Mechanical heterogeneity and roles of parallel microtubule arrays in governing meiotic spindle length

Metaphase spindles are arrays of microtubules whose architecture provides the mechanism for regulated force generation required for proper segregation of chromosomes during cell division. Whereas long-standing models are based on continuous antiparallel microtubule arrays connecting two spindle poles and overlapping at the equator, spindles typically possess a more complex architecture with randomly arranged short filaments. How these heterogeneous multifilament arrays generate and respond to forces has been mysterious, as it has not been possible to directly measure and perturb spindle force while observing relevant filament motility. Here, we combined microneedle-based quantitative micromanipulation with high-resolution microtubule tracking of Xenopus egg extract spindles to simultaneously examine the force and individual filament motility in situ. We found that the microtubule arrays at the middle of the spindle half are considerably weak and fluid-like, being more adaptable to perturbing forces as compared to those near the pole and the equator. We also found that a force altering spindle length induces filament translocation nearer the spindle pole, where parallel microtubules predominate, while maintaining equatorial antiparallel filaments. Molecular perturbations suggested that the distinct mechanical heterogeneity of the spindle emerges from activities of kinesin-5 and dynein, two key spindle motor proteins. Together, our data establish a link between spindle architecture and mechanics, and highlight the importance of parallel microtubule arrays in maintaining its structural and functional stability.