The meiotic differentiation program uncouples S-phase from cell size control in Saccharomyces cerevisiae.

Cell size homeostasis in proliferating cell populations is maintained in part by linking the initiation of a cell division cycle to growth such that a specific "size threshold" must be achieved prior to cell cycle initiation. In Saccharomyces cerevisiae the size threshold is responsive to environmental conditions and cell cycle entry is initiated at smaller sizes in poor growth medium and larger sizes in rich medium. In response to starvation for nitrogen and glucose diploid S. cerevisiae abandon proliferation and initiate gametogenesis (sporulation). The ability of both small and large cells to sporulate was investigated and the data reported here indicate that very small S. cerevisiae cells derived from the SK1 genetic background are able to sporulate. Although larger cells progress through the process more rapidly than small cells, sporulation of very small cells appears to be normal as they undergo meiotic recombination and form viable spores. Genetic analysis indicates that this trait is determined by multiple genes and may explain why some S. cerevisiae strains appear to display a minimal size threshold for sporulation. These data support a model where cell size controls can be bypassed or profoundly altered specifically to allow cellular differentiation in response to environmental signals.