CONFRONTATION OF SCABROUS EXPRESSING AND NON-EXPRESSING CELLS IS ESSENTIAL FOR NORMAL OMMATIDIAL SPACING IN THE DROSOPHILA EYE

The establishment of neural precursor cells in Drosophila depends on c ell-cell interactions and lateral inhibition. Scabrous (scea) is invol ved in this process by preventing an excess of cells from adopting a n eural precursor fate. Specifically in eye development, Sca protein fun ction has been implicated in the spacing pattern that is essential for the ordered appearance of the ommatidial array. During this process s ca expression is restricted to neurogenic groups of cells and later to the neural precursors. We report that ectopic sca expression in the m orphogenetic furrow results in a rough eye phenotype with oversized an d fused ommatidia. These defects in adult eyes are due to the generati on of too many ommatidial preclusters in the morphogenetic furrow. Str ikingly, sca loss-of-function mutants have an almost identical phenoty pe. Our results suggest that Sca plays a positive role in establishing the spacing pattern within the furrow and that the quantitative diffe rence in sea expression between neighboring groups of cells is a deter mining factor in this process. Ectopic expression of Sca also represse s endogenous sea expression in the furrow, suggesting that Sca is invo lved in a feedback loop affecting its own transcription. Interestingly , sea shares homology to a group of extracellular matrix proteins that have been implicated in neuronal differentiation. We present a model for sea function based on its phenotypic and molecular features.