Sonic hedgehog signaling in the chick retina accelerates Meis2 downregulation simultaneously with retinal ganglion cell genesis.

The spatial-temporal expression and function of Drosophila melanogaster homothorax (hth) and its vertebrate homologue Meis2 in the eye anlage show a high degree of evolutionary conservation. Both are expressed by progenitor cells ahead of the neurogenic wave front, promote rapid proliferation of these cells, and are downregulated before cells exit the cell cycle and differentiate. Here, we show that downregulation of Meis2 accompanies, but is not required, for retinal differentiation. In addition, we provide evidence that the mechanisms that terminate expression of both genes differ. Hth expression in the fly eye imaginal disc is primarily repressed by the transforming growth factor beta family protein decapentaplegic, whereas Meis2 expression in the chick eye cup is terminated in response to sonic hedgehog signaling.