Abstract
The yeast Saccharomyces cerevisiae typically divides asymmetrically to give a large mother cell and a smaller daughter cell. As mother cells become old, they enlarge and produce daughter cells that are larger than daughters derived from young mother cells. We found that occasional daughter cells were indistinguishable in size from their mothers, giving rise to a symmetric division. The frequency of symmetric divisions became greater as mother cells aged and reached a maximum occurrence of 30% in mothers undergoing their last cell division. Symmetric divisions occurred similarly in rad9 and ste12 mutants. Strikingly, daughters from old mothers, whether they arose from symmetric divisions or not, displayed reduced life spans relative to daughters from young mothers. Because daughters from old mothers were larger than daughters from young mothers, we investigated whether an increased size per se shortened life span and found that it did not. These findings are consistent with a model for aging that invokes a senescence substance which accumulates in old mother cells and is inherited by their daughters.