Figure 1

The possible influences of environmental stresses on the effects of mutations on fitness. Shown are schematic reaction norms of a wild-type strain (solid line) and three different mutants (dashed lines). The wild-type growth rates in favorable and stressful conditions are represented by ν_F and ν_S, respectively. The growth rates of each specific mutant in these environments is represented by μ_F and μ_S, respectively. The effects of mutations in favorable and stressful environments are illustrated; they are defined as α_F ≡ log(ν_F/μ_F) and α_S ≡ log(ν_S/μ_S), respectively. The effect of a specific mutation could be alleviated (α_S < α_F, green), unchanged (α_S = α_F, black) or aggravated (α_S > α_F, red) under stressful conditions. The average mutation effects under favorable and stressful conditions and , are calculated by averaging α_F and α_S over a set of random mutations. We define a stress as alleviating (or aggravating) mutation effects if the average mutation effect is decreased, < (or increased, >) by the stress.