Natural variation in fruiting body morphology in the amoeba Dictyostelium discoideum

Reproductive altruism, where some individuals reproduce and others do not, is considered one of the pinnacles of cooperative societies. However, the optimal level of reproductive altruism is likely to depend on inclusive fitness considerations, including the relatedness of reproducing to non-reproducing individuals, as well as the benefits and costs accruing to each, respectively. In the social amoeba Dictyostelium discoideum, thousands of cells aggregate to form a multicellular fruiting body. During this process, some cells die, forming a rigid stalk that supports the rest of the cells, which become viable spores. The level of stalk investment by the social group can therefore be considered a metric of altruism investment. Importantly, genetically unrelated cells can co-aggregate to produce chimeric fruiting bodies, and selection can favour genotypes that behave selfishly by preferentially forming spores and avoiding forming the stalk. Owing to the extreme differences in fitness consequences of stalk cells versus spores, the level of altruism investment is likely to be under strong selection. Here we examined clonal fruiting body morphology in four natural populations to assess the extent to which stalk investment varies within populations and is maintained to different extents among populations. We found variation in fruiting body size and stalk investment, at both a cm-scale and between geographically isolated populations. These findings indicate the divergent evolution of altruism investment with and among populations and demonstrate widespread potential for cheating.