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 <i>Saccharomyces cerevisiae</i> 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 <i>CLN3</i> and the Greatwall kinase <i>RIM15</i> 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.