Experimental evolution and intragenomic homologous recombination in Escherichia coli
Intragenomic homologous recombination (IHR) is a critical mechanism of DNA repair that can result in genome rearrangements. Like all mutations, genome rearrangements, i.e., large-scale deletions, inversions, duplications, and translocations, can be harmful, of neutral effect, or beneficial. To evaluate the role of genome rearrangements in bacterial evolution, I inserted the bacteriophage P1 ref gene into the Escherichia coli lac operon to create a strain with an elevated rate of IHR. I then maintained small populations of E. coli initiated either with the constructed strain (high IHR populations) or with the wildtype (low IHR populations) for 500 generations in a novel environment, characterized by glycerol minimal medium and periodic heat stress. Fitness assays using the generation 500 populations revealed that fitness gains were rapid, but did not differ in magnitude between the low and high IHR treatments. However, the high IHR populations followed adaptive trajectories that were significantly less variable than the low IHR populations. This reduced variability suggests that the evolution of the high IHR populations may have been more deterministic than that of the low IHR populations, due to an increased mutation supply rate. To elucidate the genetic basis of the adaptation to the novel evolution experiment environment, I fully sequenced the genomes of two clones, one low ref- and one ref+, from the endpoint of the evolution experiment. I found that the two clones had acquired non-overlapping sets of point mutations and rearrangement breakpoints, suggesting that they took alternate routes to adaptation. These studies represent the first steps in validating the lac<>ref system for use in studying IHR, and in assessing whether IHR rate has the potential to modulate adaptive evolution in small bacterial populations.
Genetics|Cellular biology|Microbiology|Evolution and Development
Fetridge, Evelyn Dey, "Experimental evolution and intragenomic homologous recombination in Escherichia coli" (2014). ETD Collection for Fordham University. AAI3611854.