Experimental *evolution of microsatellites in Saccharomyces cerevisiae and investigations into natural populations of Saccharomyces yeasts

The first part of this dissertation presents the results of a project designed to test among the proposed evolutionary mutational models of microsatellites. Twenty replicate populations of the budding yeast Saccharomyces cerevisiae , that started with no genetic variation were propagated for 2,000 generations with cycles of sporulation and mating. The accumulated microsatellite mutations were to be compared to results of computer simulations. Due to unforeseen evolved changes in the populations during the last few hundred generations of the experiment (loss of sporulation ability), reliable information was only available after 1,000 generations. Six populations were chosen at random and assayed at six microsatellite loci. The results at each locus were consistent among all populations. however, the amount and type of variation observed differed between loci. Most of the observed mutations were single-step, although at one variable locus, multi-step mutations were observed as well. Using a novel approach, the mutation rates were estimated and ranged from 1.0 x 10 -6 to 5.0 x 10-5. No firm conclusions could be drawn regarding the mutational model. The sporulation deficient strains provide a promising resource for future studies regarding the evolution and maintenance of sex. The second part of this dissertation presents work conducted to further explore natural populations of Saccharomyces cerevisiae and Saccharomyces paradoxus isolated in Northeast American woodlands. Characterization of ten microsatellite loci confirmed the previously reported population genetic structures (Kuehne 2005). However, genetic variation that corresponded with geographic origin was observed within clonal lineage, indicating that these populations have existed in the woodland environment long enough to begin diverging. These microsatellite data were also used to estimate population differentiation and effective population sizes. Moving past describing patterns of genetic variation, the biological interaction of these sympatric populations was investigated using individual mate choice trials and mating propensity assays. The results strongly suggest that prezygotic reproductive isolation exists in the form of species-specific mating kinetics. These mating differences may also explain the vastly different population genetic structures previously reported