Experimental Tests of the Influence of Interactions Between Beneficial Mutations on Adaptation and Reproductive Isolation

Newly arising mutations create genetic variation which natural selection can act on to favor organisms better suited to the environment. The effect of any mutation can, however, not necessarily be determined independently, but may depend on complex interactions with the broader genetic background. Despite the clear importance of such interactions, known as epistasis, the understanding of epistasis between beneficial mutations and overall genetic background is limited. In this dissertation, I present three studies to examine the basis and extent of epistatic interaction between beneficial mutations and their genetic backgrounds. First, I introduced each of four beneficial mutations selected in a long-term evolution experiment into a set of natural isolates of Escherichia coli and measured the fitness of the constructed strains. I found that the fitness effect of beneficial mutations highly depended on their genetic backgrounds, and that there was a negative relationship between the fitness effect of beneficial mutations and the initial fitness of the genetic background in which the mutation was introduced. These results suggest that the ability of a strain to adapt in environment is, at least in part, determined by the current fitness of recipient strains Second, to understand how the genetic background affects the epistatic interactions between focal beneficial mutations, two beneficial mutations, in the genes pykF and topA, were introduced individually and in combination into seven natural isolates of E. coli. I found that the epistasis between these two mutations strongly depends on the genetic backgrounds and again the magnitude of epistasis tended to decrease as the fitness of the recipient strain increased. Finally, I tested how parallel and divergent adaptation affects reproductive isolation by examining the extent of epistasis between mutations obtained in different populations evolved either in the same or different environments. I found a prevailing negative deviation from expected fitness in recombinants from parents evolved in both the same and different environment, indicating that negative interactions between independently evolved mutations is one potential contributor to development of reproductive isolation. Overall, my work contributes to a growing body of work that demonstrates the importance of epistasis between beneficial mutations in major evolutionary processes.