The Effects of Spatial Scale on the Evolution of Social Behavior

Conflict can arise within social groups as cheating behaviors, where individuals reap the benefits of public goods without sharing the cost of acquiring or producing them. To limit the spread of cheating, cooperators can employ mechanisms that reduce or eliminate cheaters. In turn, selection can favor alternative cheating behaviors that circumnavigate these resistance mechanisms, leading to co-evolutionary change. In principle, prolonged social conflict between cheaters and cooperators could act as a driving force for the divergent evolution of social behaviors within populations, generated by a selection mosaic that leads to social barriers between populations. The social amoeba, Dictyostelium discoideum, is an excellent system for the study of social evolution owing to its unusual form of multicellularity. Upon starvation, cells of D. discoideum aggregate together to form a multicellular structure that ultimately develops into a fruiting body in which around 20% of cells sacrifice themselves to form a stalk that supports the other 80% as spores. Stalk formation is thought to be an act of altruism, as it can promote dispersal of the reproductive spores. Furthermore, cheating has been observed to occur between pairs of genetically distinct strains where one strain was overrepresented in the spores, suggesting a reduced contribution to the stalk. In order to determine whether natural populations of D. discoideum evolve in the context of a selection mosaic for social behavior, I examined social behaviors of strains isolated from two naturally occurring populations both clonally and in co-development with strains from both the same and different populations. Analysis of the individual strains revealed within-population polymorphism in spore-stalk allocation and divergence in stalk length between populations. Co-development of genetically distinct strains showed that chimeric development negatively impacts group level social fitness, measured as spore production, and population-specific effects of genetic distance that negatively affects spore production when strains co-develop. Differences in social success, measured as increases in relative fitness, between populations suggests that social behavior between populations follows divergent trajectories. These results indicate a geographic mosaic of social behavior in this species and provide insight into how complex social traits evolve across a geographic range.Biology and Biochemistry, Department o