Normal mutation rate variants arise in a Mutator (Mut S) Escherichia coli population

Abstract

The rate at which mutations are generated is central to the pace of evolution. Although this rate is remarkably similar amongst all cellular organisms, bacterial strains with mutation rates 100 fold greater than the modal rates of their species are commonly isolated from natural sources and emerge in experimental populations. Theoretical studies postulate and empirical studies teort the hypotheses that these "mutator" strains evolved in response to selection for elevated rates of generation of inherited variation that enable bacteria to adapt to novel and/or rapidly changing environments. Less clear are the conditions under which selection will favor reductions in mutation rates. Declines in rates of mutation for established populations of mutator bacteria are not anticipated if such changes are attributed to the costs of augmented rates of generation of deleterious mutations. Here we report experimental evidence of evolution towards reduced mutation rates in a clinical isolate of Escherichia coli with an hyper-mutable phenotype due a deletion in a mismatch repair gene, (ΔmutS). The emergence in a ΔmutS background of variants with mutation rates approaching those of the normal rates of strains carrying wild-type MutS was associated with increase in fitness with respect to ancestral strain. We postulate that such an increase in fitness could be attributed to the emergence of mechanisms driving a permanent "aerobic style of life", the negative consequence of this behavior being regulated by the evolution of mechanisms protecting the cell against increased endogenous oxidative radicals involved in DNA damage, and thus reducing mutation rate. Gene expression assays and full sequencing of evolved mutator and normo-mutable variants supports the hypothesis. In conclusion, we postulate that the observed reductions in mutation rate are coincidental to, rather than, the selective force responsible for this evolution.

Figure 1. Distribution of mutation frequencies of E. coli ECU24 mutator strain along 62 serial passages.

In the y axis, mutation frequencies; in the x axis, numbers of the serial passages in which mutation frequencies were determined (three colonies per passage, 186 in total); note that for clarity not all passages are numbered in the figure. Top box in black continuous line refers to the range of mutation frequencies (80) determined for the original mutator strain; down box in black broken line refers to the range of mutation frequencies determined in the mutator strain after complementation with the wild-type mutS gene. Grey squares correspond to colonies with high mutation frequencies where mutation rates were determined (Luria-Delbrück method); black circles, to colonies with low mutation frequencies where mutation rates were obtained.

Figure 2. Mutation rates of colonies with high and low mutation frequencies obtained in the same passage, along time.

In the y axis, mutation rates (Luria-Delbrück method); in the x axis, numbers of the serial passages in which mutation rates were determined. Bars in grey correspond to colonies with high mutation frequencies; white dotted bars correspond to colonies with low mutation frequencies.

Figure 3. Distribution of the proportion of colonies with high and low mutation frequencies along four successive periods.

On the top, distribution of f values in 20 colonies of the ancestor strain; below, distributions of f values in 45–51 colonies from 15 different passages belonging to each one of the 1^st (from 13^th to 53^rd passage), 2^nd (from 58^th to 93^rd passage), 3^rd (from 97^th to 138^th passage) and 4^th (from 141^st to 180^th passage) periods respectively. Black triangles and grey circles correspond to colonies with high- or low frequencies of mutation respectively.

Figure 4. Mutation rates of cultures along the long-term sequential passages experiment.

In the y axis, mutation rates (Luria-Delbrück method) in the x axis, numbers of the serial passages in which mutation rates were determined. Bars reflect the corresponding ranges. The last column on the right side is the mutation rate of the original mutator strain after complementation with the wild-type mutS gene, serving as a control for non-mutator mutation rate.