Wingless eliminates ommatidia from the edge of the developing eye through activation of apoptosis

The Drosophila compound eye is formed by selective recruitment of undifferentiated cells into clusters called ommatidia during late larval and early pupal development. Ommatidia at the edge of the eye, which often lack the full complement of photoreceptors and support cells, undergo apoptosis during mid-pupation. We have found that this cell death is triggered by the secreted glycoprotein Wingless, which activates its own expression in peripheral ommatidia via a positive feedback loop. Wingless signaling elevates the expression of the pro-apoptotic factors head involution defective, grim and reaper, which are required for ommatidial elimination. We estimate that approximately 68% of the total photoreceptor pool in each eye is removed by this mechanism. In addition, we show that the retinal apoptosis previously reported in apc1 mutants occurs at the same time as the peripheral ommatidial cell death and also depends on head involution defective, grim and reaper. We consider the implications of these findings for eye development and function in Drosophila and other organisms.