Flora of North America species comparison

J. Kuijt (2003) used the name Phoradendron serotinum, based on the name Viscum serotinum Rafinesque (1820), not P. leucarpum, which is based on the earlier name by the same author, V. leucarpum (1817). A proposal to conserve the later name (D. L. Nickrent et al. 2010b) was not accepted, thus the name P. leucarpum has priority.

Phoradendron leucarpum has a convoluted taxonomic history, reflecting not only various species concepts but also complex evolutionary and ecological processes. Among the 234 species of Phoradendron, J. Kuijt (2003) recognized subspecies only in P. leucarpum (as P. serotinum). In addition to the typical subspecies from eastern Texas eastward, they are subsp. augustifolium from Mexico, subsp. macrophyllum from eastern Texas through New Mexico and Arizona to California and Oregon, and subsp. tomentosum, with about the same distribution as subsp. macrophyllum but also extending into Mexico. Kuijt noted that in some geographic areas, such as east-central Texas, the putative subspecies show a continuum of morphological intergradation.

A population genetic and morphometric study of this complex was undertaken by A. K. Hawkins (2010). Principal component analyses using the characters that J. Kuijt (2003) considered to be diagnostic of the subspecies, such as leaf size, color, and venation, as well as the type and density of hairs present on young vegetative and reproductive tissues, in addition to host species, did not result in clusters corresponding to the four described subspecies. Moreover, FST analyses of microsatellites showed significant interpopulational differentiation that did not match the subspecies that Kuijt recognized. Because morphological and molecular analyses show that subspecies, at least as defined by Kuijt, cannot be differentiated in Phoradendron leucarpum, no subspecies are accepted here.

Phoradendron leucarpum is the only species of the genus found east of Texas. It parasitizes over 60 species of native and introduced trees, especially Acer, Fraxinus, Juglans, Nyssa, Platanus, Populus, Quercus, Salix, and Ulmus.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Genera 7, species ca. 575 (3 genera, 15 species in the flora).

To people in the northern hemisphere, the word mistletoe most often calls to mind a branch parasite in Viscaceae. In Europe, the Christmas mistletoe is typically Viscum album; whereas in North America, it is more often a species of Phoradendron. Mistletoe is associated historically with ancient Teutonic myths and Druidic rituals, but vestiges of these associations survive today as Yuletide kissing, a custom that dates to sixteenth-century England. Viscaceae represent just one of five clades that contain mistletoes, the others being Misodendraceae J. Agardh, Loranthaceae Jussieu, Santalaceae, and Amphorogynaceae Nickrent & Der (R. L. Mathiasen et al. 2008; D. L. Nickrent et al. 2010). Thus, the term mistletoe describes both a habit (an aerial parasite) and a taxonomic association (a member of Santalales). Early twentieth-century works (for example, A. Engler and K. Krause 1935) often treated Viscaceae as a subfamily of Loranthaceae, and indeed this practice persists today in some floras, herbaria, and popular treatments. But molecular phylogenetic work has greatly clarified relationships among the five mistletoe clades, showing that they originated independently at different time periods (R. Vidal-Russell and Nickrent 2008; J. P. Der and Nickrent 2008). Thus, the mainly tropical family Loranthaceae, whose members often have large, bird-pollinated flowers, is only distantly related to Viscaceae with small, insect-pollinated flowers.

In North America, Arceuthobium and Phoradendron are native and Viscum was introduced purposely and is now naturalized. By far, Arceuthobium is the most economically significant viscaceous mistletoe. In 1982, timber losses caused by species of Arceuthobium for the western North America were estimated as 130 million cubic meters (D. B. Drummond 1982; F. G. Hawksworth and D. Wiens 1996). Although these losses are severe, particularly in areas managed for timber production, species of Arceuthobium also play important ecological roles in natural forest ecosystems, for example its witches’ brooms provide nesting habitat for the northern spotted owl (R. Everett et al. 1997). Native trees as well as cultivated fruit and nut trees can be negatively impacted by Phoradendron in North America, Mexico, and Central and South America, but the extent of damage has not been quantified. In addition to its use as a Christmas holiday novelty, Viscum is also being explored for a variety of medicinal uses including treatments for some forms of cancer (A. Bussing 2000).

Viscaceae have been shown by molecular phylogenetic studies to be sister to Amphorogynaceae Der & Nickrent (previously a tribe of Santalaceae), which includes root and stem parasites (J. P. Der and D. L. Nickrent 2008; Su H. J. et al. 2015). Resolution of some relationships among the genera of Viscaceae had been seen from molecular studies conducted over a decade ago, but only recently has a clear picture emerged. Viscum and Notothixos Oliver are sister to the remaining genera, which include a clade consisting of Korthalsella Tieghem and Ginalloa Korthals and another of Phoradendron/Dendrophthora and Arceuthobium. From a biogeographic perspective, Arceuthobium is interesting because it is the only mistletoe to occur naturally in both the Old and New Worlds. A hypothesis by F. G. Hawksworth and D. Wiens (1972) is that Arceuthobium evolved in Asia and, following a trans-Beringian migration, underwent a massive radiation in the New World. Chloroplast spacer sequences show, however, that the New World species are most similar to Phoradendron whereas the Old World species have accumulated numerous deletions (Nickrent and M. A. García 2009). The most parsimonious hypothesis is that both Arceuthobium and Phoradendron first evolved in the New World, possibly in the Mexican highlands, where both genera are today quite diverse, and Arceuthobium then colonized the Old World.

(Discussion copyrighted by Flora of North America; reprinted with permission.)