We have used MPM-2, a monoclonal antibody raised against mitotic HeLa cells, to stain a Drosophila cell line, whole mounts of Drosophila embryos, and sectioned tissue from embryonic and larval stages of development. MPM-2 recognizes a major phosphoprotein of approximately 125 X 10(3) Mr in Drosophila tissue culture cells that, like the mammalian MPM-2 antigen, appears to be recognized only in mitotic cells. During early embryogenesis, when the embryonic nuclei divide as a syncytium with a very short nuclear division time, MPM-2 antigen is observed within the spindle compartment at all stages of the nuclear division cycle. Upon cellularization of the embryo and lengthening of the duration of the cycle, the antigen is predominantly seen in mitotic cells. Drosophila larvae contain both diploid and polytene tissues: in diploid tissue MPM-2 staining is specifically observed over mitotic cells, as expected from its distribution in cellularized embryos. Surprisingly, antigen is also detected in the nuclei of polytene cells that replicate their DNA but do not undergo mitosis.

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