in mesodermal derivatives in adults and/or earlier stages of development. We have identified a zebrafish (Danio rerio) gene, six8, which shows the greatest similarity to murine Six4. The deduced proteins of these two genes have an overall sequence identity of 41%, while the homeodomains and Six domains are highly conserved, 90% and 81%, respectively. The spatiotemporal expression pattern of six8 was analyzed by RT-PCR and in situ hybridization. Transcripts were detected in a wide range of embryonic stages and in adults. Notably, the strongest expression was observed in head mesoderm of late gastrula and early neurula stages.