Mutations in Drosophila myb cause abnormal mitoses in abdominal epidermal cells. S-M. Fung , G. Ramsay , A. Katzen. Dept Genetics, Univ Illinois, Chicago, IL.

   Drosophila myb (Dm myb) encodes a transcriptional activator and is related to the vertebrate proto-oncogene, c-myb. We have previously shown that myb is essential for viability and that in temperature-sensitive mutants raised at permissive temperatures, phenotypic defects are observed in adult wings and abdomens. The abdominal defects in Dm myb mutants phenocopy mutations escargot (esg) and Drosophila cdc2, where defects have been attributed to a failure to suppress endoreplication in abdominal histoblast during larval development. We previously showed that Dm myb is required for progression through the G2/M transition and for suppression of endoreduplication in wing cells during development. Therefore, it seemed likely that basis of the abdominal phenotype in myb mutants would be the same as in esg and cdc2 mutants. Contrary to these expectations, no defects are observed in the abdominal histoblasts of mutant myb larvae. To monitor abdominal histoblast proliferation during pupal development, we have used reagents that stain DNA, centrosomes, mitotic spindles, and cell boundaries. During early pupation, mutant abdominal histoblasts proliferate more slowly than wild type, but appear otherwise normal. Later in development, when proliferation of histoblasts and replacement of larval abdominal epidermal cells is virtually complete in wild type animals, mutant myb cells continue to proliferate and exhibit a variety of abnormalities. The mitotic-specific PH3 antibody reveals that the mutant cells progress abnormally slow through the early stages of mitosis. Many mitotic cells have more than two functional centrosomes which form multipolar spindles. In interphase, cells with large misshapen nuclei or two partially fused nuclei are observed. FISH and quantitative confocal microscopy of myb mutant nuclei revealed aneploid and polyploid cells, Our findings indicate that Dm myb plays an important role in maintainig genomic stability.