Effects of competition on resistance gene polymorphism in a plant/pathogen system

Puccinia striiformis, a fungal pathogen, has been shown to cause direct, negative frequency-dependent selection on its host, wheat (Triticum aestivum). This disease-induced frequency-dependent selection was not sufficient to maintain polymorphism for resistance genes in the host populations. The present study examines whether interactions between disease and competition could impact upon the maintenance of genetic polymorphism in a highly self-pollinated species such as wheat, where strong associations between traits are likely. Four different two-way mixtures of wheat genotypes, susceptible to different races of P. striiformis, were planted at different frequencies in both the presence and absence of disease. In order to examine the influence of competition and disease on the maintenance of genetic polymorphism, relationships between host absolute fitness and host frequency were studied for each genotype in the mixtures of plants both in the presence and in the absence of disease. In the absence of disease, the absolute fitness of the stronger competitor was often negatively frequency-dependent, or else it did not vary with host frequency; the absolute fitness of the weaker competitor was often positively frequency-dependent. Disease typically rendered the slopes between absolute fitness and genotype frequency more negative for the stronger competitor. However, the influence of disease was not strong enough to reverse the sign of the slope between absolute fitness and genotype frequency for the genotype that was the weaker competitor in the absence of disease. Thus, disease was unable to reverse the relative ranking of the two genotypes caused by competition and create the negative frequency dependence on both genotypes in a mixture that is required for the maintenance of genetic polymorphism.