GAIN-OF-FUNCTION ALLELES OF BEARDED INTERFERE WITH ALTERNATIVE CELL FATE DECISIONS IN DROSOPHILA ADULT SENSORY ORGAN DEVELOPMENT

We have isolated a novel class of gain-of-function mutations at the Be arded (Brd) locus which specifically affect the development of adult s ensory organs in Drosophila, These Brd alleles cause bristle multiplic ation and bristle loss phenotypes resembling those described for the n eurogenic genes Notch (N) and Delta (Dl). We have found that supernume rary sensory organ precursor (SOP) cells develop in the proneural clus ters of Brd mutant imaginal discs; like normal SOPs, these are depende nt on the function of the proneural genes achaete and scute, and expre ss elevated levels of ac protein. At cuticular positions exhibiting th e Brd bristle loss phenotype, we have found that the progeny of the mu ltiplied SOPs develop aberrantly, in that neurons and thecogen (sheath ) cells appear but not trichogen (shaft) and tormogen (socket) cells. This appears to represent a transformation of the pIIa secondary precu rsor cell within the SOP lineage to a pIIb secondary precursor cell fa te. These results suggest that Brd gain-of-function alleles interfere with Notch pathway-dependent cell-cell interactions at two distinct st ages of adult sensory organ development. We have also identified enhan cers and suppressors of the Brd dominant phenotypes; these include bot h previously characterized mutations and alleles of apparently novel l oci. finally, we have found that Brd null mutants are viable and exhib it no mutant phenotypes, suggesting that Brd may be a component of an overlapping function. (C) 1996 Academic Press, Inc.