In the evolutionary arms race between fungal pathogens and their plant hosts, the maintenance of genetic variation in the fungus has been presumed to represent a tradeoff between the value of increased virulence (infectivity) or increased population size. However, the existence of this tradeoff and the nature of these costs have not been extensively studied. In the 14 March Science, Peter Thrall and Jeremy Burdon at the Center for Plant Biodiversity, Canberra, Australia, demonstrate that spore production is reduced in the most virulent strains of one well-studied fungus model, and that diverse fungi genotypes are maintained within a plant population as a result (Science 299:1735-1737, March 14, 2003).

Thrall and Burdon performed greenhouse studies to characterize virulence and resistance in six strains of the fungus Melampsora lini and six varieties of its host, Linum marginale. In the field, they analyzed the natural distribution of the fungus...

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