Genetic interactions and their environmental dependence.

The genetic interactions are indicated for three inverse LacI variants. Each row details the interactions between two mutations, each indicated by an X, either in a LacIwt background (denoted by a ○), or a single mutant background (denoted by a •). We consider three types of interactions: M, magnitude epistasis; S, sign epistasis; R, reciprocal sign epistasis. The mutation that changes sign is indicated between brackets. The data shows that most genetic interactions display different types of epistasis in each of the two environments. The significance of the phenotypic effect of mutations in LacI is tested with a t-test in conjunction with a Bonferroni correction for multiple comparisons (P