Brunet J, Mundt C C
Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331-2902, USA.
Evolution. 2000 Apr;54(2):406-15. doi: 10.1111/j.0014-3820.2000.tb00043.x.
Pathogens have the potential to maintain genetic polymorphisms by creating frequency-dependent selection on their host. This can occur when a rare host genotype is less likely to be attacked by a pathogen (frequency-dependent disease attack) and has higher fitness at low frequency (negative frequency-dependent selection). In this study, we used wheat genotypes that were susceptible to different races of the pathogen Puccinia striiformis to test whether disease created frequency-selection on its host and whether such selection could maintain polymorphisms for resistance genes in the wheat populations. Four different two-way mixtures of wheat genotypes were planted at different frequencies in both the presence and absence of disease. Disease created frequency-dependent selection on its host in some populations. Unknown factors other than disease also created frequency-dependent selection in this system because, in some instances, rare genotype advantage was observed in the absence of disease. Although the pathogen created frequency-dependent selection on its host, this selection was not sufficient to maintain genetic polymorphism in the host populations. In all cases where frequency-dependent selection occurred only in the diseased plots, one of the two genotypes was predicted to dominate in the population and the same genotype was predicted to dominate in both the presence and absence of disease. Only in cases where frequency-dependent selection was not caused by disease was there evidence that genetic polymorphisms would be maintained in the population. The frequency-dependent selection described in this study is a consequence of epidemiological effects of disease and differs from the time-lagged frequency-dependent selection resulting from coevolution between hosts and parasites. The impact of this direct frequency-dependent selection on the maintenance of genetic polymorphisms in the host population is discussed.
病原体有可能通过对其宿主产生频率依赖型选择来维持遗传多态性。当一种罕见的宿主基因型不太可能受到病原体攻击（频率依赖型疾病攻击）且在低频时具有更高的适合度（负频率依赖型选择）时，就会出现这种情况。在本研究中，我们使用了对条锈菌不同生理小种敏感的小麦基因型，以测试病害是否对其宿主产生频率选择，以及这种选择是否能够维持小麦群体中抗性基因的多态性。在有病害和无病害的情况下，以不同频率种植了四种不同的小麦基因型双向混合物。病害在一些群体中对其宿主产生了频率依赖型选择。除病害外的未知因素在该系统中也产生了频率依赖型选择，因为在某些情况下，在无病害时观察到了罕见基因型优势。尽管病原体对其宿主产生了频率依赖型选择，但这种选择不足以维持宿主群体中的遗传多态性。在所有仅在病害小区中发生频率依赖型选择的情况下，预计两种基因型中的一种会在群体中占主导地位，并且预计在有病害和无病害时都是同一基因型占主导地位。只有在频率依赖型选择不是由病害引起的情况下，才有证据表明群体中的遗传多态性会得以维持。本研究中描述的频率依赖型选择是病害流行病学效应的结果，与宿主和寄生虫之间协同进化产生的时间滞后频率依赖型选择不同。讨论了这种直接频率依赖型选择对宿主群体中遗传多态性维持的影响。
