Characterization of genes involved in photoreceptor differentiation

Abstract

The one receptor - one neuron rule is a common phenomenon observed in sensory organs. In the Drosophila eye, each photoreceptor expresses one of six different rhodopsin genes. The expression patterns of inner photoreceptors divide ommatidia into three distinct subtypes: pale, yellow and DRA. In the pale subtype R7 and R8 cells express rh3 and rh5 subsequently while in the yellow subtype rh4 and rh6 are expressed in the inner PRs. Photoreceptor specification in Drosophila is a step-wise process, which requires activation of specific genes. Although certain transcription factors like spalt, prospero, senseless, ortodenticle and spineles have been identified so far, the whole picture and the pathways are still unknown. Aiming to identify novel genes playing a role in PR specification, an enhancer trap screen was previously performed. In this screen, the transposable element piggyBac was used as a vector with the aim of targeting novel genes. According to molecular functions of the genes identified and their expression patterns, 13 genes were selected for further characterization. In the course of this study the detailed expression, loss of function and gain of function analysis of those candidate genes were performed. Possible functions in eye development of five of the genes (cropped, headcase, hephaestus, stubble and taranis) were defined. Three of them (cropped, headcase and hephaestus) function possible early in eye development and eye growth. While the role of the Stubble gene is suggested as Rh6 de-repression in a subset of pale R8 cells. Taranis is believed to have a function in maintenance of rhabdomere size in photoreceptor cells. The characterization of the expression patterns of a number of enhancer-trap lines, their molecular as well as their initial functional characterization is the basis for future functional work of some of these genes in rhodopsin regulation and retinal patterning in general.