Rates of DNA sequence evolution in experimental populations of Escherichia coli during 20,000 generations.

We examined rates of DNA sequence evolution in 12 populations of Escherichia coli propagated in a glucose minimal medium for 20,000 generations. Previous work saw mutations mediated by mobile elements in these populations, but the extent of other genomic changes was not investigated. Four of the populations evolved defects in DNA repair and became mutators. Some 500 bp was sequenced in each of 36 genes for 50 clones, including 2 ancestral variants, 2 clones from each population at generation 10,000, and 2 from each at generation 20,000. Ten mutations were found in total, all point mutations including mostly synonymous substitutions and nonsynonymous polymorphisms; all 10 were found in mutator populations. We compared the observed sequence evolution to predictions based on different scenarios. The number of synonymous substitutions is lower than predicted from measured mutation rates in E. coli, but the number is higher than rates based on comparing E. coli and Salmonella genomes. Extrapolating to the entire genome, these data predict about 250 synonymous substitutions on average per mutator population, but only about 3 synonymous substitutions per nonmutator population, during 20,000 generations. These data illustrate the challenge of finding sequence variation among bacterial isolates that share such a recent ancestor. However, this limited variation also provides a useful baseline for research aimed at finding the beneficial substitutions in these populations.