A MOLECULAR PHYLOGENY OF ARCEUTHOBIUM (VISCACEAE) BASED ON NUCLEAR RIBOSOMAL DNA INTERNAL TRANSCRIBED SPACER SEQUENCES

Ribosomal DNA (rDNA) internal transcribed spacer (ITS) and 5.88 rDNA s equences were obtained from 22 species of dwarfmistletoes (Arceuthobiu m-Viscaceae) to test phylogenetic relationships. Interspecific distanc es ranged from 0 to 21.4% between New World species, values two to fiv e times higher than those measures for the ITS region in other plants. One Old World species (A. oxycedri) and one New World species (A. abi etis-religiosae) were remarkably similar to each other but exhibited u p to 41% sequence divergence from the remaining species. Minimum lengt h trees support the concept of a verticillately branched subgenus Arce uthobium; however, interspecific distances indicate this group is extr emely heterogeneous. Subgenus Vaginata, Section Vaginata, is centered in Mexico and encompasses all the taxa previously placed in this group but is expanded to include several species previously classified in S ection Campylopoda (e.g., A. divaricatum, A. rubrum, and A. strictum). The sister group relationship between A. divaricatum and A. douglasii , first seen following isozyme analysis, is supported by ITS sequence data. Section Campylopoda a. s. is now composed of 13 mainly U.S. spec ies that show a high degree of morphological and genetic similarity. T he eastern dwarf mistletoe, A. pusillum, is not closely related to A. douglasii but rather with A. bicarinatum from Hispaniola, which sugges ts that these taxa represent highly modified relicts that shared an an cestor in the early Tertiary. Two endemic species from Mexico and Cent ral America (A. guatemalense and A. pendens) formed a sister group and have been placed in a new Section (Penda). Rapid molecular evolution in Arceuthobium may be associated with the adaptive radiation of this genus on numerous conifer hosts.