XVI International Botanical Congess

Pathogens have the potential to maintain polymorphisms for resistance genes by causing frequency-dependent on their host. Using wheat genotypes, susceptible to different races of the pathogen Puccinia striiformis, we tested whether disease caused frequency-dependent selection on its host and whether such selection could maintain polymorphisms for resistance genes in the wheat populations. In some populations, negative relationships between host relative fitness and host frequency were observed in the presence of disease, but not in its absence, suggesting that disease caused frequency-dependent selection on its host. However, this selection was not sufficient to maintain genetic polymorphisms in the host populations. In all cases where disease caused frequency-dependent selection, one host genotype was predicted to dominate in the population.