A characterization of adaptive mutations in yeast

Natural selection acts on phenotypes within populations, yet it is allele frequency changes at the genetic level that enable adaptation. To properly understand the evolutionary process we thus need to understand how the genotypic and external environments affect beneficial mutations and, in turn, affect the fitness of individuals. In this thesis I used the budding yeast, Saccharomyces cerevisiae to explore the genotypic basis, phenotypic diversity, and fitness effects of beneficial mutations in a variety of genotypic and external environments. I first describe fitness experiments designed to elucidate the factor that allowed diploid mutants to overtake haploid populations during batch culture evolution. I compared haploid and diploid lines isolated at many time points using multiple growth phase and competitive fitness assays, yet diploids failed to demonstrate an advantage for any measure. I then conducted a related set of experiments that compared the rate of adaptation of haploid and diploid populations across seven different environments. I found that although haploid populations adapted faster than diploids in all environments, there was considerable variation between ploidy populations and among environments. Experimental evolution results can be difficult to explain without knowledge of the specific mutations involved. The remainder of this thesis thus focused on a set of 20 unique beneficial mutations I acquired that confer tolerance to nystatin, a fungicide. The mutations are in four different genes that act close together late in the ergosterol biosynthesis pathway. Although the genetic basis of adaptation was narrow, lines that carried mutations in different genes were not equally tolerant to nystatin and were found to exhibit different gene-by-environment interactions. Surprisingly, the mutations had a larger effect size in nystatin in a haploid background than in a homozygous diploid background. I then show that the dominance of these mutations (i.e., the degree to which mutations in a heterozygote behave like wildtype) was not constant between environments. Heterozygotes grew stochastically under nystatin stress, and resequencing uncovered rapid and pervasive loss of heterozygosity. Combined, this work demonstrates that both ploidy and the environment can have a large influence on the effect of beneficial mutations and illustrates the often-dynamic nature of evolution.Science, Faculty ofZoology, Department ofGraduat