Cytokinetic ring contraction breaks the mitotic spindle when normal disassembly mechanisms are impaired. (A) Time-lapse images of cells expressing GFP-Tub1 and the cytokinetic ring marker Myo1-GFP during mitotic exit. Yellow dots indicate when the spindle breaks, whereas blue dots indicate when the cytokinetic ring has begun contracting. The frequency of spindle disassembly events that occurred after ring contraction is indicated to the right of each image series (n = 3 separate experiments each analyzing 20 disassembly events for each strain; mean ± SD). Note that cytokinetic ring contraction appears to break the spindle in kip3Δ and cdh1Δ cells but not in wild-type cells. Bar, 5 µm. (B) Cells were monitored as in Fig. 1 A, and the location of breakdown relative to the bud neck was recorded and normalized with the length of the cell (n = 34, 24, and 24 for wild-type, cdh1Δ, and kip3Δ strains, respectively). All numbers represent absolute values. kip3Δ and cdh1Δ cells were treated with 250 µM latrunculin A to inhibit actinomyosin ring contraction (n = 30 for each strain).