Stalk size and altruism investment within and among populations of the social amoeba.

Reproductive division of labour is common in many societies, including those of eusocial insects, cooperatively breeding vertebrates, and most forms of multicellularity. However, conflict over what is best for the individual vs. the group can prevent an optimal division of labour from being achieved. In the social amoeba Dictyostelium discoideum, cells aggregate to become multicellular and a fraction behaves altruistically, forming a dead stalk that supports the rest. Theory suggests that intra-organismal conflict over spore-stalk cell fate can drive rapid evolutionary change in allocation traits, leading to polymorphisms within populations or rapid divergence between them. Here, we assess several proxies for stalk size and spore-stalk allocation as metrics of altruism investment among strains and across geographic regions. We observe geographic divergence in stalk height that can be partly explained by differences in multicellular size, as well as variation among strains in clonal spore-stalk allocation, suggesting within-population variation in altruism investment. Analyses of chimeras comprised of strains from the same vs. different populations indicated genotype-by-genotype epistasis, where the morphology of the chimeras deviated significantly from the average morphology of the strains developed clonally. The significantly negative epistasis observed for allopatric pairings suggests that populations are diverging in their spore-stalk allocation behaviours, generating incompatibilities when they encounter one another. Our results demonstrate divergence in microbial social traits across geographically separated populations and demonstrate how quantification of genotype-by-genotype interactions can elucidate the trajectory of social trait evolution in nature.