Effects of mutation rate and bias on bacterial mutator dynamics

Abstract

The impact of mutation rate on evolutionary dynamics has been well-studied. It is well-known that an increased mutation rate, seen in mutator strains of bacteria, speeds up adaptation to novel environments. However, mutators often also have an altered mutation bias i.e. the frequencies of different types of mutations sampled by the population change. Though changes in mutation rate and mutation bias are coupled, the evolutionary consequences of changes in bias are not very well understood. A recent study showed that reversing the ancestral mutation bias of a population confers a benefit, by allowing the population to access beneficial mutations from mutational space that was under-sampled by the ancestor. A key prediction emerges from this result: given similar mutation rates, a mutator with a reversed bias will sample more beneficial mutations than a mutator with an ancestral bias and would therefore be able to invade a WT population faster. In this study, we look at the rate of invasion and fixation for several mutators with similar mutation rates and opposite biases. We find that at high mutation rates, a mutator with a reversed bias invades and fixes faster than mutators with a reinforced ancestral bias, which is consistent with our predictions. More work is currently ongoing to test other consequences of bias reversal. In general, the effect of change in mutation bias could add on to the known effects of changes in mutation rate. Thus, changes in mutation rate and bias could jointly govern a population’s access to beneficial and deleterious mutations, with implications for adaptive dynamics of populations, and for genome evolution.