THE DROSOPHILA MATERNAL-EFFECT MUTATION GRAPES CAUSES A METAPHASE ARREST AT NUCLEAR-CYCLE-13

grapes (grp) is a second chromosome (36A-B) maternal-effect lethal mut ation in Drosophila melanogaster. We demonstrate that the syncytial nu clear divisions of grp-derived embryos are normal through metaphase of nuclear cycle 12. However, as the embryos progress into telophase of cycle 12, the microtubule structures rapidly deteriorate and midbodies never form. Immediately following the failure of midbody formation, s ister telophase products collide and form large tetraploid nuclei. The se observations suggest that the function of the midbody in the syncyt ial embryo is to maintain separation of sister nuclei during telophase of the cortical divisions. After an abbreviated nuclear cycle 13 inte rphase, these polyploid nuclei progress through prophase and arrest in metaphase. The spindles associated with the arrested nuclei are stabl e for hours even though the microtubules are rapidly turning over. The nuclear cycle 13 anaphase separation of sister chromatids never occur s and the chromosomes, still encompassed by spindles, assume a telopha se conformation. Eventually neighboring arrested spindles begin to ass ociate and form large clusters of spindles and nuclei. To determine wh ether this arrest was the result of a disruption in normal development al events that occur at this time, both grp-derived and wild-type embr yos were exposed to X-irradiation. Syncytial wild-type embryos exhibit a high division error rate, but not a nuclear-cycle arrest after expo sure to low doses of X-irradiation. In contrast, grp-derived embryos e xhibit a metaphase arrest in response to equivalent doses of X-irradia tion. This arrest can be induced even in the early syncytial divisions prior to nuclear migration. These results suggest that the nuclear cy cle 13 metaphase arrest of unexposed grp-derived embryos is independen t of the division-cycle transitions that also occur at this stage. Ins tead, it may be the result of a previously unidentified feedback mecha nism.