Multivariate discrimination of host use by dwarf mistletoeArceuthobium vaginatum subsp.cryptopodum: Inter- and intraspecific comparisons.

The parasitic dwarf mistletoeArceuthobium vaginatum attacksPinus ponderosa as its primary host andP. contorta as an occasional host. Within ponderosa pine stands there is also differential parasitism among individual trees. We compared biochemical features of phloem and xylem oleoresin between infected individuals of the two pine species (N=15 for each species) and also between infected (N=30) and nearby uninfected (N=30) ponderosa pine conspecifics. There were significant differences in chemical features, both at the interspecific (P. ponderosa vs.P. contorta) and intraspecific (P. ponderosa) levels. Discriminant function analysis based on chemical features of phloem correctly classified all trees used in the analysis as eitherP. ponderosa orP. contorta, and 95% of all ponderosa pine trees as either parasitized or nonparasitized. Monoterpene composition of oleoresin was distinct between species, and differences between parasitized and nonparasitizedP. ponderosa were also significant. Many of the observed chemical differences are probably constitutive, although levels of nonstructural carbohydrates and α-pinene may change in response to dwarf mistletoe infection. Biochemical differences at the intraspecific level were distinct from interspecific differences. Patterns of differential attack can have genetic consequences upon both the parasite and its hosts, and, in the process, may contribute to the evolution of host races of the parasite and to the evolution of host resistance within ponderosa pine.