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 mechanosensory bristl es), chordotonal organs (internal stretch receptors), multiple dendrit ic neurons, and photoreceptors. During development, the selection of n eural precursors for external sense organs requires the proneural gene s of the achaete-scute complex, which encode basic-helix-loop-helix tr anscription factors(1-3). These genes do not, however, control precurs or selection for chordotonal organs or photoreceptors(4,5), raising th e question of whether other proneural genes exist(6) or a different me chanism of neurogenesis operates. Here we show that atonal (ato), orig inally isolated as a proneural gene for chordotonal organs(7), is also the proneural gene for photoreceptors. Pattern formation in the Droso phila eye involves a succession of cell fate specifications. Of the ei ght photoreceptors within each ommatidium of the compound eye, the pho toreceptor R8 is the first to appear in the eye imaginal disc, right b ehind the morphogenetic furrow(8-10). The appearance of other photorec eptors (R1-7) follows in a defined sequence that is thought to arise b y induction from R8 (refs 8, 9, 11, 12). We find that photoreceptor fo rmation requires the function of atonal at the morphogenetic furrow an d that atonal is specifically required for R8 selection. Formation of other photoreceptors does not directly require atonal function, but do es depend on R8 selection by atonal. Thus, photoreceptors are selected by two mechanisms: R8 by a proneural mechanism, and R1-7 by local rec ruitment.