Department of Botany and Plant Sciences, University of California, Riverside, 900 University Avenue, Riverside, CA 92125, USA.
School of Geography, Development, and the Environment, University of Arizona, PO Box 210137, Tucson, AZ 85721, USA.
Curr Biol. 2021 Mar 22;31(6):1344-1350.e3. doi: 10.1016/j.cub.2021.01.034. Epub 2021 Feb 23.
Intraspecific competition among parasites should, in theory, increase virulence, but we lack clear evidence of this from nature. Parasitic plants, which are sessile and acquire carbon-based resources through both autotrophy (photosynthesis) and heterotrophy (obtaining carbon from the host), provide a unique opportunity to experimentally study the role of intraspecific competition for nutrients in shaping the biology of both parasite and host. Here, we manipulated the spatial position of naturally occurring individuals of desert mistletoe (Phoradendron californicum), a xylem hemiparasite, by removing parasites from co-infected branches of a common nitrogen-fixing host, velvet mesquite (Prosopsis velutina), in the Sonoran Desert. We measured physiological performance of both host and parasite individuals under differing competitive environments-parasite location along the xylem stream-through time. Performance was determined by measuring resource availability and use, given that resource demand changed with competitor removal and monsoon-driven amelioration of seasonal drought. Our principal finding was that intraspecific competition exists for xylem resources between mistletoe individuals, including host carbon. Host performance and seasonal climate variation altered the strength of competition and virulence. Hemiparasitic desert mistletoes demonstrated high heterotrophy, yet experimental removals revealed density- and location-dependent effects on the host through feedbacks that reduced mistletoe autotrophy and improved resource availability for the remaining mistletoe individual. Trophic flexibility tempered intraspecific competition for resources and reduced virulence. Mistletoe co-infections might therefore attenuate virulence to maintain access to resources in particularly stressful ecological environments. In summary, experimental field manipulations revealed evidence for intraspecific competition in a parasite species.
寄生虫之间的种内竞争按理应该会增加寄生虫的毒力，但我们缺乏自然界中明确的相关证据。寄生植物是固着生物，通过自养（光合作用）和异养（从宿主获取碳）获取基于碳的资源，为实验研究寄生虫和宿主的生物学特征提供了一个独特的机会，即种内竞争对养分的获取如何影响寄生虫和宿主。在这里，我们通过从普通固氮宿主柔毛野漆树（Prosopis velutina）的共感染枝条上移除寄生植物，人为改变了加利福尼亚叶寄生（Phoradendron californicum）的自然个体的空间位置，加利福尼亚叶寄生是一种木质部半寄生植物。我们通过测量资源的可利用性和使用情况，来衡量不同竞争环境下（寄生位置沿着木质部流）随时间变化的宿主和寄生虫个体的生理表现。由于资源需求随着竞争者的移除和季风驱动的季节性干旱缓解而发生变化，因此性能是通过测量资源的可用性和使用情况来确定的。我们的主要发现是，木质部资源存在叶寄生个体之间的种内竞争，包括宿主碳。宿主的表现和季节性气候变化改变了竞争的强度和毒力。半寄生的沙漠叶寄生植物表现出很高的异养能力，但实验性去除表明，通过反馈作用，个体密度和位置对宿主产生了依赖效应，减少了叶寄生植物的自养能力，并提高了剩余叶寄生个体的资源可利用性。营养灵活性缓和了资源竞争，降低了毒力。因此，叶寄生植物的共感染可能会减轻毒力，以维持在特别具有压力的生态环境中对资源的获取。总之，实验性野外操作揭示了寄生虫物种中种内竞争的证据。
