There is also an Ssd1-mediated increase in stress tolerance and longevity in the nondividing state, but no significant increase in trehalose levels ( Li et al., 2013). In this case the mother-daughter asymmetry persists in that the quiescent daughter cells have a longer life span than the quiescent mother cells ( Li et al., 2013).
However, introduction of a full-length SSD1 allele to this strain enables 90% of the cells to fractionate as Q cells ( Li et al., 2009). In W303 haploid prototrophs, grown to saturation in rich medium, the Q cells make up about half the population. These cells lose the ability to reenter the cell cycle rapidly and those that can reenter do so much slower than Q cells ( Lee et al., 2016). The lighter nonquiescent cells are primarily old mothers that undergo reactive oxygen species accumulation and apoptosis. This daughter enrichment is consistent with numerous studies indicating that daughter cells asymmetrically inherit higher quality proteins, mitochondria, and other organelles that promote their longevity ( Lai et al., 2002 McFaline-Figueroa et al., 2011). They are primarily daughter cells and young mothers ( Allen et al., 2006). They are more thermotolerant and long-lived than their nonquiescent siblings ( Allen et al., 2006 Li et al., 2009). These are unbudded cells with 1N DNA content that have accumulated high levels of glucose in the form of trehalose, which contributes to their high density ( Shi et al., 2010). One of these three cell types is quiescent (Q cells) and can be separated from the nonquiescent (non-Q) cells by density gradient sedimentation ( Allen et al., 2006).
These three cell types arise in part from the highly asymmetric final cell divisions that cells undergo after they have taken up all the glucose from their medium ( Johnston et al., 1977 Li et al., 2013) and the elaboration of highly fortified cell walls ( Shimoi et al., 1998).
However, stationary phase cultures contain at least three distinct cell types based on light scattering alone ( Li et al., 2013). Most of what we know about quiescence in budding yeast has come from studies of yeast cultures grown to saturation or stationary phase. Faced with a waning nutrient supply, these cells down-regulate highly conserved signaling pathways that promote proliferation and redirect their gene expression and metabolism to stockpile nutrients and induce processes that promote long-term survival ( Lillie and Pringle, 1980 Sillje et al., 1999 Francois and Parrou, 2001 De Virgilio, 2012 McKnight et al., 2015 Young et al., 2017). In Saccharomyces cerevisiae, quiescence is typically induced by nutrient limitation, but it is not a starvation state. The advantage to studying quiescence in free-living, unicellular organisms is that there is no intervention or genetic manipulation required to follow the transition in or out of quiescence. Maintenance of this protective quiescent state and recovery from it promotes the long-term survival of both multi- and unicellular species. Quiescence is a critical and conserved alternative to proliferation. The cell wall integrity pathway also promotes entry, maintenance, and recovery from quiescence through the Rlm1 transcription factor. The ability of these mutants to enter quiescence, and their long-term survival in the quiescent state, can be rescued by exogenously added trehalose. Ssd1 and another RNA-binding protein, Mpt5 (Puf5), have parallel roles in quiescence in haploids. Introduction of SSD1 to W303 diploids switches fate, in that it rescues cellular quiescence and disrupts the ability to sporulate. This is the result of diploidy, not mating type regulation. W303 haploids can enter quiescence, but their diploid counterparts cannot. Others only sporulate, but if sporulation is disabled by heterozygosity at the IME1 locus, those diploids can enter quiescence. Some wild diploids can only enter cellular quiescence, which indicates that there are conditions in which sporulation is lost or selected against. The ability to enter quiescence is highly reproducible but shows broad natural variation. Diploids can also enter a protective, nondividing cellular state or quiescence. When faced with glucose and nitrogen limitation they can undergo meiosis and sporulate. Wild Saccharomyces cerevisiae strains are typically diploid.
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