A core hypothesis in coevolutionary theory proposes that parasites adapt to specifically infect common host genotypes. Under this hypothesis, parasites function as agents of negative frequency-dependent selection, favouring rare host genotypes. This parasite-mediated advantage of rarity is key to the idea that parasites maintain genetic variation and select for outcrossing in host populations. Here, we report the results of an experimental test of parasite adaptation to common versus rare host genotypes. We selected the bacterial parasite Serratia marcescens to kill Caenorhabdiis elegans hosts in uneven mixtures of host genotypes. To examine the effect of commonness itself, independent of host identity, each of four host genotypes was represented as common or rare in experimental host mixtures. After experimental selection, we evaluated a parasite line's change in virulence—the selected fitness trait—on its rare and common host genotypes. Our results were consistent with a slight advantage for rare host genotypes: on average, parasites lost virulence against rare genotypes but not against common genotypes. The response varied substantially, however, with distinct patterns across host genotype mixtures. These findings support the potential for parasites to impose negative frequency-dependent selection, while emphasizing that the cost of being common may vary with host genotype.

共同进化理论的核心假设提出，寄生虫可以适应特定的感染宿主基因型。在这种假设下，寄生虫起负频率依赖性选择的作用，有利于罕见的宿主基因型。这种寄生虫介导的稀有优势是寄生虫保持遗传变异并选择寄主群体杂交的想法的关键。在这里，我们报告寄生虫适应常见与罕见宿主基因型的实验测试结果。我们选择了细菌寄生虫粘质沙雷氏菌来杀死秀丽隐杆线虫（Caenorhabdiis elegans）宿主的宿主基因型不均匀。为了检查通用性本身的影响，与宿主身份无关，四种宿主基因型中的每一种在实验宿主混合物中均表示为常见或罕见。经过实验选择后，我们评估了寄生虫系在其罕见和常见宿主基因型上的毒力变化（选定的适应性状）。我们的结果与稀有宿主基因型的轻微优势是一致的：平均而言，寄生虫对稀有基因型失去了毒力，但对普通基因型却没有。然而，反应在宿主基因型混合物中以明显不同的模式变化。这些发现支持寄生虫施加负频率依赖性选择的可能性，同时强调寄生虫的成本可能随宿主基因型而变化。