| The process of evolution by natural selection has been known for a century and a half, yet the mechanics of selection are still poorly understood. In most cases where natural selection has been studied, the genetic and physiological bases of fitness variation that result in population changes were not identified, leaving only a partial understanding of the forces that drive natural selection. Starved cultures of the bacterium Escherichia coli present a model system with which to address the genetic and physiological bases of natural selection. This is a model system that also reflects the “normal” state of bacteria in the natural world; due to intense competition for nutrients, microorganisms spend the majority of their lives under starvation conditions. The survivors of this prolonged starvation are evolved mutants that express a Growth Advantage in Stationary Phase (GASP) phenotype.; To explore the mechanistic bases of natural selection during prolonged starvation of microbes, my thesis focused on the genetic and physiological analysis of a survivor of an aged culture that acquired multiple advantageous mutations. This mutant has acquired four beneficial mutations, which act to increase fitness in an additive fashion. In this model system, we found that selection favors mutants with pleiotropic effects on cell physiology, including increased amino acid scavenging and metabolism. Two of the GASP mutations are in genes that encode transcription regulators, and appear to confer fitness gains through global physiological changes. One GASP mutation has been found to create a new niche for the organism by activating a cryptic operon encoding a transporter for several amino acids. The nature of the latter mutation is an inversion between the upstream regions of two genes, which activates the transporter operon while inactivating the other gene. That the inactivated gene is also involved in nutrient acquisition and stationary phase fitness demonstrates that evolution can select for forfeiture of one ecological niche for the creation of another. Finally, we have learned that epistatic effects can dictate the order of accumulation of adaptive mutations. |
