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Received Sep 7, 2017; Revised Dec 4, 2017; Accepted Jan 1, 2018
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1. Introduction
The Saccharomyces cerevisiae yeast has been one of the most frequently used model organisms in scientific studies, including studies of the mechanism of aging, as it was assumed that this mechanism is universal, at least for Fungi and Metazoa [1]. The contribution of yeast to such studies is based mainly on the analysis of replicative lifespan (RLS). This parameter is expressed as the number of daughter cells produced by a single “mother” cell during its life. This number is limited, as discovered by Mortimer and Johnston in 1959 [2], equaling an average of 20–30 generations. Having assumed that the number of daughters (buds) formed is a measure of the yeast cell’s age, it was acknowledged that factors influencing that number are associated with regulation of the aging process, which is responsible for the limited replicative lifespan of the yeast cells. Thus far, the explanation of the phenomenon of limited reproductive potential of yeast cells has mainly been based on the “senescence factor” accumulation hypothesis [3]. Such an accumulation would lead to a progressive loss of reproductive capabilities by the “mother” cell. The molecules proposed as the “senescence factor” were primarily extrachromosomal rDNA circles [4], oxidatively damaged proteins [5], protein aggregates [6], or damaged mitochondria [7, 8]. Explanations of the phenomenon were also based on genetic regulation [9]. The aforementioned reasons for the limited reproductive capacity of yeast cells inevitably suggested that this phenomenon should be attributed to the aging process.
An alternative explanation that includes reasons unrelated to aging but that are still determined by cell genotype is offered by the hypertrophy hypothesis, which emphasizes cell size and its important role in the regulation of the reproductive potential of yeast [10, 11]. There exists a clear relationship between cell size and the number of daughter cells produced by a single yeast cell [12–15]. It is connected with an inevitable increase in the cell size observed in successive reproductive cycles, which is a consequence of the evolutionary selection of budding as the...