Coevolutionary dynamics in one-to-many mutualistic systems

"One-to-many" mutualisms are often observed in nature. In this type of mutualism, each host individual can interact with many symbionts, whereas each individual symbiont can interact with only one host individual. Partner choice by the host is a potentially critical mechanism for maintaining such systems; however, its long-term effects on the coevolution between the hosts and symbionts have not been completely explored. In this study, I developed a simple mathematical model to describe the coevolutionary dynamics between hosts and symbionts in a one-to-many mutualism. I assumed that each host chooses a constant number of symbionts from a potential symbiont population, a fraction of which are chosen through preferential choice on the basis of the cooperativeness of the symbionts and the rest are chosen randomly. Using numerical calculations, I found that mutualism is maintained when the preferential choice is not very costly and the mutation rate of symbionts is large. I also found that symbionts that receive benefits from hosts without a return (cheater symbionts) and hosts that do not engage in preferential partner choice (indiscriminator hosts) can coexist with mutualist symbionts and discriminator hosts, respectively. The parameter domain of pure mutualism, i.e., free from cheater symbionts and indiscriminator hosts, can be narrower than the whole domain where the mutualism persists.