Color vision requires the comparison of the output of classes of photoreceptor cells (PRs) that respond differently to the wavelength of incoming light. In Drosophila, the primary visual system is composed of pale (p) and yellow (y) ommatidia, which are distributed in a stochastic manner with a conserved ratio of 30% (p) to 70% (y). Recent evidence has suggested that the gene spineless (ss), is responsible for the formation of the ommatidial mosaic by defining the y ommatidia. The studies described in this proposal are aimed at testing the hypothesis that the stochastic expression of spineless in a subset of PRs is the primary event that leads to the creation of the complex retinal mosaic used for color vision. To test this hypothesis, we will address three specific aims: 1) functional analysis of spineless, 2) determination of factors controlling the stochastic expression of ss, and 3) identification of new molecular phenotypes of ss. These experiments will be performed using a variety of molecular, genetic and genomic approaches. The successful completion of this work should provide a wealth of information about the basic mechanisms involved in y/p PR subtype specification. Since the mechanisms of eye development and differentiation are conserved from flies to humans, ultimately, a more detailed knowledge of the fly retinal mosaic will lead to a better understanding of the vertebrate retina.