FIG. 2.
Types of compensatory mutations. The x axis shows how many mutations the genotype has compared to the wild type. The y axis shows the replicative fitness compared to that of the wild type. The fitness in the absence of drugs is shown in black, and the fitness in the presence of antibiotic treatment is shown in red. ++ denotes the wild type (wt), R+ the genotype with only the resistance mutation, C+ the genotype carrying only the compensatory mutation, and RC the resistant-compensated genotype. Three extreme types of compensation are possible. (A) The compensatory mutation may be beneficial only in the absence of drugs (eC,no drug = 1 and eC,drug = 0). (B) Conversely, compensation may occur only in the presence of treatment (eC,no drug = 0 and eC,drug = 1). (C) Finally, compensation may have the same efficiency in both environments (eC,no drug = 1 and eC,drug = 1). Other parameters take the following values: a = 0.9, cC = 0.1, cR = 0.2, and eR = 0.9.