Date of Award
Program or Major
Master of Science
Rare, beneficial mutations that increase an organism's fitness provide the basis by which adaptation proceeds. Current theory predicts that the individual fitness effects of these beneficial mutations are exponentially distributed, suggesting that mutations conferring a small fitness increase are more numerous than those of large benefit. However, there is little empirical evidence describing measurable fitness effects of individual mutations, nor their availability or effects across a range of environments. We experimentally evolved a single strain of the cystic fibrosis pathogen Burkholderia cenocepacia under both physically structured (biofilm) and unstructured (planktonic) conditions, collected a sample of mutants, and measured the fitness effect of each in direct competition with the ancestor. Fitness was also measured in a variety of alternative environments to quantify the pleiotropic, or indirect, effects of each mutation. We found that the distribution of direct mutational effects was better modeled by an extreme value distribution with a truncated, Weibull-like domain of attraction, rather than exponential. A clustering of high fitness values and parallel evolution at the nucleotide level indicate that mutations greatly increasing fitness are more readily available to an adapting population than previously assumed. Pleiotropic effects were generally positive, although mutants did experienced a fitness trade-off under some alternative conditions, suggesting that highly beneficial mutations in a structured environment are likely specific to that biofilm environment and may ultimately narrow the organism's niche breadth. We also found that the magnitude of direct and pleiotropic fitness effects were strongly correlated, indicating that mutations of higher initial benefit in the selective environment also drastically influence fitness in alternative environments, the negative effects of which may bar their success under fluctuating conditions.
Staples, Rachel Katherine, "The distribution of direct and indirect fitness effects of beneficial mutations" (2012). Master's Theses and Capstones. 768.