ORIGIN AND DIFFERENTIATION OF SUPERNUMERARY MIDLINE GLIA IN DROSOPHILA EMBRYOS DEFICIENT FOR APOPTOSIS

Drosophila embryos deficient for programmed cell death produce 9 midli ne glia (MG) in addition to the wild-type complement of 3.2 MG/segment . More than 3 of the supernumerary MG derive from the MGP (MG posterio r) lineage and the remainder from the MGA/MGM (MG anterior and middle) lineage. There is one unidentified additional neuron in the mesectode rm of embryos deficient for apoptosis. The supernumerary MG are not di verted from other lineages nor do they arise from an altered pattern o f mitosis. Instead, these MG appear to arise from a normally existing pool of 12 precursor cells, larger than anticipated by earlier studies . During normal development, MG survival is dependent upon signaling t o the Drosophila EGF receptor. The persistence of supernumerary MG in embryos deficient for apoptosis does not alter the spatial pattern of Drosophila EGF receptor signaling. The number and position of MG which express genes dependent upon EGF receptor function, such as pointed o r argos, are indistinguishable from wild type. Genes of the spitz grou p are required for Drosophila EGF receptor function. Surviving MG in s pitz group/H99 double mutants continue to express genes characteristic of the MG, but the cells fail to differentiate into ensheathing glia and are displaced from the nerve cord. (C) 1997 Academic Press.