Genetic background affects epistatic interactions between two beneficial mutations

Abstract

The phenotypic effect of mutations can depend on their genetic background, a phenomenon known as epistasis. Many experimental studies have found that epistasis is pervasive, and some indicate that it may follow a general pattern dependent on the fitness effect of the interacting mutations. These studies have, however, typically examined the effect of interactions between a small number of focal mutations in a single genetic background. Here, we extend this approach by considering how the interaction between two beneficial mutations that were isolated from a population of laboratory evolved Escherichia coli changes when they are added to divergent natural isolate strains of E. coli. We find that interactions between the focal mutations and the different genetic backgrounds are common. Moreover, the pair-wise interaction between the focal mutations also depended on their genetic background, being more negative in backgrounds with higher absolute fitness. Together, our results indicate the presence of interactions between focal mutations, but also caution that these interactions depend quantitatively on the wider genetic background.

Figure 1.

Relative fitness effect of topA and pykF mutations individually and in combination in each genetic background. Fitness was measured relative to the corresponding natural isolate strains. REL606 is the ancestor used to found the population in which the mutations were originally isolated. Other genotypes are natural isolates. Mean and s.e.m. are shown (natural isolates n = 4; REL606 n = 25).

Figure 2.

Relationship between maximum growth rate and the magnitude of epistasis. Points indicate the mean of estimates of maximum growth rate (n = 9) and magnitude of epistasis between topA and pykF mutations for the ancestor (n = 25) and seven natural isolate strains (n = 4). Error bars indicate 95% CI.