Morphogenesis and evolution of mistletoes' haustoria

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.