Expression of three Rx homeobox genes in embryonic and adult zebrafish.

The paired-class homeobox gene, Rx, is important in eye development. In this study we analyze expression patterns of three zebrafish Rx genes (Zrx1, 2, 3) in embryos and adults. All three genes show dynamic spatiotemporal patterns of expression. Zrx3 is expressed earliest, in the anteriormost region of the neural plate, in regions that give rise to ventral diencephalon and retinae. As development proceeds, Zrx3 expression is reduced in the lateral optic primordia, and is absent in the optic cup, but is retained at the ventral midline of the diencephalon, and is expressed in hypothalamus in the adult. As the neural retina begins to differentiate, Zrx3 is re-expressed in a subset of cells in the inner nuclear layer, presumably bipolar cells, and this expression is retained in the adult. In contrast, Zrx1/2 have a slightly later onset of expression, are initially coincident with Zrx3, but then become complementary, remaining on in the optic primordia but disappearing from the ventral midline of the diencephalon. Zrx1/2 are down-regulated as the retina differentiates, except in the outer nuclear layer where they continue to be expressed at high levels in cone, but not rod, photoreceptors. This is the first transcription factor described that distinguishes between cone and rod photoreceptors.