Concurrent co-evolution of intra-organismal cheaters and resisters

The evolution of multicellularity is a major transition that is not yet fully understood. Specifically, we do not know if there are any mechanisms by which multicellularity can be maintained without a single cell bottleneck or other relatedness enhancing mechanisms. Under low relatedness, cheaters can evolve that benefit from the altruistic behaviour of others without themselves sacrificing. If these are obligate cheaters, incapable of co-operating, their spread can lead to the demise of multicellularity. One possibility, however, is that co-operators can evolve resistance to cheaters. We tested this idea in a facultatively multicellular social amoeba, Dictyostelium discoideum. This amoeba usually exists as a single cell but, when stressed, thousands of cells aggregate to form a multicellular organism in which some of the cells sacrifice for the good of others. We used lineages that had undergone experimental evolution at very low relatedness, during which time obligate cheaters evolved...