Flight muscle formation inDrosophila mosaics: requirement for normalshibire function of endocytosis.

InDrosophila, the temperature-sensitiveshibire (shi) mutation causes blockage of endocytosis. Disruption ofshi function by heat pulse at a sensitive pupal stage produces adults with altered flight muscles and motoneurons (Hummon and Costello 1988b). Thus, endocytosis is evidently required for normal development of flight muscle, but the site(s) with primary sensitivity to disruption ofshi function has not been shown. In Ring-X generated mosaic animals (wild type/shi), the inducedshi flight muscle phenotype maps to the area of the blastoderm fate map containing the presumptive thoracic mesoderm, the developmental foci for specific muscles (Hummon and Costello 1992). We use these wild type/shi mosaics to look for independence versus correlation between the pattern of muscle phenotypes and the genotype of other tissues that might control muscle phenotype, e.g. the motoneuron. For each of these tissues, we define the expected pattern of muscle phenotypes, and compare expected and observed patterns in each of a series of wild type/shi animals. We find that muscle phenotype is independent of the genotype of the motoneurons or muscle attachment sites. Control of the inducedshi muscle phenotype evidently lies within the muscle itself. The normalshi function, endocytosis, is therefore essential in this muscle tissue during a sensitive stage of myogenesis in early pupae (20 h).