U.S. Forest Service, Rocky Mountain Research Station, Shrub Sciences Laboratory, 735 North 500 East, Provo, Utah 84606 USA.
Am J Bot. 2010 Aug;97(8):1304-12. doi: 10.3732/ajb.0900261. Epub 2010 Jul 20.
Evolutionary processes that maintain genetic diversity in plants are likely to include selection imposed by pathogens. Negative frequency-dependent selection is a mechanism for maintenance of resistance polymorphism in plant-pathogen interactions. We explored whether such selection operates in the Bromus tectorum-Ustilago bullata pathosystem. Gene-for-gene relationships between resistance and avirulence loci have been demonstrated for this pathosystem. •
We used molecular markers and cross-inoculation trials to learn whether the SSR genotypes of the host exhibited resistance to co-occurring pathogen races, whether host genotypes within a population had equal disease probability, and whether a common resistance locus and its corresponding avirulence locus exhibited predicted allele frequency changes during an epidemic. •
Five of six putative resistance loci that conferred resistance to co-occurring pathogen races occurred in common host SSR genotypes. Some common genotypes within populations were more likely to be diseased than others, and genotype frequencies sometimes changed across years in patterns consistent with frequency-dependent selection. Observed changes in frequency of resistance and virulence alleles during an epidemic provided further support, but evidence was inconclusive. •
Frequency-dependent selection may operate at endemic disease levels in this pathosystem, but is difficult to detect because many susceptible plants escape infection. Most pathogen isolates were virulent on most host genotypes, minimizing the apparent importance of frequency-dependent selection even during epidemics.
维持植物遗传多样性的进化过程可能包括病原体施加的选择。负频率依赖性选择是植物-病原体相互作用中保持抗性多态性的一种机制。我们探索了这种选择是否在 Bromus tectorum-Ustilago bullata 病理系统中起作用。该病理系统已经证明了抗性和无毒基因座之间的基因对基因关系。
我们使用分子标记和交叉接种试验来了解宿主的 SSR 基因型是否对共存的病原体具有抗性，种群内的宿主基因型是否具有相同的疾病概率，以及共同的抗性基因座及其相应的无毒基因座在流行期间是否表现出预测的等位基因频率变化。
在共发生的病原体种群中，有六个假定的抗性基因座中的五个发生在常见的宿主 SSR 基因型中。种群内的一些常见基因型比其他基因型更容易患病，基因型频率有时会在几年内发生变化，这与频率依赖性选择模式一致。在流行期间观察到的抗性和毒力等位基因频率的变化提供了进一步的支持，但证据并不确定。
在这个病理系统中，频率依赖性选择可能在地方性疾病水平上起作用，但由于许多易感植物逃避感染，因此很难检测到。大多数病原体分离株对大多数宿主基因型都具有毒性，即使在流行期间，也最大限度地减少了频率依赖性选择的明显重要性。
