Established determinants of replicative lifespan, including mitochondrial DNA, play a limited role in early yeast stress-resistance decline.

Baker's yeast is a long-standing model for studying mechanisms of aging. Due to asymmetric division, yeast mother and daughter cells are distinguishable, and mother cells usually cease division after 15-25 budding events. However, yeast suspensions consist predominantly of young cells. Here we investigated whether, under harsh environmental conditions, replicative aging-defined by the number of buds produced-manifests at early replicative ages. To test this, we adapted a flow-cytometry-based pulse-chase labeling method to track the survival of cells of increasing replicative age, from daughter cells to mother cells that had produced up to eight buds on average. We found that stress-induced mortality increased with replicative age across all nine stress conditions tested. Mitochondrial dysfunction, caused by depletion of mitochondrial DNA, altered basal stress resistance for some stresses but largely preserved the age-dependent decline in stress resistance across conditions. Similarly, deletion of FOB1, which extends replicative lifespan under optimal conditions, did not markedly alter this decline under heat shock or acetic acid. In contrast, deletion of SIR2, which shortens replicative lifespan, steepened the age-mortality slope under acetic acid stress but not under heat shock. Together, our data indicate that aging manifests early in life under various extreme environments, and that the genetic determinants of maximum replicative lifespan tested here are largely dispensable for the early decline in stress resistance.