FIGURE 3:

Actin inhibits anterior pulling force generation. (A) Time-lapse series of a mitotic spindle in a control embryo, in which the midzone spindle is severed at anaphase onset (arrow). Note that the posterior spindle pole moves with a higher velocity than the anterior spindle pole. (B and C) Kymographs of mitotic spindles from a control (B) and a cytochalasin D–treated embryo (C) after midzone severing. Kymographs are taken from a single longitudinal line across the mitotic spindle. The spindle poles are visualized by GFP::TBB-2. Compared with control embryos (B), both spindle poles move with high velocities after midzone ablation in cytochalasin D–treated embryos (C). (D) Spindle pole peak velocities after severing the spindle midzone at anaphase onset with a UV laser. Values are shown for embryos treated with cytochalasin D, latrunculin A, or solvent only (control Cyto. D: egg buffer + 1% EtOH; control Lat. A : egg buffer plus 1% DMSO). Pole velocities in the latter control embryos were slightly higher than normal (average values ± SEM). Anterior pole velocity is significantly increased in the presence of cytochalasin D (p < 0.0007 for solvent compared with cytochalasin D; p < 0.3 for solvent compared with latrunculin A. Pole velocities in the latter control embryos were slightly higher than normal. n ≥ 10 embryos for each treatment.