Selection for faster microbial expansion on surfaces causes heritable changes in cell shape that feedback on the population genetics of expanding colonies

Spatially structured microbial populations are ubiquitous in nature and display evolutionary dynamics that are not yet well characterized, despite their importance to human health and technology. Within dense, surface-associated microbial populations, mechanical interactions among cells can affect their evolutionary dynamics, for example by altering the rate at which beneficial mutations spread in these populations. We performed an evolutionary experiment with the budding yeast, Saccharomyces cerevisiae, to investigate how cells evolve when selected for surface-associated growth. We found that cells selected for faster expansion on surfaces evolved an elongated cell shape and a bipolar budding pattern, in which daughter cells bud at the pole opposite to the birth scar. In addition to an increased expansion speed, evolved populations are characterized by an increased genetic drift compared to the ancestor, that is, an increased stochasticity of the competition dynamics between different alleles, which we attribute to the change in cell shape. We are currently investigating the genetic changes responsible for the evolved phenotypes, via whole genome sequencing and reconstruction of putative causative mutations in the ancestor strain. In parallel, we are investigating how the evolved phenotypes allow cells to spread faster on surfaces, via numerical simulation of expanding colonies.