Expected and unexpected phenotypes from the proneural gene, amos. P. zur Lage , D.R.A. Prentice , A.P. Jarman. The Wellcome Trust Centre for Cell Biology, ICMB, Univ Edinburgh, Edinburgh, Scotland.

   In the PNS Drosophila, bHLH-encoding proneural genes are required for the selection of sense organ precursors. When they are mutated, specific subsets of sense organ precursors fail to form. Proneural genes include the achaete-scute complex for external sense organs and atonal for chordotonal organs, R8 photoreceptors and sensilla coeloconica. We recently isolated a further proneural gene, called amos. Indirect evidence suggested that amos is required for two kinds of olfactory sensilla. amos may be the last of the proneural genes required for the PNS, but there was no definitive evidence for its function.
    We have now isolated amos mutations. Our preliminary analysis of one mutation suggests that is very severe. It is viable, but the adult flies completely lack sensilla basiconica and trichodea, demonstrating that amos is absolutely required for these sensilla. This mutation, however, also has unexpected features. Many of the presumed amos-dependent olfactory precursors still seem to appear in the mutant antennal discs. Strikingly, some of these differentiate inappropriately to form ectopic external sense organs on the antenna. These features are not characteristic of a proneural gene mutation. We are testing the hypothesis that this particular mutation results in a partially active protein: one that can still trigger neurogenesis (create sense organ precursors), but is unable to confer subtype specificity (tell the precursors to follow an olfactory fate), so they differentiate as default external sense organs. Potentially, it is the first demonstration that the subtype specificity function of a proneural protein is separable from its neural precursor determination function.