DROSOPHILA MYB IS REQUIRED FOR THE G(2) M TRANSITION AND MAINTENANCE OF DIPLOIDY/

The myb proto-oncogenes are thought to have a role in the cell divisio n cycle. We have examined this possibility by genetic analysis in Dros ophila melanogaster, which possesses a single myb gene. We have descri bed previously two temperature-sensitive, recessive lethal mutants in Drosophila myb (Dm myb). The phenotypes of these mutants revealed a re quirement for myb in diverse cellular lineages throughout the course o f Drosophila development. We now report a cellular explanation for the se findings by showing that Dm myb is required for both mitosis and pr evention of endoreduplication in wing cells. Myb apparently acts at or near the time of the G(2)/M transition. The two mutant alleles of Dm myb produce the same cellular phenotype, although the responsible muta tions are located in different functional domains of the gene product. The mutant phenotype can be partially suppressed by ectopic expressio n of either cdc2 or string, two genes that are known to promote the tr ansition from G(2) to M. We conclude that Dm myb is required for compl etion of cell division and may serve two independent functions: promot ion of mitosis, on the one hand, and prevention of endoreduplication w hen cells are arrested in G(2), on the other.