The cost of being big: local competition, importance of dispersal and experimental evolution of reversal to unicellularity

Multicellularity provides multiple benefits. Nonetheless, unicellularity is ubiquitous and there have been multiple cases of evolutionary reversal to a unicellular organization. In this paper, we explore some of the costs of multicellularity as well as the possibility and dynamics of evolutionary reversals to unicellularity. We hypothesize that recently evolved multicellular organisms would face a high cost of increased competition for local resources in spatially structured environments because of larger size and increased cell densities. To test this hypothesis we conducted competition assays, computer simulations, and selection experiments using isolates of Saccharomyces cerevisiae that recently evolved multicellularity. In well-mixed environments, multicellular isolates had lower growth rates relative to their unicellular ancestor due to limitations of space and resource acquisition. In structured environments with localized resources, cells in both multicellular and unicellular iso...