COST OF RESISTANCE AND THE MAINTENANCE OF GENETIC-POLYMORPHISM IN HOST-PATHOGEN SYSTEMS

By using models which incorporate both numerical and gene-frequency dy namics, we investigate the conditions for a stable polymorphism in hos t disease resistance when there is a genetically uniform pathogen. We show that polymorphism is more likely when the difference in resistanc e conferred by alternative alleles is large rather than small. This co nforms with the frequent observation of major gene effects on resistan ce. Moreover, when allelic differences in resistance are large, polymo rphism is possible over a wide range of costs, including situations wh ere costs approach values close to zero. The actual resistance cost th at can be sustained in such polymorphic populations is dependent on th e transmission mode and the intensity of disease-independent populatio n regulation. Expectations regarding resistance costs in any particula r host-pathogen system will be dependent on knowledge of the epidemiol ogical and genetic characteristics of that system.