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The role of pleiotropy in the maintenance of sex in yeast Hill, Jessica Anne

Abstract

Sexual reproduction is widespread, yet no comprehensive explanation for its ubiquity exists, despite a multitude of theories for sex. In order to determine the plausibility of these theories, it would be useful to know how easily sexual function can be lost. We have estimated the total rate of mutations that are deleterious to sexual function in a facultatively sexual organism, the yeast Saccharomyces cerevisiae. We propagated replicate lines of yeast at small population size (SMA) for 800 generations, so that all but the most deleterious of mutations were maintained. We estimated the sexual fitness (sporulation rate) of the evolved and ancestral SMA lines, thus allowing us to apply maximum-likelihood and Bateman-Mukai techniques to estimate the rate and nature of mutations affecting sexual function. The deleterious mutation rate per diploid per genome with respect to sexual function (Usex) was estimated as 2.3 x 10⁻⁴ while the average reduction in fitness of heterozygous mutants with respect to sexual function (Ssex) was estimated as 0.24, which is approximately double the value of estimates of mutational parameters in S. cerevisiae with respect to asexual growth. In a facultatively sexual organism like yeast, pleiotropy between asexual and sexual function is likely to occur. To determine what effect pleiotropy with asexual function was having on sexual function, we propagated replicate lines of yeast asexually at large population size (LMA). We determined the direction of pleiotropy (positive or negative) between asexual and sexual function by comparing the rate of decline in sexual function in the LMA and SMA lines. We found evidence for positive pleiotropy in our lines. This indicates that sexual function in yeast may be maintained by asexual function. We find that sex is lost at a slow rate in yeast and that the costs of sex are mitigated by pleiotropy with asexual function. In a separate study, I explored the interaction between DNA methylation and associated protein attachment, which is important for proper transcription and cell function. A model that examines the dynamics of DNA methylation and methyl-CpGbinding domain protein attachment in the context of selection at the level of the cell was developed. The approximate equilibrium values of the four possible states of the system were determined, and numerical simulations were performed, using realistic parameter values from prior studies when known. Two general conclusions emerge from this model: 1) selection among cells can alter epigenetic signals such as proteination attachment and methylation status, but 2) the efficacy of selection is dramatically reduced by high transition rates in methylation and proteination status.

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