Host individuals are often coinfected with diverse parasite assemblages, resulting in complex interactions among parasites within hosts. Within hosts, priority effects occur when the infection sequence alters the outcome of interactions among parasites. Yet, the role of host immunity in this process remains poorly understood. We hypothesized that the host response to the first infection could generate priority effects among parasites, altering the assembly of later-arriving strains during epidemics. We tested this by infecting sentinel host genotypes of Plantago lanceolata with strains of the fungal parasite Podosphaera plantaginis and measuring susceptibility to subsequent infection during experimental and natural epidemics. In these experiments, prior infection by one strain often increased susceptibility to other strains, and these facilitative priority effects altered the structure of parasite assemblages, but this effect depended on host genotype, host population and parasite genotype. Thus, host genotype, spatial structure and priority effects among strains all independently altered parasite assembly. Using a fine-scale survey and sampling of infections on wild hosts in several populations, we then identified a signal of facilitative priority effects, which altered parasite assembly during natural epidemics. Together, these results provide evidence that within-host priority effects of early-arriving strains can drive parasite assembly, with implications for how strain diversity is spatially and temporally distributed during epidemics.

寄主个体经常被各种寄生虫组合共同感染，导致寄主内寄生虫之间复杂的相互作用。在宿主内，当感染顺序改变了寄生虫之间相互作用的结果时，就会产生优先效应。然而，宿主免疫在该过程中的作用仍知之甚少。我们假设宿主对首次感染的反应可能在寄生虫之间产生优先效应，从而在流行期间改变了后来到达的菌株的装配。我们通过用真菌寄生植物Podosphaera plantaginis的菌株感染车前草的前哨宿主基因型来测试这一点。并在实验和自然流行期间测量对随后感染的易感性。在这些实验中，一种菌株先前感染通常会增加对其他菌株的易感性，并且这些促进的优先效应改变了寄生虫组合的结构，但这种效应取决于宿主基因型，宿主种群和寄生虫基因型。因此，菌株的宿主基因型，空间结构和优先级效应均独立地改变了寄生虫的装配。通过对几个种群中的野生宿主进行的大规模调查和感染抽样，我们确定了有利的优先效应信号，该信号在自然流行期间改变了寄生虫的装配。综合起来，这些结果提供了证据，即早期到达菌株的宿主内优先效应可以驱动寄生虫组装，