Social dynamics in natural populations of Dictyostelium discoideum : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Evolution at Massey University, Auckland, New Zealand

Abstract

Evolutionary theory predicts that selection should favour individuals who act in their self-interest, resulting in widespread selfish behaviour. Nonetheless, cooperative behaviours that provide benefits to others are common in nature. Over the past 50 years, evolutionary biologists have established a framework to explain how cooperation can be maintained despite the threat of selfish individuals who fail to cooperate but benefit from the cooperation of others. This framework has mostly been built around social behaviours in animal societies such as social insects. However, the more recent discoveries of social behaviours in microbes, analogous to those observed in animals, allow us to examine the generality and importance of proposed mechanisms that limit selfishness in non-animal societies. The social amoeba Dictyostelium discoideum has become an established model system in social evolution studies. Upon starvation, genetically unrelated cells can co-aggregate to form a ‘chimeric’ fruiting body in which some cells altruistically die to form a supportive stalk while others become viable spores. Theory and empirical work have shown that altruistic stalk formation can select for cheaters: genotypes that preferentially allocate cells to the spores and exploit stalk formation by their social partner. This finding has led to a considerable number of studies that demonstrated proof-of-principle for mechanisms that both promote and limit cheating in this organism. In contrast, few studies have examined whether cheating is widespread in nature, nor assessed the importance of the previously identified mechanisms that may limit this behaviour. In this thesis, I aimed to bridge the gap between the laboratory and a more natural situation. Using a diverse collection of natural strains of D. discoideum, I investigated the frequency and intensity of cheating among natural strains, the mechanisms that limit this behaviour, and the potential evolutionary consequences of interactions between cheaters and cooperators. Overall, this thesis will contribute to the growing body of work that examines how cooperative behaviours can be maintained in non-animal societies.