The qualitative difference between stresses in their influence on the effect of mutations. (a,b) The growth rates of the individual mutants (dots) and the parental strain (square) under (a) acidic stress and (b) chloramphenicol stress, compared to their growth in the favorable environment. The acidic stress is seen to aggravate the effect of most mutations, while the chloramphenicol stress alleviates their effects. (c) Schematic representation of the possible impacts of stress on mutations. The main diagonal represents the geometric locus of mutants whose absolute growth rates are not affected by the stress (μS = μF). The equal-effect line represents the geometric locus of mutants whose relative growth rates are not altered by the stress (μS/νS = μF/νF, or αS = αF). Mutations above (or below) this line, shown in green (or red) are alleviated (or aggravated) under stress. (d) The distribution of distances of mutations from the equal-effect line. The area below the lines is normalized to 1. Lethality or very slow growth under the stress is represented by 'L' on the x axis. Positive (or negative) distance corresponds to mutations alleviated (or aggravated) under the stress.