The influence of perenniality and seed banks on polymorphism in plant-parasite interactions.

Antagonistic interactions, such as diseases, play an important role in natural populations. Understanding the mechanisms that promote long-term polymorphism at loci that are involved in host-parasite recognition is a fundamental problem in evolutionary ecology. Coevolution implies the existence of indirect frequency-dependent selection because the fitnesses of parasite genotypes depend on the frequencies of host genes and vice versa. Polymorphism can be maintained in both organisms if there is also negative, direct, frequency-dependent selection, when natural selection for host resistance or parasite virulence declines with increasing frequency of that trait itself. In this article, using the gene-for-gene relationship as a model, we show that two plant life-history traits, seed banks and perenniality with parasite density-dependent disease transmission, generate frequency-dependent selection on host resistance and are thus capable of stabilizing frequencies of coevolving host and parasite genes. The host population's response to selection by the parasite is modified by the contribution of past selective events stored in long-lived seed banks or in a growing population of perennial plants that have a long life span in the absence of disease. While fitness costs determine whether coevolutionary cycles occur in interacting host and parasite populations, the ecology of the two organisms determines whether stable polymorphism is maintained.