THE ROLE OF YAN IN MEDIATING THE CHOICE BETWEEN CELL-DIVISION AND DIFFERENTIATION

An allele of the yan locus was isolated as an enhancer of the Ellipse mutation of the Drosophila epidermal growth factor receptor (Egfr) gen e. This yan allele is an embryonic lethal and also fails to complement the lethality of anterior open (aop) mutations. Phenotypic and comple mentation analysis revealed that aop is allelic to yan and genetically the lethal alleles act as null mutations for the yan gene. Analysis o f the lethal alleles in the embryo and in mitotic clones showed that l oss of yan function causes cells to overproliferate in the dorsal neur oectoderm of the embryo and in the developing eye disc. Our studies su ggest that the role of yan is defined by the developmental context of the cells in which it functions. An important role of this gene is in allowing a cell to choose between cell division and differentiation. T he relationship of the Egfr and Notch pathways to this developmental r ole of yan is discussed.