9701320 Coley The relationships between plants and their pathogens are not well understood, yet these systems present fascinating questions which bear upon fundamental issues of evolutionary ecology. This research will address one such topic, the maintenance of resistance and virulence polymorphisms. This study will undertake an integrated investigation of various hypotheses for the maintenance of polymorphism in the native wild sunflower (Helianthus annuus) and its fungal rust (Puccinia helianthi). First, a series of garden experiments will determine whether there are detectable fitness costs associated with bearing unnecessary resistance or virulence alleles. Such costs might balance the benefits of resistance or virulence, resulting in polymorphic populations. In addition, both direct and indirect fitness benefits of resistance will be examined in natural populations. A frequency-dependent advantage for rare resistance or virulence genotypes might also be important to the maintenance of polymorphism, and a controlled garden experiment will be performed to test this hypothesis. Finally, surveys of natural populations in Utah will compare polymorphism in areas favorable or unfavorable for the rust, and will examine polymorphism in populations of differing sizes and degrees of isolation. The proposed research will critically test a variety of hypotheses for the maintenance of virulence and resistance polymorphisms, and will make a valuable contribution to our understanding of the evolutionary dynamics of natural plant-pathogen systems.
