Developmental and cell cycle progression defects in drosophila hybrid males

Matings between D. melanogaster females and males of sibling species in the D. melanogaster complex yield hybrid males that die prior to pupal differentiation. We have reexamined a previous report suggesting that the developmental defects in these lethal hybrid males reflect a failure in cell proliferation that may be the consequence of problems ill miotic chromosome condensation. We also observed a failure ill cell proliferation, but find in contrast that the frequencies of mitotic figures and of nuclei staining For the mitotic marker phosphohistone H3 in the brains of hybrid male larvae are extremely low We also found that very few of these brain cells in male hybrids are in S phase, as determined by BrdU incorporation. These data suggest that cells in hybrid males are arrested in either the G(1) or G(2) phases of the cell cycle. The cells in hybrid male brains appear to he particularly sensitive to environmental stress; our results indicate that certain in vitro incubation conditions induce widespread cellular necrosis ill these brains, causing all abnormal nuclear morphology noted by previous investigators. We also document. that hybrid larvae develop very slowly particularly during the second larval instar. Finally, We found that. I-lie frequency Of mitotic figures ill hybrid male larvae mutant for Hybrid male resuce (Hmr) is increased relative to lethal hybrid males, although not to wild-type levels, and that chromosome morphology ill Hmr(-) hybrid males is also not completely normal.