Calibrating bacterial evolution

Abstract

Estimations of the rate of mutation in bacteria have been based on the assumption that the rates of accumulation of DNA sequence divergence are similar in prokaryotes and in higher eukaryotes. Unfortunately, the factors that determine the rate of molecular evolution, such as population size, generation time and mutation rate per generation, differ in bacteria and higher eukaryotes. Furthermore, the data estimated for bacterial species for which molecular evolution can be dated from ecological or geological evidence have sometimes been used as a reference, yet comparisons of the relative rates of mutagenesis at different classes of genomic loci indicate that they cannot be universally applied to all bacteria. However, there is evidence based on laboratory observations that microorganisms have a constant mutation rate per genome per generation. It has been demonstrated that, although the genome sizes and average mutation rates per base pair vary by 3–4 orders of magnitude, the mutation rate per genome is remarkably stable. Ochman et al.1 Ochman H. et al. Calibrating bacterial evolution. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 12638-12643 Crossref PubMed Scopus (327) Google Scholar compared the mutation rates estimated by sequence comparisons with those determined from laboratory experiments. They found that the genomic mutation rate for Escherichia coli and Buchnera does indeed seem to be constant across an evolutionary time scale. The estimated mutation rate is 0.0001–0.0002 per genome per replication, which is 30 times lower than the rate based on laboratory studies. If this pattern is true for other bacterial taxa, it will be valuable for calibrating bacterial evolution.