Kniskern Joel M, Rausher Mark D
Department of Biology, Duke University, Durham, North Carolina 27708, USA.
Evolution. 2007 Feb;61(2):377-87. doi: 10.1111/j.1742-4658.2007.00032.x.
Although disease-resistance polymorphisms are common in natural plant populations, the mechanisms responsible for this variation are not well understood. Theoretical models predict that balancing selection can maintain polymorphism within a population if the fitness effects of a resistance allele vary from a net cost to a net benefit, depending upon the extent of pathogen damage. However, there have been a few attempts to determine how commonly this mechanism operates in natural plant-pathogen interactions. Ipomoea purpurea populations are often polymorphic for resistance and susceptibility alleles at a locus that influences resistance to the fungal pathogen, Coleosporium ipomoeae. We measured the fitness effects of resistance over three consecutive years at natural and manipulated levels of damage to characterize the type of selection acting on this locus. Costs of resistance varied in magnitude from undetectable to 15.5%, whereas benefits of resistance sometimes equaled, but never exceeded, these costs. In the absence of net benefits of resistance at natural or elevated levels of disease, we conclude that selection within individual populations of I. purpurea probably does not account completely for maintenance of this polymorphism. Rather, the persistence of this polymorphism is probably best explained by a combination of variable selection and meta-population processes.
尽管抗病多态性在天然植物种群中很常见，但导致这种变异的机制尚未得到充分理解。理论模型预测，如果抗性等位基因的适合度效应根据病原体损害程度从净成本变为净收益，那么平衡选择可以在种群中维持多态性。然而，很少有人尝试确定这种机制在天然植物 - 病原体相互作用中普遍起作用的程度。牵牛属植物种群在一个影响对真菌病原体旋花科鞘锈菌抗性的位点上，抗性和易感等位基因通常是多态的。我们在连续三年的时间里，在自然和人为控制的损害水平下测量了抗性的适合度效应，以表征作用于该位点的选择类型。抗性成本的幅度从不可检测到15.5%不等，而抗性的益处有时与这些成本相当，但从未超过这些成本。在自然或疾病水平升高的情况下，抗性没有净益处，我们得出结论，牵牛属植物单个种群内的选择可能无法完全解释这种多态性的维持。相反，这种多态性的持续存在可能最好用可变选择和集合种群过程的组合来解释。
