All organisms engage in parasitic relations, as either parasites or hosts. Some species may even play both roles simultaneously. Among flowering plants, the most widespread form of parasitism is characterized by the development of an intrusive organ called the haustorium, which absorbs water and nutrients from the host. Despite this functionally unifying feature of parasitic plants, haustoria are not homologous structures; they have evolved 12 times independently. These plants represent ca. 1% of all extant flowering species and show a wide diversity of life histories. A great variety of plants may also serve as hosts, including other parasitic plants. This phenomenon of parasitic exploitation of another parasite, broadly known as hyper- or epiparasitism, is well described among bacteria, fungi, and animals, but remains poorly understood among plants. Here, we review empirical evidence of plant hyperparasitism, including variations of self-parasitism, discuss the diversity and ecological importance of these interactions, and suggest possible evolutionary mechanisms. Hyperparasitism may provide benefits in terms of improved nutrition and enhanced host-parasite compatibility if partners are related. Different forms of self-parasitism may facilitate nutrient sharing among and within parasitic plant individuals, while also offering potential for the evolution of hyperparasitism. Cases of hyperparasitic interactions between parasitic plants may affect the ecology of individual species and modulate their ecosystem impacts. Parasitic plant phenology and disperser feeding behavior are considered to play a major role in the occurrence of hyperparasitism, especially among mistletoes. There is also potential for hyperparasites to act as biological control agents of invasive primary parasitic host species.

中文翻译：

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寄生虫上的寄生虫：开花植物中的超寄生、外寄生和自寄生

所有有机体都参与寄生关系，无论是寄生虫还是宿主。有些物种甚至可能同时扮演这两种角色。在开花植物中，最普遍的寄生形式的特征是形成一种称为吸器的侵入性器官，它从宿主那里吸收水分和养分。尽管寄生植物具有这种功能统一的特征，但吸器并不是同源结构。它们已经独立进化了12次。这些植物代表约。占所有现存开花物种的 1%，并显示出广泛的生活史。种类繁多的植物也可以作为宿主，包括其他寄生植物。这种对另一种寄生虫的寄生利用现象，广泛称为过度寄生或外寄生，在细菌、真菌和动物中得到了很好的描述，但在植物中仍然知之甚少。在这里，我们回顾了植物过度寄生的经验证据，包括自寄生的变化，讨论这些相互作用的多样性和生态重要性，并提出可能的进化机制。如果伴侣有关系，过度寄生可能会在改善营养和增强宿主 - 寄生虫兼容性方面提供好处。不同形式的自寄生可能促进寄生植物个体之间和内部的营养共享，同时也为过度寄生的进化提供了潜力。寄生植物之间的超寄生相互作用案例可能会影响单个物种的生态并调节其对生态系统的影响。寄生植物物候和分散摄食行为被认为在过度寄生的发生中起主要作用，尤其是在槲寄生中。超寄生虫也有可能作为侵入性初级寄生宿主物种的生物控制剂。