A unique mutation in the Enhancer of split gene complex affects the fates of the mystery cells in the developing Drosophila eye

An unusual recessive allele of the Drosophila groucho gene, which encodes a transducinlike protein, affects the fates of specific cells in the eye disc groucho is one of several transcription units in the Enhancer of split complex. Most groucho mutations are zygotic lethal due to the proliferation of embryonic neural cells at the expense of epidermal cells. In contrast, flies homozygous for the mutant allele described here, gro^BFP2, are viable but have abnormal eyes. The Drosophila compound eye is composed of several hundred identical facets, or ommatidia, each of which contains eight photoreceptor cells, R1-R8. In gro^BFP2 mutant retinas, most of the facets contain eight normally determined photoreceptor cells and one or two additional R-cells of the R3/4 subtype. The extra photoreceptors appear to arise from the mystery cells, which are part of the precluster that initiates the ommatidium, but do not normally become neurons. gro^BFP2 behaves as a partial loss-of-function mutant. Analysis of ommatidia mosaic for wild-type and gro^BFP2 mutant cells suggests that the focus of action of the gro^BFP2 mutation is outside of the photoreceptor cells. These results imply that one function of groucho is in a pathway whereby neuralization of the mystery cells is inhibited by other non-neural cells in the eye disc. In addition, determination of R3/4 photoreceptors usually requires contact with R2 and R5. Specification of the mystery cells as ectopic R3/4 subtype photoreceptors in gro^BFP2 mutant eye discs implies that induction by R2 or R5 is not absolutely necessary for R3/4 cell determination.