Mitotic spindle width scales with genome size across eukaryotes following a universal power law with exponent ~1/3, driven by inter-chromosome pushing forces arising from chromosome crowding within the metaphase plate, while spindle length is independently regulated by microtubule-associated proteins, establishing that chromosome mechanical properties rather than microtubule self-organization determine spindle geometry.

Cells regulate the size of their internal structures to maintain function in diverse biological settings. The mitotic spindle, a molecular micro-machine responsible for chromosome segregation, must scale to accommodate genomes varying in size by over 10,000-fold across eukaryotes. Yet, how spindle biomechanics adapts to vastly different genome sizes remains unknown. Here, we uncover a universal spindle scaling law, where metaphase plate width scales with genome size following a power law with an exponent of ∼1/3. We hypothesize that chromosome crowding within the metaphase plate generates compressive forces as chromosomes push against each other, thereby determining spindle size and shape. Our experiments with altered chromosome number and mechanical properties in healthy and cancerous human and mouse cells, together with a theoretical model based on inter-chromosome pushing forces and mechanical manipulations of cells, confirm this hypothesis. Extending these insights across eukaryotes, we demonstrate that chromosome crowding predicts the observed power-law scaling. The biophysical constraint of chromosome crowding offers a mechanistic explanation for the evolution of open mitosis and mitotic cell rounding, enabling the division of larger genomes. Spindle adaptability to larger genomes may promote the proliferation of polyploid cells, driving not only tumor progression but also speciation during evolution.