Constructing individuality: Ecological and evolutionary dynamics in experimental transitions to multicellularity

Abstract

The evolution of multicellularity transformed the history of life on our planet. Multicellularity involves the reorganization of previously autonomous cells into a more complex organism, however the ecological and evolutionary consequences of this reorganization remain poorly understood. This work explores experimentally the implication of reorganization for environmental change, as well as the costs and benefits of this transition in novel environments (UV radiation, and spatial structure), using a simple experimental system that evolved in our laboratory as the result of selection for larger size. Ten replicate populations of the unicellular strain Saccharomyces cerevisiae evolved under selection for faster settling, and selection in all of these populations resulted in the evolution of incipient multicellular phenotypes. Here, I evaluate how and under what conditions does adaptation occurs at a multicellular vs. unicellular levels. I expected that multicellularity would facilitate adaptation to UV radiation because multiple cells provide layers of protection for the internal cells. However, I found that the costs of a larger size are pervasive and adaptation at a multicellular level requires strong selective pressures. This work highlights the importance of space and the organization of cells as a consequence of the evolution of multicellularity: I show here how this organization has the potential to create novel ecological opportunities, new challenges for adaptation (like strong local competition) and different cellular micro-environments providing the potential for cellular differentiation.