Use of gain-of-function study to delineate the roles of crumbs in Drosophila eye development.

The cell polarity gene, crumbs (crb)), has been shown to participate in the development and degeneration of the Drosophila retina. Mutations in CRB1, the human homologue of Drosophila crb, also result in retinitis pigmentosa and Leber congenital amaurosis. In this study, we used the gain-of-function approach to delineate the roles of CRB in developing Drosophila eye. In the third-instar larval stage, eye development is initiated with photoreceptor differentiation and positioning of photoreceptor nuclei in the apical cellular compartment of retinal epithelium. In the pupal stage, differentiated photoreceptors begin to form the photosensitive structures, the rhabdomeres, at their apical surface. Using GMR-Gal4 to drive overexpression of the Crb protein at the third-instar eye disc, we found that differentiation of photoreceptors was disrupted and the nuclei of differentiated photoreceptors failed to occupy the apical compartment. Using HS-Gal4 to drive Crb overexpression in pupal eyes resulted in interference with extension of the adherens junctions and construction of the rhabdomeres, and these defects were stage-dependent. This gain-of-function study has enabled us to delineate the roles of Crb at selective stages of eye development in Drosophila.