Complex interactions underpin social behaviour in Dictyostelium giganteum

In the wild, social groups of the cellular slime mould amoeba Dictyostelium giganteum are genetically heterogeneous more often than not. When studied as 1:1 binary mixes, amoebae of one strain almost always form more spores than the other, an observation that leads one to wonder what might be responsible for the long-term persistence of different strains in nature. We have monitored a number of individual and collective traits bearing on reproductive fitness in chimaeras of Dictyostelium giganteum obtained by mixing pairs of starved amoebae belonging to distinct wild-type strains in proportions ranging from 1:9 to 9:1. The main findings are that intercellular interactions take place at more than one stage of the life cycle and that generalisations drawn after mixing cells only in a 1:1 ratio can be misleading. A strain that does better than another in respect of some component of fitness (for example, spore formation) may do worse in respect of a different component (for example, growth rate). Also, a strain that is a more efficient sporulator than a second strain in one context may be less efficient in another context. In addition to such trade-offs, spore formation in chimaeras can exhibit negative frequency dependence, which too can lead to stable co-existence.Significance statementHumans live in social groups whose members perform different but equally important tasks. When they do so freely, the long-term stability of the group depends on give and take (trade-offs') between individuals who are generally unrelated. In contrast, studies of cooperation in non-human animals have focussed on relatedness through common descent as responsible for the maintenance of group cohesion. But animal groups too contain unrelated individuals. Using the example of a social amoeba, we show that this need not rule out a high level of cooperation including so-called altruism, as long as inter-individual interactions permit it. In both genetically homogeneous and heterogeneous groups, interactions can result in trade-offs between different traits. In genetically heterogeneous groups, they can also lead to a negative correlation between the efficiency with which cells of a genotype reproduce and the proportion of cells belonging to that genotype