Figure 3. ETI Mother Cells Age with Terminal Cellular and Mitochondrial Morphologies Distinct from LTI Senescence but Similar to those of Normal Mother Cell Aging.

(A) Three possible terminal death morphologies were observed in WT, ETI, and/or LTI (see text for definition) mother cells: small budded (type i), elongated (type ii), and a G2/M large-budded (“dumbbell” shape) (type iii). Mitochondrial volume was measured using mitochondrially localized GFP (mtGFP). (B) ETI and LTI populations of tlc1Δsml1Δ cells were prepared to distinguish cell death from normal mother cell aging and LTI senescence. tlc1Δsml1Δ LTI strains senesced and showed reduced lifespan as expected. (C) ETI cells terminally arrest in a manner similar to WT mother cells and distinct from LTI senescence. Most of the cells in ETI tlc1Δ and ETI tlc1Δ sml1Δ show type i or type ii death morphologies (>95%), similar to terminal WT mother cells. In LTI cells, a major fraction (~70%) displayed type iii morphology (p-value < 1e-4 compared to ETI tlc1Δ sml1Δ by Fisher’s exact test) indicative of senescence induced by critically short telomeres.