In Drosophila, color vision is achieved by the two inner photoreceptors R7 and R8 which express different rhodopsins and compare their outputs in the medulla part of the optic lobe. Three classes of ommatidia with different functions form a mosaic in the retina. In p ommatidia, R7 contains the UV-Rh3 while R8 contains blue-Rh5. In y ommatidia, R7 contains UV-Rh4 and R8 green-Rh6. These two classes are randomly distributed in the retina with a 30:70 ratio. This results from a stochastic decision made in R7 to express rh4 and to exclude rh3. Coordinate expression in R8 allows the formation of ommatidia that specialize in the discrimination of shorter (p) or longer (y) wavelengths of light. Ommatidia located at the dorsal rim area (DRA) specialize in the detection of the vector of polarized light. We have identified many of the factors that contribute to the choice of rhodopsins in the various classes of photoreceptors. In the following aims, we propose to further investigate the mechanisms regulating the stochastic choice of spineless (ss) expression as well as the spatial control of specific subsets of photoreceptors. 1. Regulation of photoreceptor choice in the larval eye and adult brain eyelet: The larval eye contains two types of photoreceptors: four express blue-Rh5 while eight contain green-Rh6, reminiscent of the Rh5:Rh6 ratio in adult R8 cells. We will compare the regulation of these rhodopsins in these two systems. At metamorphosis, the larval eye becomes the extra-retinal eyelet that contains four green-Rh6 photoreceptors. We will investigate how Ecdysone signaling leads to the death of the eight green-Rh6 photoreceptors and to the trans-determination of the four blue-Rh5 photoreceptors into green-Rh6. 2. Regulation of the stochastic expression of spineless: We will pursue our attempts to understand how transcription of ss is activated in a subset of R7 cells. We will explore the possibility that lozenge represses ss and might thus be responsible for its stochastic activation. We will dissect the ss promoter and identify factors that control its expression in the retina. 3. Localized distribution of specialized ommatidia: We will also analyze the function of an effector of ss, dve, a homeobox gene that appears to be involved in the repression of rh3 in cells that express ss. We will also investigate its relationship to Iro-C genes that control the development of two subtypes of ommatidia (DRA and a novel type hat co-expresses Rh3 and Rh4 in yR7 in the dorsal part of the eye). 4. An FRT-piggyBac screen to identify novel genes regulating R7 subtype specification. We have devised a genetic screen using a collection of 4,000 FRT-piggybac insertion lines provided by the lab of Liqun Luo. We will generate whole mutant eyes for each insertion and will evaluate subtype ratio visually with rhodopsin GFP reporters using water immersion fluoroscopy. This unbiased approach will allow us to complete our understanding of the regulatory pathway that leads to the formation of the retinal mosaic.
The Drosophila eye has served as a very powerful model system to understand how different classes of photoreceptors develop to form the retinal mosaic that is responsible for the function of the eye: motion detection, dim light or color vision. We offer to pursue our investigations of the events that allow photoreceptors to take on specific fates, express one of the rhodopsin photopigments and exclude all others. The identification of regulators of these processes will lead to a better understanding of genes whose mutations cause blindness in humans.
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