Experimental evolution of cellular miniaturization reveals a mechanism for cell size evolution

Cell volume, a key determinant of physiology, is maintained by cell size homeostasis. Large deviations from typical size are often harmful, yet cell sizes have diverged drastically in evolution. How does size homeostasis evolve to support such diversity without impairing physiology? To address this, we used experimental evolution to select progressively smaller Saccharomyces cerevisiae cells. Over 1,500 generations, we achieved a six-fold volume reduction compared to wild type. Size homeostasis remained robust and evolutionary stable, with cells maintaining competitive fitness, indicating that dramatic volume changes need not compromise physiology. We investigated the genetic basis of cellular miniaturization through whole-genome sequencing of the evolving populations. We show that genetic manipulations of the G1 cyclin CLN3 and the Greatwall kinase RIM15 signaling cascades produce a fourteen fold change in cell size, with adaptive mutations recapitulating the evolved volumes and loss of-function mutations yielding enlarged cells. Our results demonstrate the evolutionary plasticity of cell size homeostasis and reveal a mechanism for eukaryotic cell size evolution. Significance statementCell size affects nearly every aspect of cell function and varies enormously among eukaryotes. How such diversity arises while cells remain stable and healthy is not well understood. Using experimental evolution in budding yeast, we selected for progressively smaller cells and identified mutations in conserved cellular pathways that enable stable size reduction without major fitness costs. These results show that gradual evolutionary tuning of core regulatory networks can support large changes in cell size while preserving normal function, providing an experimental framework to explore how fundamental cellular traits evolve.