Asymmetric and uncertain interactions within mutualisms

Although understanding mutualism stability has advanced over the last few decades, two fundamental problems still remain in explaining how mutualisms maintain stable. 1) What resolves conflict between mutualists over resources and 2) in the presence of less cooperative and / or un-cooperative symbionts, what prevents symbiont populations from becoming dominated by un-cooperative individuals? Many past explanations of mutualism stability have assumed that interactions between mutualists are symmetrical. However, in most mutualisms, interactions between hosts and symbionts show varying degrees of asymmetry at different levels. Here we review three major types of asymmetric interactions within obligate mutualisms: i) asymmetric payoffs, which is also defined as individual power differences, ii) asymmetric potential rates of evolutionary change, and iii) asymmetric information states between hosts and symbionts. We suggest that these asymmetries between mutualists help explain why cooperation and conflict is inherent in the evolution of mutualisms, and why both hosts and symbionts present diversified phenotypes whilst cooperation predominates.