Polymorphism in multi-locus host-parasite co-evolutionary interactions

Numerous loci in host organisms are involved in parasite recognition, such as MHC genes in vertebrates or genes involved in gene-for-gene (GFG) relationships in plants. Diversity is commonly observed at such loci, and at corresponding loci encoding antigenic molecules in parasites. Multi-locus theoretical models of host-parasite co-evolution predict that polymorphism is more likely than in single locus interactions because recurrent co-evolutionary cycles are sustained by indirect frequency-dependent selection as rare genotypes have a selective advantage. These cycles are stabilised by direct frequency-dependent selection, resulting from repeated re-infection of the same host by a parasite, a feature of most diseases. Here, it is shown that for realistically small costs of resistance and virulence, polycyclic disease and high auto-infection rates, stable polymorphism of all possible genotypes is obtained in parasite populations. Two types of epistatic interactions between loci tend to increase the parameter space in which stable polymorphism can occur with all possible host and parasite genotypes. In the parasite, the marginal cost of each additional virulence allele should increase, while in the host, the marginal cost of each addition resistance allele should decrease. It is therefore predicted that GFG polymorphism will be stable (and hence detectable) when there is partial complementation of avirulence genes in the parasite and of resistance genes in the host.

Key Words: epistasis, frequency-dependent selection, gene-for-gene relationship, population dynamics, resistance

Online ISS 1091-6490
Copyright 2008 by the Genetics Society of America
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