The Coexistence of Hosts with Different Abilities to Discriminate against Cheater Partners: An Evolutionary Game-Theory Approach

Abstract

Evolutionary theory predicts that mutualisms based on the reciprocal exchange of costly services should be susceptible to exploitation by cheaters. Consistent with theory, both cheating and discrimination against cheaters are ubiquitous features of mutualisms. Several recent studies have confirmed that host species differ in the extent that they are able to discriminate against cheaters, suggesting that cheating may be stabilized by the existence of susceptible hosts (dubbed "givers"). We use an evolutionary gametheoretical approach to demonstrate how discriminating and giver hosts associating with mutualist and cheater partners can coexist. Discriminators drive the proportion of cheaters below a critical threshold, at which point there is no benefit to investing resources into discrimination. This promotes givers, who benefit from mutualists but allow cheater populations to rebound. We then apply this model to the plant-mycorrhizal mutualism and demonstrate it is one mechanism for generating host-specific responses to mycorrhizal fungal species necessary to generate negative plant-soil feedbacks. Our model makes several falsifiable, qualitative predictions for plant-mycorrhizal population dynamics across gradients of soil phosphorus availability and interhost differences in ability to discriminate. Finally, we suggest applications and limitations of the model with regard to coexistence in specific biological systems.

Preferential allocation, physio‐evolutionary feedbacks, and the stability and environmental patterns of mutualism between plants and their root symbionts

“…In fact, heterogeneous populations of symbionts that differ in their growth promotion commonly occur in nature (Bever, ; Bever et al ., , ; Heath & Tiffin, ; Porter & Simms, ). The physio‐evolutionary negative feedback in this model is analogous to the negative community feedback generated by an interspecific tradeoff because of the cost of preferential allocation (Steidinger & Bever, ). In the present framework, the maintenance of variation in symbiont growth promotion is an expected result of physio‐evolutionary feedbacks between the rates of plant allocation and the relative abundance of mutualists in the symbiont population, not an evolutionary paradox (Heath & Stinchcombe, ).…”

Host discrimination in modular mutualisms: a theoretical framework for meta-populations of mutualists and exploiters

“…[31,37]). Our model also does not consider the impact of imperfect mixing in the partner community-a process that could lead to positive associations and repeat encounters among the offspring of individual modules.…”

Host investment into symbiosis varies among genotypes of the legume Acmispon strigosus, but host sanctions are uniform

“…Under the host variation hypothesis, we predicted that hosts would exhibit genetic differences in host control traits. Our work contributes to an emerging theoretical framework to explain the maintenance of variation in populations of microbial mutualists (Sachs et al ., ; Heath & Stinchcombe, ; Steidinger & Bever, , ; Bever, ; Pahua et al ., ).…”