Evolution and stasis in plant-pathogen associations

The impact of disease on plant populations may change as a result of coevolutionary interactions. The Red Queen hypothesis suggests that parasites provide a selective advantage to host sexual recombination sufficient to explain its persistence despite the twofold advantage of asexual reproduction. The constant production of resistant host genotypes by sexual recombination favors parasite genotypes that can overcome host resistance, resulting in a coevolutionary ''arms race.'' Systemic fungal parasites differ from nonsystemic fungi in ways that affect plant-pathogen interactions. Many systemic fungal parasites castrate their host plants, which reduces the potential for production of genetically variable, resistant progeny. Moreover, host genotypes susceptible to infection by the maternal pathogen are perpetuated through parasite-induced asexual reproduction or enforced selfing. Grasses in the genus Danthonia infected by Atkinsonella hypoxylon represent a model system for investigating evolution and stasis in plant-pathogen associations. Infected plants, unlike uninfected plants, reproduce only by self-fertilized seed through which the fungus is vertically transmitted. Other similar associations exist, suggesting that parasites can often enforce genetic uniformity within host populations.