Population Genetics of Single-cell Variation in Microbial Growth

Microbial populations undergo multiple stages of growth, including a lag phase, an exponential growth phase, and a stationary phase. Mutations can therefore improve the frequency of a genotype not only by increasing its exponential growth rate, but also by decreasing the lag time or adjusting the yield (resource efficiency). However, recent experiments in E. coli and yeast have shown that these traits can also vary across genetically-identical single cells. Both the mean and variance of these distributions can evolve under mutation, selection, and genetic drift. To interpret this data in an evolutionary context, we develop a framework for population genetics with single-cell variation across multiple phases of microbial growth. We show how variation in single-cell lag times creates large fluctuations in lineage dynamics, allowing a mutation to fix more rapidly than would otherwise be expected. We also quantify how selection acts on this variability, which we use to predict patterns of coevolution for these growth traits.