atonal is the proneural gene for Drosophila photoreceptors

THE Drosophila peripheral nervous system comprises four major types of sensory element: external sense organs (such as mechano-sensory bristles), chordotonal organs (internal stretch receptors), multiple dendritic neurons, and photoreceptors. During development, the selection of neural precursors for external sense organs requires the proneural genes of the achaete–scute complex, which encode basic-helix–loop–helix transcription factors1–3. These genes do not, however, control precursor selection for chordotonal organs or photoreceptors4,5, raising the question of whether other proneural genes exist6 or a different mechanism of neurogenesis operates. Here we show that atonal (ato), originally isolated as a proneural gene for chordotonal organs7, is also the proneural gene for photo-receptors. Pattern formation in the Drosophila eye involves a succession of cell fate specifications. Of the eight photoreceptors within each ommatidium of the compound eye, the photoreceptor R8 is the first to appear in the eye imaginal disc, right behind the mor-phogenetic furrow8–10. The appearance of other photoreceptors (Rl–7) follows in a defined sequence that is thought to arise by induction from R8 (refs 8, 9, 11, 12). We find that photoreceptor formation requires the function of atonal at the morphogenetic furrow and that atonal is specifically required for R8 selection. Formation of other photoreceptors does not directly require atonal function, but does depend on R8 selection by atonal. Thus, photo-receptors are selected by two mechanisms: R8 by a proneural mechanism, and Rl–7 by local recruitment.