The homeobox gene irx1a is required for the propagation of the neurogenic waves in the zebrafish retina

Neurogenesis in the compound eyes of Drosophila and the camera eyes of vertebrates spreads in a wave-like fashion. In both phyla, waves of hedgehog expression are known to drive the wave of neuronal differentiation. The mechanism controlling the propagation of hedgehog expression during retinogenesis of the vertebrate eye is poorly understood. The Iroquois homcobox genes play important roles in Drosophila eye development; they are required for the up-regulation of hedgehog expression during propagation of the morphogenetic furrow. Here, we show that the zebrafish Iroquois homolog irxla is expressed during retinogenesis and knockdown of irxla results in a retinal phenotype strikingly similar to those of sonic hedgehog (shh) mutants. Analysis of shh-GFP transgene expression in irxla knockdown retinas revealed that irxla is required for the propagation of shh expression through the retina. Transplantation experiments illustrated that the effects of irxla on shh expression are both cell-autonomous and non-cell-autonomous. Our results reveal a role for Iroquois genes in controlling hedgehog expression during vertebrate retinogenesis. (c) 2005 Elsevier Ireland Ltd. All rights reserved.