This experimental study is designed to test several assumptions and predictions of genetic models used to explain the coevolution of plant resistance to pathogens and pathogen virulence on hosts. These models indicate that stabilizing selection can maintain genetic variation in host-plant resistance and pathogen virulence when these traits are each accompanied by pleiotropic fitness costs. Unfortunately, little evidence is available about the costs to plants of pathogen-resistance; nor is there conclusive evidence that virulence is costly to pathogens. Moreover, genetic modifier theory suggests that many such traits may not be costly. The experiments have two main objectives: (1) determine the genetic mechanisms underlying virulence in the fungal pathogen Colletorichum dematium and resistance in its host, the tall morning glory, Ipomoea purpurea, and (2) observe patterns of selection on virulence and resistance traits in these species. Results from these experiments will test three important assumptions of plant-pathogen coevolutionary theory: (1) Pathogens impose selection on their host plants, (2) Stabilizing selection maintains genetic variation for both resistance and virulence characters, (3) Resistance to pathogens is costly. Findings will contribute directly to ecological theory and will add substantially to basic knowledge which can be used for problems in the applied ecology of disease management. //
