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Experiments of sexual selection and ploidy-specific effects of mutations in yeast Sandell, Nathalie Linnea Helene

Abstract

For organisms to genetically adapt, they need to have or acquire mutations that affect their survival and reproductive success. The effects of mutations depend not only on the abiotic factors extrinsic to the organism, but also its biotic interactions as well as the intrinsic state of the organism. Preexisting mutations in the genome will either favor or prevent certain adaptive paths. This thesis contributes knowledge of two aspects of life cycle evolution using the budding yeast Saccharomyces cerevisiae. My evolutionary experiment of phenotypic responses to sexual selection showed that the two mating types of yeast, though often considered indistinguishable, respond differently to increased amounts of mating competition. These results have implications for our understanding of mating system evolution. In a bioinformatic project, I found that changes to the molecular function of proteins do not directly correspond to fitness differences among lines with multiple known mutations in the lab. These results affirm that mutations in proteins act on a system of molecular reactions which determine fitness, and caution against assuming a direct relationship between effects on molecular function and fitness. Finally, I conducted an experiment to measure the fitness effects of mutations in haploids versus diploids. The experimental method used to produce the three genotypic states led to large changes in fitness, impeding our investigation. Most notably I observed a large loss in respiratory function among my experimental replicates that correlated strongly with the presence of the wildtype TID1/RDH54 gene. While unable to answer the question originally stated, the experimental lines produced are a valuable resource for investigations into how the nuclear recombination machinery may increase the likelihood of mitochondrial mutations. Together, these results illustrate the utility of laboratory experiments to both answer and pose new evolutionary questions about such fundamental phenomena as the evolution of sex and the fitness effect of mutations.

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