The cell cycle oscillator and spindle length set the speed of chromosome separation in Drosophila embryos.

Anaphase is tightly controlled spatiotemporally to ensure proper separation of chromosomes. The mitotic spindle, the self-organized microtubule structure driving chromosome segregation, scales in size with the available cytoplasm. Yet, the relationship between spindle size and chromosome movement remains poorly understood. Here, we address this relationship during the cleavage divisions of the Drosophila blastoderm. We show that the speed of chromosome separation gradually decreases during the four nuclear divisions of the blastoderm. This reduction in speed is accompanied by a similar reduction in spindle length, ensuring that these two quantities are tightly linked. Using a combination of genetic and quantitative imaging approaches, we find that two processes contribute to controlling the speed at which chromosomes move in anaphase: the activity of molecular motors important for microtubule depolymerization and sliding and the cell cycle oscillator. Specifically, we found that the levels of multiple kinesin-like proteins important for microtubule depolymerization, as well as kinesin-5, contribute to setting the speed of chromosome separation. This observation is further supported by the scaling of poleward flux rate with the length of the spindle. Perturbations of the cell cycle oscillator using heterozygous mutants of mitotic kinases and phosphatases revealed that the duration of anaphase increases during the blastoderm cycles and is the major regulator of chromosome velocity. Thus, our work suggests a link between the biochemical rate of mitotic exit and the forces exerted by the spindle. Collectively, we propose that the cell cycle oscillator and spindle length set the speed of chromosome separation in anaphase.