Sequence and expression pattern ofpax-6are highly conserved between zebrafish and mice

Despite obvious differences in the patterns of early embryonic development, vertebrates share a number of developmental mechanisms and control genes, suggesting that they use similar genetic programs at some stages of development. To examine this idea, we isolated and characterized one such gene, pax-6, a member of the pax gene family, from the zebrafish Brachydanio rerio and determined the evolutionary conservation in the structure and expression of this gene by comparison to its homolog in mice. We found two alternatively spliced forms of the zebrafish pax-6 message. Sequence and expression pattern of the zebrafish pax-6 gene are remarkably similar to its murine homolog. pax-6 expression begins during early neurulation. A stripe of cells in the neuroectoderm, including the prospective diencephalon and a part of the telencephalon, expresses pax-6 as well as the hindbrain and the ventral spinal cord extending from the level of the first rhombomere to the posterior end of the CNS. During later development more limited regions of the brain including the eye, the olfactory bulb and the pituitary gland express pax-6. Cells at the midbrain-hindbrain junction express eng genes and are separated from the neighboring pax-6 regions by several cells that express neither gene, indicating a complex subdivision of this region. pax-6 expression appears during processes when cell-to-cell signalling is thought to be important, for example during induction of the eye and regionalization of the spinal cord and brain, suggesting that it may be one component mediating the response to inductive interactions.