Origin and Differentiation of Supernumerary Midline Glia inDrosophilaEmbryos Deficient for Apoptosis

Drosophilaembryos deficient for programmed cell death produce 9 midline 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 posterior) lineage and the remainder from the MGA/MGM (MG anterior and middle) lineage. There is one unidentified additional neuron in the mesectoderm of embryos deficient for apoptosis. The supernumerary MG are not diverted from other lineages nor do they arise from an altered pattern of 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 to theDrosophilaEGF receptor. The persistence of supernumerary MG in embryos deficient for apoptosis does not alter the spatial pattern ofDrosophilaEGF receptor signaling. The number and position of MG which express genes dependent upon EGF receptor function, such aspointedorargos,are indistinguishable from wild type. Genes of the spitz group are required forDrosophilaEGF receptor function. Surviving MG in spitz group/H99double 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.