Theory on the evolution of niche width argues that resource heterogeneity selects for niche breadth. For parasites, this theory predicts that parasite populations will evolve, or maintain, broader host ranges when selected in genetically diverse host populations relative to homogeneous host populations. To test this prediction, we selected the bacterial parasite Serratia marcescens to kill Caenorhabditis elegans in populations that were genetically heterogeneous (50% mix of two experimental genotypes) or homogeneous (100% of either genotype). After 20 rounds of selection, we compared the host range of selected parasites by measuring parasite fitness (i.e. virulence, the selected fitness trait) on the two focal host genotypes and on a novel host genotype. As predicted, heterogeneous host populations selected for parasites with a broader host range: these parasite populations gained or maintained virulence on all host genotypes. This result contrasted with selection in homogeneous populations of one host genotype. Here, host range contracted, with parasite populations gaining virulence on the focal host genotype and losing virulence on the novel host genotype. This pattern was not, however, repeated with selection in homogeneous populations of the second host genotype: these parasite populations did not gain virulence on the focal host genotype, nor did they lose virulence on the novel host genotype. Our results indicate that host heterogeneity can maintain broader host ranges in parasite populations. Individual host genotypes, however, vary in the degree to which they select for specialization in parasite populations.

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异质宿主种群中寄生虫宿主范围的演变

生态位宽度演化理论认为资源异质性选择生态位宽度。对于寄生虫，该理论预测寄生虫种群在相对于同质宿主种群的遗传多样性宿主种群中进行选择时，将进化或维持更广泛的宿主范围。为了测试这一预测，我们选择了细菌寄生虫粘质沙雷氏菌来杀死遗传异质（两种实验基因型的 50% 混合）或同质（任一基因型的 100%）种群中的秀丽隐杆线虫。经过 20 轮选择后，我们通过测量两种焦点宿主基因型和一种新型宿主基因型的寄生虫适应性（即毒力，选定的适应性特征）来比较所选寄生虫的宿主范围。正如预测的那样，为具有更广泛宿主范围的寄生虫选择的异质宿主种群：这些寄生虫种群获得或保持对所有宿主基因型的毒力。这一结果与在一种宿主基因型的同质群体中的选择形成对比。在这里，宿主范围缩小，寄生虫种群对焦点宿主基因型获得毒力，而对新宿主基因型失去毒力。然而，这种模式并没有在第二宿主基因型的同质种群中重复选择：这些寄生虫种群没有获得对焦点宿主基因型的毒力，也没有失去对新宿主基因型的毒力。我们的结果表明宿主异质性可以在寄生虫种群中保持更广泛的宿主范围。然而，个体宿主基因型在它们选择专攻寄生虫种群的程度方面有所不同。