Title: Understanding the mechanism of pigment rim formation at the periphery of the eye in Drosophila melanogaster
Description: The Drosophila eye periphery undergoes peripheral patterning in response to a graded Wingless signal emanating from the surrounding head capsule. High levels of Wg signaling lead to the formation of the Pigment Rim. The pigment rim is a thick band of pigment cells that serves to optically insulate the eye from extraneous light rays. It is composed mainly of the pigment cells that surrounded the outermost row of ommatidia in the developing pupal eye. These peripheral ommatidia undergo timed developmental apoptosis, leaving the remaining pigment cells to coalesce and form the pigment rim. Earlier work showed that high levels of Wingless signaling induced the expression of Escargot, Wingless and Notum in a subset of the cells of the peripheral ommatidia, namely the cone cells. But the mechanism of apoptosis of the entire ommatidia remained unclear. My work focuses on the mechanism by which Wingless leads to the apoptosis of the different cell types of the ommatidia in a concerted manner. In this thesis, I show that the peripheral apoptosis follows a precisely timed sequence of events.

I also show that ectopic expression of Wingless at high levels causes the entire eye to respond in a manner similar to the peripheral ommatidia. In order to elucidate the mechanism of Wingless induced apoptosis, I analyzed the effects of manipulations of the Wingless signaling pathway in the subsets of the cells of the ommatidia. I found that the expression of Escargot in the cone cells is required for their collapse, while the Wingless expression appears to be a booster signal for the apoptosis of the remaining cells of the ommatidia. I also show that the activation of Wingless signaling in the cone cells alone is insufficient for apoptosis of the ommatidia, thereby suggesting a combinatorial response of all the cell types. Lastly, I present a logical conundrum in the response of the photoreceptors to manipulations in Wingless signaling. In conclusion, I present a possible model of a concerted response of the different cell types of the ommatidia to lead to their apoptosis.