GENETIC POLYMORPHISM IN THE MORPHOLOGICALLY REDUCED DWARF MISTLETOES (ARCEUTHOBIUM,VISCACEAE): AN ELECTROPHORETIC STUDY

Arceuthobium is a well defined genus of obligate parasites of conifers. Assessment of taxonomic and phylogenetic relationships in this group is difficult owing to morphological reductions and complex host relationships. In the present study, genetic relationships within and among 19 taxa were examined using starch gel electrophoresis of triploid seed endosperm tissue. Allelic frequency data for eleven polymorphic loci were derived from analysis of 40 natural populations collected from the U.S. and Mexico. The genus showed remarkably high levels of genetic diversity: averaged across the 19 taxa, 66.7% of the loci were polymorphic with an average of 2.23 alleles per locus. This level of polymorphism is approximately double the average value reported for many dicotyledons and stands in contrast to the overall uniform morphology of these parasites. Unweighted pair group cluster analysis (UPGMA) of genetic similarity measures was conducted for all sampled populations and the results compared to a phenetic system of classification for the genus. Similarities between the two studies include the recognition of two subgenera based upon the segregation of the verticillately branched Arceuthobium americanum from the remainder of the taxa examined. Analysis of isozyme data supported a grouping of six taxa: A. vaginatum ssp. cryptopodum, A. vaginatum ssp. durangense, A. gillii, A. rubrum, A. divaricatum, and A. douglasii. The placement of the latter three taxa in other groups by phenetic criteria provides evidence for molecular divergence not seen using morphological features. The Campylopodum group of taxa comprised eleven members that were linked at similarity values of 80% or greater. Populations were not unambiguously grouped according to species as defined by the phenetic study or at similarity levels comparable to other well defined species in this study. This group is either not reproductively isolated or molecular differentiation is cryptic due to rapid adaptive radiation onto numerous host tree species.