Mutation bias is a key predictor of adaptation rate

A growing body of work indicates that mutation bias — whereby some types of mutations occur more often than others — can influence the genetic basis of adaptation and the distribution of fitness effects of new mutations, with potentially important evolutionary consequences. Specifically, reversing an ancestral mutation bias (e.g., in Escherichia coli , shifting from a transition-biased to a transversion-biased mutation spectrum) enhances access to under-sampled variants, increasing the supply of beneficial mutations. However, the effect of mutation bias relative to other factors influencing adaptation remains unclear. By experimentally evolving wild type and mutator E. coli strains in different growth media, we show that mutation bias reversal facilitates early adaptation, altering the genetic basis of adaptation and the type of mutations accumulated under selection. Together, mutation bias, mutation rate, and initial fitness correctly predicted relative adaptation rates for any pair of strains. Our results highlight multiple ways in which mutational processes can shape genetic and phenotypic outcomes of adaptation.