Resource competition, cooperation, and the selective dynamics of the TI plasmid

Explaining the evolution of cooperative traits is a fundamental challenge in evolutionary theory. Cooperative traits impose a cost on individuals exhibiting the trait to the benefit of other individuals. Consequently, within-group natural selection disfavors cooperative individuals, favoring instead individuals that cheat or freeload by avoiding the costs of cooperation while continuing to receive the benefits provided by cooperative individuals. Despite this, cooperation is commonplace in nature, being found in a variety of biological systems, suggesting that the costs of cooperation can be overcome. My dissertation research examines the role of ecological factors such as resource competition and population elasticity on the evolution of agrobacterial cooperative pathogenesis. An ecological model of competition between cooperative and non-cooperative strains of Agrobacterium tumefaciens predicts that avirulent, non-cooperative strains are competitively dominant when the nutritional benefits of cooperative pathogenesis---in the form of opines---are in low supply; however virulent strains dominate when opines are available in high supply. In testing the predictions of this resource competition model, I have demonstrated that there is a low cost associated with maintaining the virulence plasmid but a large cost associated with expressing the plasmid-borne virulence genes. As predicted by the model when opines are available cells bearing the virulence plasmid have a fitness advantage but are at a competitive disadvantage when opines are not present. Moreover, the cooperative pathogenesis of A. tumefaciens can promote increased local carrying capacity of cells bearing the plasmid. There is a widespread notion that kin competition is a powerful antagonist of the evolution of cooperation because it limits cooperative benefits. However, these effects on population size limit the power of kin competition as a constraint on the evolution of cooperation. This work highlights the importance of understanding how ecological factors influence the costs and benefits of cooperative traits.