Xrx1, a novel Xenopus homeobox gene expressed during eye and pineal gland development.

We have isolated a novel Xenopus homeobox gene, Xrx1, belonging to the paired-like class of homeobox genes. Xrx1 is expressed in the anterior neural plate, and subsequently in the neural structures of the developing eye (neural retina and pigmented epithelium), and in other forebrain structures deriving from the anterior neural plate: in the pineal gland, throughout its development, in the diencephalon floor and in the hypophysis. Its rostral limit of expression corresponds to the chiasmatic ridge, which some authors consider as the anteriormost limit of the neural tube: thus, Xrx1 may represent one of the most anteriorly expressed homeobox genes reported to date. Moreover, its expression in organs implicated in the establishment of circadian rhythms, may suggest for Xrx1 a role in the genetic control of this function. Finally, analysis of Xrx1 expression in embryos subjected to various treatments, or microinjected with different dorsalizing agents (noggin, Xwnt-8), suggests that vertical inductive signals leading to head morphogenesis are required to activate Xrx1.