ENVIRONMENT-DEPENDENT CAUSES AND CONSEQUENCES OF MUTATION IN SACCHAROMYCES CEREVISIAE

Abstract

Environmental effects on mutation have been documented for many years but have concentrated on agents that directly interact with DNA. Mutation research in its early history investigated a variety of more mundane environmental factors at levels that inhibited biological function and attempted to characterize their mutagenicity. This thesis revisits these old questions armed with more modern methods. It consists of one review chapter and three experimental chapters. The review chapter proposes that biological organization itself acts to direct mutation pressure, and that many mutations are context dependent within this organization. Experimentally, I performed an approximately 1,500-generation mutation accumulation (MA) experiment using the budding yeast Saccharomyces cerevisiae as an evolutionary genetic model. This thesis investigates the rates and distribution of effects of new mutations on fitness when they accumulate under a moderate salt stress. The first experimental section describes the production of the MA lines, measures the diploid fitness traits mitotic growth rate and sporulation, and uses changes in fitness among replicate lines to infer mutation parameters affecting these traits. Mutation rate estimates for these traits were roughly doubled in the salt stress treatment. The proportion of beneficial mutations was high for mutations affecting sporulation in both MA treatments but zero for growth rate. Measurements of haploid viability and haploid growth rate on strains derived from the diploid MA lines were used to infer mutation parameters. Mutation rates affecting haploid growth were ten-fold higher in our salt-line derivatives than those derived from the non-stress treatment. Variance component analysis identified a large fraction of genetic variation arising from differences among haploids within the same tetrad. This component was significantly larger in the salt MA treatment than the non-stress treatment. MA lines were subjected to a novel weak-acid stress. Mutation rate estimates were 38-fold higher in the salt MA treatment when lines were tested under acid stress. Cross-environmental genetic correlation for growth in acid stress versus standard media was significantly different between the two MA treatments suggesting that both MA environment and test environment are important factors when considering mutational effects on fitness.