Expression domains of the medaka (Oryzias latipes) Ol-Gsh 1gene are reminiscent of those of clustered and orphan homeobox genes

Screening of a medaka (Oryzias latipes) adult brain cDNA library, with a degenerated probe corresponding to the most conserved region of helix III of the homeodomain, led to the isolation of a gene homologous to a murine orphan Hoxgene, named Gsh-1. We have called this gene Ol-Gsh 1(Oryzias latipes-Gsh 1). Molecular analysis of the Ol-Gsh 1 putative protein points to potential functional domains which are highly conserved between fish and mouse genes. Whole-mount in situ hybridization shows that Ol-Gsh 1is expressed in several waves during embryonic development. Transcripts are found in many regions of the central nervous system: the spinal cord, dorsal rhombencephalon, optic tectum, dorsal diencephalon, hypothalamus anlagen and rostral telencephalon. This multimodal expression pattern, strikingly conserved between fish and mammals, is reminiscent of both clustered and orphan homeobox genes. In addition, each expression wave is initiated in the fish embryo earlier than in the mammalian embryo, relative to the time scale defined by somitogenesis. We propose that Ol-Gsh 1may be involved in conserved developmental pathways and in particular may be linked to proliferation events. Mouse Gsh-1was shown to participate in neuro-endocrine functions of the hypothalamus. From late developmental stages onwards, Ol-Gsh 1expression is also restricted to the hypothalamus. The expression pattern in this structure raises interesting questions concerning a fully or partially conserved function for these genes.