DELAYS IN ANAPHASE INITIATION OCCUR IN INDIVIDUAL NUCLEI OF THE SYNCYTIAL DROSOPHILA EMBRYO

The syncytial divisions of the Drosophila melanogaster embryo lack som e of the well established cell-cycle checkpoints. It has been suggeste d that without these checkpoints the divisions would display a reduced fidelity. To test this idea, we examined.division error frequencies i n individuals bearing an abnormally long and rearranged second chromos ome, designated C(2)EN. Relative to a normal chromosome, this chromoso me imposes additional structural demands on the mitotic apparatus in b oth the early syncytial embryonic divisions and the later somatic divi sions. We demonstrate that the C(2)EN chromosome does not increase the error frequency of the late larva neuroblast divisions. However, in t he syncytial embryonic nuclear divisions, the C(2)EN chromosome produc es a 10-fold increase in division errors relative to embryos with a no rmal karyotype. During late anaphase of the neuroblast divisions, the sister C(2)EN chromosomes cleanly separate from one another. In contra st, during late anaphase of the syncytial divisions in C(2)EN-bearing nuclei, large amounts of chromatin often lag on the metaphase plate. L ive analysis of C(2)EN-bearing embryos demonstrates that individual nu clei in the syncytial population of dividing nuclei often delay in the ir initiation of anaphase. These delays frequently lead to division er rors. Eventually the products of the nuclei delayed in anaphase sink i nward and are removed from the dividing population of syncytial nuclei . These results suggest that the Drosophila embryo may be equipped wit h mechanisms that monitor the fidelity of the syncytial nuclear divisi ons. Unlike checkpoints that rely on cell cycle delays to identify and correct division errors, these embryonic mechanisms rely on cell cycl e delays to identify and discard the products of division errors.