Quantification of social behavior in D. discoideum reveals complex fixed and facultative strategies.

Understanding the maintenance of cooperation requires an understanding of the nature of cheaters and the strategies used to mitigate their effects. However, it is often difficult to determine how cheating or differential social success has arisen. For example, cheaters may employ different strategies (e.g., fixed and facultative), whereas other causes of unequal fitness in social situations can result in winners and losers without cheating. To address these problems, we quantified the social success of naturally occurring genotypes of Dictyostelium discoideum during the formation of chimeric fruiting bodies, consisting of dead stalk cells and viable spores. We demonstrate that an apparent competitive dominance hierarchy of spore formation in chimera is partly due to a fixed strategy where genotypes exhibit dramatically different spore allocations. However, we also find complex, variable facultative strategies, where genotypes change their allocation in chimera. By determining the magnitude and direction of these changes, we partition facultative cheating into two forms: (1) promotion of individual fitness through selfish behaviour ("self-promotion") and (2) coercion of other genotypes to act cooperatively. Our results demonstrate and define social interactions between D. discoideum isolates, thus providing a conceptual framework for the study of the genetic mechanisms that underpin social evolution.