Dwarf mistletoe [Greek arceuthos, juniper, and bios, life, alluding to A. oxycedri, which parasitizes that host] Dwarf mistletoe [Greek arceuthos, juniper, and bios, life, alluding to A. oxycedri, which parasitizes that host]

Herbs or subshrubs, dioecious; parasitic on branches of Pinaceae [Cupressaceae], infections localized, nonsystemic, or systemic, sometimes inducing witches’ brooms. Stems multiple; primary branching fanlike, secondary branching fanlike or whorled. Leaves scalelike, connate. Inflorescences axillary or terminal, spikelike thyrses; flowers borne singly or in cymes in axils of subtending scales, not in cavities or grooves. Staminate flowers: petals 3–4(–6), triangular, distinct, radially symmetric or slightly asymmetric; stamens 3–4(–6); anthers 1-locular, dehiscing by transverse slits; nectary (= central cushion) present. Pistillate flowers: petals 2(–3), deltate, connate basally; ovary 1-locular; style very short to absent; stigma 2-lobed, secreting pollination droplet. Berries borne on recurved pedicels (when mature), explosively dehiscent from pedicellar end, 2-colored, smooth, glaucous, petal remnants not persisting. Seeds sticky when fruit dehisces, mucilaginous later after imbibing water; endosperm globose to pyriform; embryo oriented longitudinally. x = 14.

Species 29 (7 in the flora): North America, Mexico, West Indies, Central America, Eurasia, Africa, Atlantic Islands.

Species of Arceuthobium have received much attention because of their negative impact on commercially important conifers, and also because of their fascinating life history. Most mistletoes rely on birds to disperse their seeds, but in all but one species of Arceuthobium (A. verticilliflorum Engelmann from Durango, Mexico), seeds are dispersed by explosive dehiscence. Another feature considered an evolutionary advancement is reduced photosynthesis accompanying greater dependence on host carbon. Arceuthobium can be considered on the brink of becoming a holoparasite (D. L. Nickrent and M. A. García 2009). It can form three basic types of infections on the host. The first, called a localized infection, is present as the mistletoe shoots emerge from an often swollen host branch at the original site of seedling penetration. In some species, the infections remain localized, but in many species they later develop into witches' brooms, with the parasite inducing prolific branching of the host distal to the point of infection. With nonsystemic witches' brooms, the mistletoe endophyte remains localized; whereas with systemic witches’ brooms, the parasite endophyte occurs within the broom, sometimes existing adjacent to and dividing in synchrony with the host apical meristem (J. Kuijt 1960).

Arceuthobium has long been considered a taxonomically difficult group, mainly owing to losses and reductions in morphologic features used for classification. L. S. Gill (1935) was the first to propose a comprehensive treatment of the genus in the United States. In their classification, F. G. Hawksworth and D. Wiens (1972) utilized many of the same features as Gill, such as branching pattern, but did not agree with his host-form concept. Host specificity in Arceuthobium ranges from specialists (only one host known) to generalists that parasitize over a dozen species. Hawksworth and Wiens and D. L. Nickrent (1996) strongly advocated that mistletoe morphologic integrity was maintained irrespective of host being parasitized, and recognized 28 and 42 species, respectively.

A classification of Arceuthobium, revised from D. L. Nickrent (1996), was given by Nickrent et al. (2004), who recognized 26 species in two subgenera and 11 sections. That system is followed here, considering only subg. Vaginata Hawksworth & Wiens because all North American Arceuthobium reside there. This classification differs most from F. G. Hawksworth and D. Wiens (1972) with regard to sect. Campylopoda Hawksworth & Wiens; whereas the latter authors recognized 13 species in this section, Nickrent et al. recognized only two.

SELECTED REFERENCES Hawksworth, F. G. and D. Wiens. 1972. Biology and Classification of Dwarf Mistletoes (Arceuthobium). Washington. [Agric. Handb. 401.] Hawksworth, F. G. and D. Wiens, eds. 1996. Dwarf Mistletoes: Biology, Pathology, and Systematics. Washington. [Agric. Handb. 709.] Kuijt, J. 1955. Dwarf mistletoes. Bot. Rev. (Lancaster) 21: 569–628. Kuijt, J. 1960. Morphological aspects of parasitism in the dwarf mistletoes (Arceuthobium). Univ. Calif. Publ. Bot. 30: 337–436. Nickrent, D. L. 1996. Molecular systematics. In: F. G. Hawksworth and D. Wiens, eds. 1996. Dwarf Mistletoes: Biology, Pathology, and Systematics. Washington. Pp. 155–170. Nickrent, D. L. et al. 2004. A phylogeny of all species of Arceuthobium (Viscaceae) using nuclear and chloroplast DNA sequences. Amer. J. Bot. 91: 125–138. Nickrent, D. L. and M. A. García. 2009. On the brink of holoparasitism: Plastome evolution in dwarf mistletoes (Arceuthobium, Viscaceae). J. Molec. Evol. 68: 603–615. Nickrent, D. L., K. P. Schuette, and E. M. Starr. 1994. A molecular phylogeny of Arceuthobium based upon rDNA internal transcribed spacer sequences. Amer. J. Bot. 81: 1149–1160.