ATONAL IS A PRONEURAL GENE THAT DIRECTS CHORDOTONAL ORGAN FORMATION IN THE DROSOPHILA PERIPHERAL NERVOUS-SYSTEM

In the Drosophila peripheral nervous system, proneural genes of the ac haete-scute complex (ASC) are required for formation of the precursors of external sense organs but not of chordotonal organs. We report the isolation of a gene, atonal (ato), with evidence that it is a proneur al gene for the formation of chordotonal organs. This gene is expresse d in the proneural clusters and sense organ precursors that give rise to the embryonic and adult chordotonal, but not external sense, organs . Chordotonal organs are eliminated in embryos carrying chromosomal de ficiencies that remove ato. Like the ASC products, ato protein contain s a basic-helix-loop-helix region and heterodimerizes with daughterles s protein to bind to E boxes. Moreover, ectopic expression of ato prom otes the formation of extra sense organs. Despite similar proneural pr operties, we find that ectopic expression of the ASC genes promotes ex ternal sense organ formation exclusively, whereas ato promotes chordot onal organ formation preferentially. Thus, proneural genes are major d eterminants of neuronal identity.