Evolutionary stability of social commitment

Conflict resolution between individual cells and a group is essential for multicellularity. The social amoeba Dictyostelium discoideum switches between solitary growth and social fruitification depending on nutrient availability. Under starvation, cells form fruiting bodies consisting of spores and non-viable altruistic stalk cells. Once cells socially committed, they complete fruitification even with a renewed source of nutrients. This social commitment is puzzling because it deprives individual cells of benefits of quickly resuming solitary growth. One idea posits that traits that facilitate premature de-commitment are somehow hindered from being selected. We studied outcomes of premature de-commitment by forced refeeding. We show that when refed cells resume sociality together with non-refed cells, besides some becoming solitary outside of fruiting bodies, a large fraction was redirected to a sub-region of altruistic stalk regardless of their original fate. The refed cells exhibited reduced cohesivity and were sorted out to the altruistic positions in morphogenesis. Furthermore, a theoretical model considering evolution of cell-cell association revealed a valley in the fitness landscape that prevents invasion of de-committing mutants. Our results provide a general scheme that naturally penalizes withdrawal from a society by evolving a specific division of labor that less cohesive individuals become altruists. Significance StatementEvolution of unicellular to multicellular organisms must resolve conflicts of reproductive interests between individual cells and the group. In the social amoeba Dictyostelium, a transition from a solitary to multicellular group occurs under starvation. Once cells commit themselves to multicellular organization, the process continues even when shifting to an environment that favors solitary growth. Our study revealed that cells forced to partially revert to a de-committed state take an altruistic role through interaction with socially committed cells. The de-committed cells exhibited reduced cohesivity and were sorted out to altruistic positions in morphogenesis. This inevitably penalizes selfish cells that revert to solitary growth too quickly. Our results explain group-level behavior that is apparently difficult to understand from an individual-level fitness.