Electrophoretic evidence for genetic differentiation in two host races of hemlock dwarf mistletoe ( Arceuthobium tsugense)

Abstract

Three host races of hemlock dwarf mistletoe have been described: the western hemlock race (mainly parasitic on Tsuga heterophylla), the shore pine race (mainly on Pinus contorta ssp. contorta) and the mountain hemlock race (mainly on Tsuga mertensiana and P. monticola). Mistletoe shoots from 21 populations representing the three host races and eight host species were obtained and analysed using starch gel electrophoresis. Over 900 individuals were examined and 13 electrophoretic loci were used. On average, 84% of the loci were polymorphic and mean heterozygosity was 0.260. The mistletoes from the Pacific Northwest (Washington, Vancouver Island, B.C. and Orcas Island) were less diverse genetically than mainland populations. Mistletoe populations parasitic on two more hosts per population had higher numbers of alleles per locus and a higher percentage of polymorphic loci than populations colonizing only one host. The UPGMA phenogram showed a strong correspondence between geographic location and genetic distance indicating a clinal pattern from north to south. The most distinct cluster is composed of four populations from the Sierra Nevada in California. The population from Juneau, Alaska, is also distinct not shows greater affinity to the more northern populations using UPGMA analysis. The Oregon Cascade populations referred to the mountain hemlock race were allied with the western hemlock populations with UPGMA analysis but showed affinity to the Sierra Nevada populations when analysed with the distance Wagner procedure. Cluster and cladistic analyses did not result in discrete clusters of populations composed of either the western hemlock or shore pine races. Thus, the mountain hemlock dwarf mistletoe appears to be a separate taxon deserving of taxonomic recognition, possibly at the subspecies level. Isozyme data do not support the recognition of the shore pine race as distinct from the dwarf mistletoes on western hemlock.