Microbial Evolutionary Dynamics and Transport on Solid and Liquid Substrates

Abstract

The interplay between a population's spatial structure and transport can dramatically alter its evolutionary dynamics. In this thesis, we study this interplay by growing microbial range expansions, or microbial colonies expanding into unoccupied territory, on nutrient-laden agar plates and extremely viscous liquids. On agar plates, the main source of microbial population transport is diffusion mediated by nearby jostling due to dividing microbes. The genetic history of microbial range expansions is encoded in their spatial structure because diffusive transport does not move daughters far away from mothers within a generation. We study how selection and genetic drift at frontiers are impacted by the diffusive transport of four different genetically labeled strains of E. coli with differing expansion velocities on agar plates and show that a random walk model can predict their evolutionary dynamics. Next, we create an experimental system to study the impact of advective transport on the evolutionary dynamics of populations by growing microbial range expansions on the surface of a nutrient-laden fluid 10^4 to 10^5 times more viscous than water. The extreme viscosity allows microbes to live at the air-liquid interface as they settle extremely slowly. We find that microbes metabolizing at liquid surfaces generate intense, buoyancy-driven fluid flows that dramatically impact their colony morphology and evolutionary dynamics.