Abstract

Cellular populations expanding in space often exhibit very crowded environments, where cells have to push onto each other to replicate, leaving little space in between. In these conditions, the spatial position of an individual can be the major determinant of its evolutionary success, as pioneers living at the front of the expansion are much more likely to leave offspring in this favorable position. This phenomenon, called allele surfing, has been shown to lead to counterintuitive effects, such as the accumulation of deleterious mutations and waste of potentially beneficial mutants. Here, I will present an experimental quantification of the effect of crowding on the fate of neutral mutations in flat E. coli colonies, which I will explain using simulations where cells behave similarly to granular materials. I will then show how more complex biofilms, which exhibit a three dimensional architecture, are able to bypass the problem and release mutants that are temporarily stuck in the core of the biofilm. Finally, I will switch perspective, put ourselves in the shoes of a virus and show how, by contrast, viral evolution can strongly benefit from a crowded cellular environment.