Expression of the medaka (Oryzias latipes) Ol-Rx3 paired-like gene in two diencephalic derivatives, the eye and the hypothalamus.

Here we report the expression pattern of the homeobox Ol-Rx3 gene, a medaka gene homologous to the mouse, Xenopus, zebrafish and Drosophila Rx genes. Ol-Rx3 starts to be expressed, at late gastrula stages, in the presumptive territories of the anterior brain. Subsequently, transcripts are localised in an antero-ventral region of the prosencephalon and in the primordia of the optic vesicles. During organogenesis, distribution of Ol-Rx3 transcripts are gradually restricted to the floor of the diencephalon, the prospective territory of the hypothalamus and the neurohypophysis. During late development and in adult, Ol-Rx3 expression is maintained in hypothalamic nuclei bordering the third ventricle. In the optic vesicles, Ol-Rx3 expression is temporarily switched off when the eye cup morphogenesis is complete, but it is turned on again in the inner nuclear layer of the retina. Thus, the early expression pattern of Ol-Rx3 is in agreement with a conserved role in the specification of the ventral forebrain and eye field. Putative functions linked to late expression domains are discussed in light of the different hypothesis concerning the involvement of vertebrate Rx genes in the maintenance of particular cell fate. Copyright 1998 Elsevier Science S.A. All rights reserved