After finding that cells in old yeast cultures die with the typical membranous and nuclear markers of apoptosis, the authors looked for a motive. Cells overexpressing Yap1p, which mediates the yeast stress response, enjoy prolonged survival in aging cultures, indicating that the death response is triggered by stress. Deleting the caspase YCA1 improves short-term survival, but YCA1 deletants fail to regrow when transferred from an old culture to fresh medium, and wild-type cells out-compete YCA1 deletants in cocultures. Yeast cells undergoing apoptosis release low molecular weight substances that improve the growth of old cultures.
Herker et al. argue that, in the wild, apoptosis allows a clonal population of yeast to ensure the survival of its genes through lean times, by killing off less fit individuals to conserve nutrients, and promoting the survival of only the healthiest descendants. The results also explain why yeasts die in minimal medium, but survive in distilled water. In the complete absence of any nutrients, apoptosis is presumably inhibited. ▪