Chromosome numbers can vary widely in nature, even between closely related species. Despite this, the mitotic machinery must accurately segregate each chromosome every cell cycle. We sought to understand the physical constraints dictating chromosome number evolution. 2/9

Using a set of budding yeast strains in which the native chromosomes have been successively fused, we show that cells can tolerate large changes in karyotype. But, only up until a critical point: cells with fewer than 5 chromosomes experience a mitotic delay. 3/9