Can Unicellular Organisms Sequester a Germline? The Yeast-Germline Hypothesis.

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Germline mutations can affect future generations, while somatic mutations cannot. This germline-soma distinction does not seem to make sense for unicellular organisms. We challenge this view, arguing that baker's yeast (Saccharomyces cerevisiae) has a germline. Under aerobic conditions yeast cells use mainly fermentation of glucose to produce ethanol. Only when glucose is exhausted, cells switch to full respiration of the produced ethanol. We hypothesize that only a subset of the cells continue dividing and switch to respiration. A change from exponential to linear growth is consistent with asymmetrical cell division, where a senescing mother cell produces quiescent daughter cells. We thus propose that most cells produced during fermentation are "somatic," that is, they rapidly lose reproductive capacity, while the cells continuing to divide constitute the germline, as they exclusively produce rejuvenated quiescent cells. We discuss biased DNA-template strand inheritance by the mother cell as a potential adaptive explanation for germline sequestration to reduce the mutation rate.