Data from: Infrapopulation size and mate availability influence reproductive success of a parasitic plant

1. Aggregated distributions of parasite individuals across host individuals are nearly ubiquitous among parasitic taxa. The size and sex ratio of the population of one parasite species infecting a single host (hereafter “infrapopulation”) can influence parasite fitness through intraspecific competition, mate availability, and the ability to attract vectors for transmission of parasite propagules. Competition for both resources and for pollen and seed vector services may limit reproductive success (pollen receipt, fruit production, and seed dispersal) in large infrapopulations of parasitic plants, while mate limitation or reduced ability to attract vectors may limit this success in small infrapopulations. 2. Using a dioecious parasitic plant, desert mistletoe (Phoradendron californicum), we experimentally removed reproductive tissue from male parasites in whole infrapopulations to test for independent effects of infrapopulation size and within-host mate availability on female fitness. As desert mistletoe requires both pollen and seed vectors for successful reproduction, the species provides the opportunity to test how infrapopulation characteristics affect multiple components of parasite fitness. 3. We found that insect-mediated pollen receipt decreased for parasites on treated hosts, consistent with within-host mate limitation. Additionally, the relationship between mate availability and fruit production per flower ranged from neutral to positive depending on year of the experiment. 4. As expected if competition for host resources limits reproductive success more than mate availability in larger infrapopulations, the greater pollen receipt to females in large infrapopulations did not generally translate into increased mistletoe fruit production. Relationships between mistletoe fruit production per flower and infrapopulation size ranged from negative to neutral. 5. Both pollen receipt and pollinator visitation increased with infrapopulation size, indicating that larger populations can be more attractive to pollen vectors independent of mate availability. However, we found no relationship between infrapopulation size and fruit removal by dispersers and, thus, no evidence that attraction of seed dispersal vectors increases with infrapopulation size. 6. Synthesis: These results highlight the interactive roles of within-host processes (resource competition, mate availability, and vector attraction) in determining the fitness of biotically-transmitted parasite individuals.1. 寄生物个体在寄主个体间的聚集分布，在各类寄生物类群中几乎普遍存在。侵染单一寄主的单种寄生物种群的规模与性比（以下简称“寄主体内种群（infrapopulation）”），可通过种内竞争、可获得交配对象情况以及吸引媒介传播寄生物繁殖体的能力，影响寄生物的适合度。寄生植物的大型寄主体内种群中，对寄主资源、花粉媒介与种子传播媒介服务的竞争，可能会限制其繁殖成效（花粉接收、果实产出与种子扩散）；而小型寄主体内种群则可能因交配限制或吸引媒介能力下降，导致繁殖成效受限。 2. 本研究以雌雄异株寄生植物沙漠槲寄生（Phoradendron californicum）为对象，通过实验移除整个寄主体内种群中的雄性寄生物的繁殖组织，以此检验寄主体内种群规模与寄主体内可获得交配对象情况，对雌性寄生物适合度的独立影响。由于沙漠槲寄生的成功繁殖同时依赖花粉媒介与种子传播媒介，该物种为探究寄主体内种群特征如何影响寄生物适合度的多个维度提供了研究契机。 3. 研究发现，经实验处理的寄主上的寄生物，其昆虫介导的花粉接收量有所下降，这与寄主体内交配限制的预期相符。此外，可获得交配对象情况与单花果实产出量之间的关系，随实验年份不同，呈现中性至正向相关的变化趋势。 4. 若大型寄主体内种群中，寄主资源竞争较交配限制对繁殖成效的限制作用更强，则雌性寄生物在大型种群中获得的更高花粉接收量，通常不会转化为槲寄生果实产出的提升，这与预期一致。单花果实产出量与寄主体内种群规模之间的关系，呈负相关至中性相关的变化。 5. 花粉接收量与传粉者访花频次均随寄主体内种群规模增大而提升，这表明无需考虑交配对象情况，更大的寄主体内种群对花粉媒介的吸引力更强。但本研究未发现寄主体内种群规模与传播者移除果实的情况存在关联，因此尚无证据表明种子传播媒介的吸引力会随寄主体内种群规模增大而提升。 6. 综合讨论：本研究结果凸显了寄主体内过程（资源竞争、可获得交配对象情况与媒介吸引）在决定生物媒介传播的寄生物个体适合度中的交互作用。