Expression of three spalt (sal) gene homologues in zebrafish embryos

Three homologues of the Drosophila region-specific homeotic gene spalt (sal) have been isolated in zebrafish, sall1a, sall1b and sall3. Phylogenetic analysis of these genes against known sal DNA sequences showed zebrafish sall1a and sall1b to be orthologous to other vertebrate sal-1 genes and zebrafish sall3 to be orthologous to other vertebrate sal-3 genes, except Xenopus sall3. Phylogenetic reconstruction suggests that zebrafish sall1a and sall1b resulted from a gene duplication event occurring prior to the divergence of the ray-finned and lobe-finned fish lineages. Analysis of the expression pattern of the zebrafish sal genes shows that sall1a and sall3 share expression domains with both orthologous and non-orthologous vertebrate sal genes. Both are expressed in various regions of the CNS, including in primary motor neurons. Outside of the CNS, sall1a expression is observed in the otic vesicle (ear), heart and in a discrete region of the pronephric ducts. These analyses indicate that orthologies between zebrafish sal genes and other vertebrate sal genes do not imply equivalence of expression pattern and, therefore, that biological functions are not entirely conserved. However we suggest that, like other vertebrate sal genes, zebrafish sal genes have a role in neural development. Also, expression of zebrafish sall1a in the otic vesicle, heart sac and the pronephric ducts of zebrafish embryos is possibly consistent with some of the abnormalities seen in Sall1-deficient mice and in Townes-Brocks Syndrome, a human disorder which is caused by mutations in the human spalt gene SALL1.