Modulation of mutation rates and adaptation of bacteria

Abstract

The level of genetic variability that maximises the fitness of population varies with the degree of its adaptation to the environment. It is low when environment is stable and high when environment is unstable and hostile. Since environmental conditions are changing, the adaptation is never permanent. Consequently, because the genetic variability in bacteria is first generated by mutagenesis, it could be expected that populations with high mutation rates would have better chance for successful evolution. Indeed, bacteria with elevated mutation rates are frequently found among natural isolates. Experimental observations and theoretical calculations suggest that there must be positive selection for higher mutation rate in spite of the fact that majority of newly generated mutations are deleterious or lethal. Mutator alleles rise to a high frequency through their association with the favourable mutations they generate that counterbalance the load of deleterious mutations. However, when adaptation is achieved, the load of deleterious mutations counterselects high-mutation rates. Therefore, evolution of bacterial populations may happen through alternating periods of high and low mutation rates that provide a remarkable potential for the fine tuning of the rates of generation of genetic variability in the function on the adaptation to environmental conditions. © 2006 Springer.