Microtubule perturbation by drugs and local laser ablation reveals an important role of centrosomal asters for nuclear separation. (A and B) Low doses of colcemid (0.5–1.0 µM) allow chromosome segregation but reduce the size of microtubule asters in preblastoderm embryo extract, as shown by fluorescence microscopy (A) and as a graph of quantified aster radius (black; n ≥ 20, representative out of three repeats; B) compared with untreated extract (gray; from Fig. 1 G). Nuclei rotated away from the spindle axis (yellow arrowheads in A). (C) Distance–time plot of daughter DNA masses in colcemid-treated extract (solid lines) showing a significant reduction (P < 0.001) of both the fast and slow phase of DNA separation, as compared with the control (gray; from Fig. 1 E). (D) Fluorescence microscopy images of central spindle (midzone) ablation by a UV laser beam during telophase (blue arrowhead). (E) Graph illustrating continued nuclear separation between DNA masses (black lines and arrows indicate ablation; red arrows indicate repeated ablation). (F and G) Additional single centrosomal aster ablation caused the associated nucleus to pause, reducing nuclear separation (dashed black lines). Additional combined ablation (arrows) of both centrosomes abolished postanaphase separation (solid line and G). Each distance–time curve represents an independent experiment. DNA is shown in red, and microtubules are shown in green. Time is shown in minutes/seconds. Bars, 5 µm.