Gain-of-Function Alleles ofBeardedInterfere with Alternative Cell Fate Decisions inDrosophilaAdult Sensory Organ Development

Abstract

We have isolated a novel class of gain-of-function mutations at theBearded(Brd) locus which specifically affect the development of adult sensory organs inDrosophila.TheseBrdalleles cause bristle multiplication and bristle loss phenotypes resembling those described for the neurogenic genesNotch(N) andDelta(Dl). We have found that supernumerary sensory organ precursor (SOP) cells develop in the proneural clusters ofBrdmutant imaginal discs; like normal SOPs, these are dependent on the function of the proneural genesachaeteandscute,and express elevated levels of ac protein. At cuticular positions exhibiting theBrdbristle loss phenotype, we have found that the progeny of the multiplied 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 precursor cell within the SOP lineage to a pIIb secondary precursor cell fate. These results suggest thatBrdgain-of-function alleles interfere withNotchpathway-dependent cell–cell interactions at two distinct stages of adult sensory organ development. We have also identified enhancers and suppressors of theBrddominant phenotypes; these include both previously characterized mutations and alleles of apparently novel loci. Finally, we have found thatBrdnull mutants are viable and exhibit no mutant phenotypes, suggesting thatBrdmay be a component of an overlapping function.