Epistasis between new mutations and genetic background and a test of genetic canalization

The importance for fitness of epistatic interactions among mutations is poo rly known, yet epistasis can exert important effects on the dynamics of evo lving populations. We showed previously that epistatic interactions are com mon between pairs of random insertion mutations in the bacterium Escherichi a coli. In this paper, we examine interactions between these mutations and other mutations by transducing each of twelve insertion mutations into two genetic backgrounds, one ancestral and the other having evolved in, and ada pted to, a defined laboratory environment for 10,000 generations. To assess the effect of the mutation on fitness, we allowed each mutant to compete a gainst its unmutated counterpart in that same environment. Overall, there w as a strong positive correlation between the mutational effects on the two genetic backgrounds. Nonetheless, three of the twelve mutations had signifi cantly different effects on the two backgrounds, indicating epistasis. Ther e was no significant tendency for the mutations to be less harmful on the d erived background. Thus, there is no evidence supporting the hypothesis tha t the derived bacteria had adapted, in part, by becoming buffered against t he harmful effects of mutations.