Analyses of bent spindles reveal the mechanics of anaphase B in fission yeast
The mitotic spindle in the fission yeast Schizosaccharomyces pombe is a single bundle of microtubules which elongates to segregate the chromosomes during anaphase B. The mechanical properties of the spindle and the forces driving its elongation remain poorly defined. Here, we analyzed how spindles react to mechanical and genetic perturbations to uncover their mechanical properties. Treatment of cells with osmotic oscillations and blue light led to a consistent phenotype of spindle buckling and breakage in mid-anaphase. The stalling of pole separation and reduced rates of spindle elongation indicated that spindles elongate and buckle under increased mechanical load. The structural integrity of the bent spindles was dependent on Ase1 (PRC1), while the spindle elongation rate was dependent on motor proteins Klp9 (kinesin-6) and Cut7 (kinesin-5). Modeling of bent spindle shapes revealed that most spindles behave mechanically as a beam with a two-fold increase in rigidity in the midzone. Upon reaching a threshold size, bent spindles broke at a specific fragile site near the edge of the spindle midzone. Our findings in this simple fission yeast spindle are relevant to the mechanics of more complex metazoan spindles. Significance statementO_LIThe anaphase B spindle in S. pombe consists of a microtubule bundle that elongates to move the chromosomes apart. The various forces and mechanical properties of the spindle remain poorly quantified. C_LIO_LIThe authors establish a method to induce spindle buckling in mid-anaphase. Time-lapse imaging shows that these spindles elongate at reduced rates, buckle as a non-homogeneous beam under mechanical load, and break at a fragile site adjacent to the midzone. C_LIO_LIThese results provide quantitative and molecular insights into spindle force regulation and structural integrity that are relevant to mitosis in other cell types. C_LI