A size-invariant bud-duration timer enables robustness in yeast cell size control

Cell populations across nearly all forms of life generally maintain a characteristic cell type-dependent size, but how size control is achieved has been a long-standing question. Prior work has uncovered diverse size control strategies operating at distinct cell cycle stages, but it is unclear how these numerous pathways are integrated to provide robust, systems-level cell size control for any organism. Here, we probe cell growth and size control in budding yeast that can be reversibly blocked from bud initiation. While blocked, cells continue to grow isotropically, increasing their volume by more than an order of magnitude over unperturbed cells. Upon release, these giant yeast resume budding and the population returns to its initial volume distribution within a few cell division cycles. Size control under these conditions does not require an explicit molecular size sensor. Instead, our observations are consistent with a size-invariant bud growth timer specifying the duration of S/G2/M to limit daughter cell size.