Adaptive Mutation: How Growth under Selection Stimulates Lac + Reversion by Increasing Target Copy Number

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Abstract
From the time of Darwin until about 1950, a controversy continued over whether selective stress induces mutations or only affects the relative reproductive success of organisms with different genotypes (30). The controversy was resolved by the classic experiments of Luria and Delbruck (27) and of Leder- berg and Lederberg (25), who showed that some bacterial mutants arise prior to application of the selection that allows their detection and thus could not have been caused by selec- tive conditions. However, these experiments used lethal selec- tions and therefore did not eliminate the possibility that an- other fraction of total mutations might be formed in response to stress and be detected only by nonlethal selection. Shapiro and Cairns et al. reopened the controversy by pointing out this caveat and presenting data that seemed to support stress-in- duced mutation (7, 45). Because very few genetic systems behave in ways that sug- gest stress-induced mutation, the rare cases that seem to ex- hibit such behavior have attracted close attention. In one case, mutants were later shown to preexist selection (14, 28, 29, 44). For the system devised by Cairns and Foster (5), we suggest that reversion occurs by a multistep process initiated prior to selection and the appearance of stress-induced mutagenesis results from growth under strong selection.