Merotelic KTs shorten after MT severing. (A) Diagrammatic representation of the in vivo assay in which MTs attached to a merotelic KT are severed by laser. Laser ablation was used to sever one (single ablation) or both (double ablation) MT bundles attached to a merotelic KT in anaphase cells, releasing the forces acting on the KT. Change of the KT length after laser severing reflects its mechanical response. (B) Maximum intensity projection of time-lapse images of a merotelic KT (Hec1-GFP, green) in a PtK1 cell and the severing of one of the two MT bundles (α-tubulin–X-Rhodamine, magenta). Time 0 indicates the frame before ablation. The white lightning sign indicates the ablation site. Images were acquired with a time resolution Δt = 2.5 s. Close-up views show lagging KTs (2×). (C) Time projection of the area containing the merotelic KT from the cell in B. The boxed region indicates the time interval shown in B. (D) Relaxation kinetics of the merotelic KT from B and C. The red dashed line indicates the severing time. (E) Maximum intensity projections of time-lapse images of a merotelic KT (Ndc80-GFP, green) in a S. pombe cell and the severing (lightning sign) of the spindle (α-tubulin–mCherry, magenta) close to one spindle pole body. The time 0 is the frame before ablation. The spindle breakage occurred immediately after ablation. Images were acquired with a time resolution Δt = 2.8 s. Close-up views show lagging KTs (1.35×). (F) Time projection of an area containing the merotelic KT from the cell in E. The boxed region indicates the time interval shown in E. (G) Relaxation kinetics of the merotelic KT from E and F. The red dashed line indicates the severing time.