Identification of chick rax/rx genes with overlapping patterns of expression during early eye and brain development.

We have isolated chick rax/rx cDNAs, cRaxL (chick Rax/Rx-like) and cRax, (chick Rax) and examined their expression patterns during early eye and brain development. The cRaxL cDNA encodes a 228 amino acid protein that is most closely related to the zebrafish Rx1 and Rx2. The cRax cDNA encodes a 317 amino acid protein, which shares higher homology with the Xenopus Rx. In addition to the homeodomain, the octapeptide and paired tail domains are conserved between the cRax and other vertebrate Rax/Rx, while cRaxL lacks the octapeptide containing N-terminal region which is conserved among all other members of the rax/rx gene family identified so far. The chick rax/rx genes are expressed in overlapping domains in the anterior neural ectoderm which corresponds to the forebrain and retina field, and later in the optic vesicle. cRax mRNA can be detected earlier than cRaxL prior to the formation of the notochord and its expression domain appears broader than that of cRaxL.