Expression of the zebrafish paired box gene pax[zf-b] during early neurogenesis.

The paired box-containing (pax) gene family encodes a group of putative transcription factors differentially expressed during embryonic development. In this study, we describe the cloning and expression of a zebrafish gene pax[zf-b], which most probably is a direct homologue to the mouse Pax2 gene. The putative protein encoded by pax[zf-b] contains a paired box, an octapeptide, but no homeobox. However, a region of homology to the N-terminal half of paired-type homeoboxes is detected C-terminal to the pax[zf-b] paired domain. In zebrafish embryos, pax[zf-b] transcripts are first seen during the formation of the neural keel. At 9-10 h of development, two laterally located transverse stripes of cells expressing the gene appear in the rostral 1/3 of the embryo. The two areas subsequently move towards the midline and form the posterior portion of the midbrain. In the following stages of development, at 10-12 h, transcripts are detected in the otic placode, the Wolffian duct including the nephritic primodium and in the optic stalk. At a later time point, beginning at 14-15 h, single cells along the spinal cord, presumably interneurons, start to express the gene. The characteristic expression pattern of pax[zf-b] in the neural tube suggests an involvement of this gene in the regionalization of the midbrain as well as in the specification of neuronal cell fates at early embryonic stages.