Interactions between Cdk1-cyclin B and the cytoskeleton networks in the syncytial Drosophila embryo. J.Y. Ji , C. Trusty , G. Schubiger. Dept Zoology, Univ Washington, Seattle, WA.

   Microtubule (MT) and microfilament (MF) play pivotal roles in many cellular processes, such as mitosis, cytokinesis and organelle transport. The syncytial embryo is an ideal system to study the cytoskeleton networks. We have reported that compared to wild-type controls, preblastoderm embryos (cycles 4-7) from mothers with 4 or 6 cyclin B gene copies have higher Cdk1 activity, shorter microtubules, slower nuclear movement and longer metaphase with the same overall cell cycle time. Later at blastoderm stage, in the 6 cyclin B embryos, we observe chromosome bridges, abnormal nuclear distribution, small and large nuclei and local extra divisions. We did a loss-of-function genetic screen to detect suppressors and enhancers of the 6 cyclin B blastoderm phenotypes. Among the suppressors, we identified cdk1 and six MF regulatory genes (chic, dia, sqh, sced, Cdc42 and qua), all of which have normal blastoderm. How is this suppression of the 6 cyclin B-phenotype achieved? We analyzed MT morphology at the preblastoderm stage and found that in all but dia and Cdc42, MT volume and Cdk1 activities are more normal than those of 6 cyclin B embryos. Therefore, normalization of the blastoderm phenotype can result from the corrected MT morphology and function at the preblastoderm stage. On the other hand, dia and Cdc42 normalize blastoderm without correcting Cdk1 activity and MT morphology. Thus, there are at least two ways to restore a normal blastoderm. How does reducing a gene product that regulate MF dynamics affect Cdk1 activities and MT morphology? We see two alternative explanations. First, it was reported that lowering the MF stability affects stability of MT, where cyclin B is degraded. Therefore, a weaker MF network could increase MT volume and decrease cyclin B, thus reduces Cdk1 activities. Second and more speculatively, reducing MF regulating proteins such as Chic could indirectly promote cyclin B degradation and thus lower Cdk1 activity while increasing MT volume.