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All organisms engage in parasitic relations, as either parasites or hosts. Some species may even play both roles simultaneously. Among flowering plants, parasitism is characterized by the development of an intrusive organ called a haustorium, which absorbs water and nutrients from host plants. Despite this functionally unifying feature, haustoria are not homologous structures, as parasitic plants have evolved at least 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 epi- parasitism, 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, and 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.

Parasitic plants have evolved independently at least 12 times among the angiosperms, adding up to ca. 1% of the diversity of this group. Mistletoes, shrubby to arboreal parasitic plants infesting the above-ground organs of their hosts, have evolved at least five times within the Santalales. Currently accounted to include 89 genera, representing nearly half of the diversity within the order, this parasitic life form has been successful in infesting a wide array of hosts and colonizing all forested continents. Such success can be partially attributed to the emergence and specialization of different types of host-parasite connections via an organ generically known as haustorium. Details obtained from broad studies on the morphogenesis of mistletoes' haustoria in different species were used for a phylogenetic analysis of ancestral character state reconstruction, which was coupled with divergence time estimations for all clades within Santalales. Our results suggest that the change from root to aerial parasitism could have been facilitated by a common background for haustorium development shared by root parasites and epicortical root-forming mistletoes. From an ancestor with either root or epicortical root haustoria, drier climates may have acted as a driver for the expansion and specialization of mistletoe's endophytic tissue, leading to the emergence of a structure known as cortical strands, which help proliferating parasitic tissue within the host bark. This crucial evolutionary step has probably conferred great advantages to mistletoes, enabling groups such as Visceae to expand to a wider distribution, regardless of climatic conditions. The development of cortical strands could have also promoted host tissue proliferation, leading to the formation of convergent haustorium types known as woody galls and woodrose formations.