A genetic method for generating Drosophila eyes composed exclusively of mitotic clones of a single genotype

The genetic analysis of a gene at a late developmental stage can be impeded if the gene is required at an earlier developmental stage. The constructio n of mosaic animals, particularly in Drosophila, has been a means to overco me this obstacle. However, the phenotypic analysis of mitotic clones is oft en complicated because standard methods for generating mitotic clones rende r mosaic tissues that are a composite of both mutant and phenotypically nor mal cells. We describe here a genetic method (called EGUF/hid) that uses bo th the GAL4/UAS and FLP/FRT systems to overcome this limitation for the Dro sophila eye by producing genetically mosaic flies that are otherwise hetero zygous but in which the eye is composed exclusively of cells homozygous for one of the five major chromosome arms. These eyes are nearly wild type in size, morphology, and physiology. Applications of this genetic method inclu de phenotypic analysis of existing mutations and F-1 genetic screens to ide ntify as yet unknown genes involved in the biology of the fly eye. We illus trate the utility of the method by applying it to lethal mutations in the s ynaptic transmission genes synaptotagmin and syntaxin.