Figure 1.

Model structure. (A) Payoff matrix for partners that invest I or do nothing D. As long as B(I) > C(I), the payoffs satisfy the assumptions of the Prisoner’s Dilemma. (B) Benefit B(I) and cost C(I) functions used in simulations. (C) Strategy for repeated interactions, defined by the initial offer (intercept a) and by the reward rate (slope b). (D) Spatial lattices for host and symbiont; example illustrated shows 30 host-symbiont pairs; heavy vertical arrow symbolizes mutualistic (between lattice) interaction; horizontal arrows symbolize competitive (within lattice) interactions.