Arceuthobium tsugense (Viscaceae): four subspecies with contrasting morphologies and host distributions

Abstract

There have been four subspecies of hemlock dwarf mistletoe (Arceuthobium tsugense: Viscaceae) described by various investigators of this complex group of parasitic flowering plants: subsp. tsugense, subsp. amabilae, subsp. contortae, and subsp. mertensianae. As suggested by their subspecific epithets, these taxa differ in their host affinities; parasitizing different and the same hosts to varying degrees. Although these taxa also have morphological differences, their classifica-tion has been under debate for many years. Therefore, we compared the morphological characteristics of each subspecies using both univariate and multivariate statistical analyses in order to better assess their differences. Because some investigators have grouped hemlock dwarf mistletoe with western dwarf mistletoe (A. campylopodum), we also compared the subspecies of hemlock dwarf mistletoe with western dwarf mistletoe. Our morphometric analyses demonstrated that all of the subspecies are morphologically distinct from western dwarf mistletoe and that subsp. contortae is the most morphologically differentiated of the subspecies. Overlap in the morphological characters across two of the other three subspecies was evident; yet, subsp. amabilae and subsp. mertensianae were also consistently delimited using female and male plant morphologies. Statistical comparisons of female or male plants via standard and stepwise discriminant function analyses demonstrated that without consideration of host plant, female and male A. tsugense subsp. tsugense are morphologically similar to corresponding plants of subsp. amabilae and mertensianae but not subsp. contortae or A. campylopodum.