ROLE OF THE PRONEURAL GENE, ATONAL, IN FORMATION OF DROSOPHILA CHORDOTONAL ORGANS AND PHOTORECEPTORS

The Drosophila gene atonal encodes a basic helix-loop-helix protein si milar to those encoded by the proneural genes of the achaete-scute com plex (AS-C). The AS-C are required in the Drosophila PNS for the selec tion of neural precursors of external sense organs. We have isolated m utants of atonal, which reveal that this gene encodes the proneural ge ne for chordotonal organs and photoreceptors. In atonal mutants, all o bservable adult chordotonal organs, and almost all embryonic chordoton al organs fail to form; all adult photoreceptors are missing. For both types of sense organ, this defect is already apparent at the level of precursor formation. Therefore it is a failure in the epidermal-neura l decision process i.e. a proneural defect. The failure to form photor eceptors results in atrophy of the atonal mutant imaginal disc, due to apoptosis and lack of stimulation of division. Lack of photoreceptors should also eliminate signalling that arises from differentiating pho toreceptors and is required for morphogenetic furrow movement in the w ild-type eye disc. Nevertheless, a remnant morphogenetic furrow is sti ll observed in the atonal mutant disc. This presumably reflects the pr ocess of furrow initiation, which would not depend on signals from dev eloping photoreceptors.