We developed inducible synthetic gene circuits that generate varying degrees of expression stochasticity of an antifungal resistance gene. We initiated laboratory evolutionary experiments by exposing the cells to gradually increasing antifungal stress. From the final day of evolution, we randomly selected 4-4 independently evolved clones, to get an insight into the molecular mechanisms of evolved fluconazole resistance and its dependence on phenotypic heterogeneity.. Phenotypic heterogeneity guides adaptive evolution

Genetically identical cells frequently display substantial heterogeneity in gene expression, cellular morphology and physiology and such phenotypic variation is occasionally transmitted across cell generations. It has been suggested that by rapidly generating a sub-population with novel phenotypic traits, phenotypic heterogeneity accelerates the rate of adaptive evolution in populations facing extreme environmental challenges. This issue is important as cell-to-cell phenotypic heterogeneity may initiate key steps in microbial evolution of drug resistance and cancer progression. Here, we study how stochastic transitions between cellular states influence evolutionary adaptation to a stressful environment in yeast Saccharomyces cerevisiae. We developed inducible synthetic gene circuits that generate varying degrees of expression stochasticity of an antifungal resistance gene. We initiated laboratory evolutionary experiments with genotypes carrying different versions of the genetic circuit by exposing the corresponding populations to gradually increasing antifungal stress. Phenotypic heterogeneity altered the evolutionary dynamics by transforming the adaptive landscape that relates genotype to fitness. Specifically, it enhanced the adaptive value of beneficial mutations through synergism between cell-to-cell variability and genetic variation. Remarkably, phenotypic heterogeneity evolved rapidly under directional selection pressure. We conclude that phenotypic heterogeneity shapes evolutionary trajectories and facilitates evolutionary rescue from a deteriorating environmental stress.