The ecology and evolution of social aggregations: case study Dictyostelium discoideum

Corina Tarnita

Department of Ecology & Evolutionary Biology, Princeton University, USA

Cooperation, in which individuals provide benefits to others at a cost to themselves, has been studied extensively and mechanisms have been proposed for its persistence in the face of free-riders. Often, however, especially in microbes, these studies focus on one fitness component, with little information about or attention to the ecological context. A well-studied example of microbial cooperation is the social amoeba Dictyostelium discoideum, whose life cycle includes both a single-cellular and a multicellular stage. To become multicellular, amoebae aggregate upon starvation and form a fruiting body made of dead stalk cells and reproductive spores. During aggregation, amoebae do not discriminate against non-kin, and chimeric fruiting bodies can form. The lab-measured reproductive skew in the spores of chimeras indicates strong social antagonism which should result in low genotypic diversity that is inconsistent with observations from nature. I will discuss how the inconsistency stems from the one-dimensional assessment of fitness (spore production) and how the solution lies in the multidimensional nature of fitness arising from tradeoffs between non-social traits, such as spore number versus viability, and investment in spores versus staying vegetative. Apparent winners in one dimension are in fact underrepresented in another. I will argue that the complexities of life histories, including social behavior, can only be understood in the appropriate multidimensional ecological context.