Résumé
Genotypes exhibiting an increased mutation rate, called hypermutators, can
propagate in microbial populations because they can have an advantage due
to the higher supply of beneficial mutations needed for adaptation.
Although this is a frequently observed phenomenon in natural and
laboratory populations, little is known about the influence of parameters
such as the degree of maladaptation, stress intensity and the genetic
architecture for adaptation on the emergence of hypermutators. To address
this knowledge gap, we measured the emergence of hypermutators over ~1000
generations in experimental Escherichia coli populations exposed to
different levels of osmotic or antibiotic stress. Our stress types were
chosen based on the assumption that the genetic architecture for
adaptation differs between them. Indeed, we show that the size of the
genetic basis for adaptation is larger for osmotic stress compared to
antibiotic stress. During our experiment, we observed an increased
emergence of hypermutators in populations exposed to osmotic stress but
not in those exposed to antibiotic stress, indicating that hypermutator
emergence rates are stress-type-dependent. These results support our
hypothesis that hypermutator emergence is linked to the size of the
genetic basis for adaptation. In addition, we identified other parameters
that covaried with stress type (stress level and IS transposition rates)
that might have contributed to an increased hypermutator provision and
selection. Our results provide a first comparison of hypermutator
emergence rates under varying stress conditions and point towards complex
interactions of multiple stress-related factors on the evolution of
mutation rates.