Molecular studies of Six class homeobox genes involved in zebrafish embryogenesis

The high degree of both structural and functional conservation between insect and mammalian homeobox genes has suggested common mechanisms guiding developmental processes in different species. One of these processes with profound interests is the molecular cascade leading to eye formation. At the initiation of this study, the vertebrate Pax6 gene, a homologue of the Drosophila gene eyeless, was considered a 'master control gene' of this regulatory network. These assumptions was partly based on the eye deficiencies seen in mice (Small eye) and humans (Aniridia) carrying mutant alleles of Pax6, and the fact that targeted expression of either Pax6 or eyeless were able to induce ectopic eyes in Drosophila. Pax6 was also found to be expressed in the eye primordia in several other vertebrate species including Xenopus, quail and zebrafish . However, additional areas of Pax6 expression exist outside the retinal fields, suggesting that other genes are involved in setting up the borders of the eye primordia. Hence, further analysis of vertebrate homologues of Drosophila eye regulatory genes seemed necessary to gain further information about this complex process. One obvious group of genes to study was homologues of the sine oculis gene known to be essential for proper development of the visual system in Drosophila. Three vertebrate members of this family, Six1, Six7 and Six3 had just been isolated in mouse, and the Six3 gene showed expression during murine eye development. Based on these studies, we initiated a screen for Six gene homologues in zebrafish in order to analyze their expression during zebrafish embryogenesis and elucidate phylogenetic and functional conservation of this gene family during evolution.