Partner choice in symbiotic mutualisms: The role of population structure
Mutualisms, where two species provide benefits to each other, are among the most important ecological interactions. Many mutualisms, such as the one between legumes and rhizobia, involve costly investments: traits that are directly costly to their carriers, and beneficial only to the partner species. For such costly investments to evolve, some sort of feedback mechanism must exist that more than recoups the direct costs. Multiple feedback mechanisms are known to operate in mutualisms, yet there is relatively little known about the interaction of these mechanisms with each other. Two feedback mechanisms in particular have long been recognized as important: (i) partner choice, where one mutualist preferentially interacts with, or rewards, more cooperative partners, and (ii) population structure among mutualists, which allows costly investments to evolve through kin- (or group-, depending on one’s inclination) selection. Yet, how partner choice operates in structured populations, and how the interaction between the two feedback mechanisms affects the overall course of selection remains understudied. In this talk, I present a model that explores the effects of host partner choice on structured symbiont populations.
I model two different interaction scenarios: one in which the host’s investment into the symbiont is fixed and one where the host investment increases with the total benefit from the symbiont population, a condition termed partner fidelity feedback. In the former case, I find that population structure (i.e., increased relatedness amongst symbionts) generally reduces the benefit from partner choice to the more cooperative symbiont. This is because population structure reduces the available variation for the host to choose from. On the other hand, partner fidelity feedback works in the opposite direction, allowing returned benefits to go to relatives. The overall effect of partner choice depends on the balance of the two types of host investment. Interestingly, under both scenarios, partner choice and population structure can generate both positive and negative frequency dependence, with implications for the maintenance of variation among symbionts. These results point to the importance of integrating multiple selective mechanisms and suggest new empirical questions to probe the evolution and maintenance of cooperation in mutualisms.