Polyploidization can be an impetus for evolutionary change, bringing about novel organisms that are adapted to niches unoccupied by parental species. Unlike diploid derivative species, which often show reduced levels of genetic diversity relative to their progenitors, allopolyploid species are expected to harbor higher levels of diversity than their progenitors due to the incorporation of multiple genomes and origination from multiple source populations. In this study, evolutionary origins and population genetic structure of Platanthera huronensis are examined using data from RAPD, ISSR, and chloroplast RFLP markers. Because of its morphological intermediacy, P. huronensis is thought to be a hybrid between Platanthera dilatata and P. hyperborea. An analysis of 305 RAPD and ISSR markers supports this hypothesis; P. huronensis contains markers present in each of the putative parental species, and not detected in other closely related species. Variation in the chloroplast genome in the trnT- trnF intergenic spacers and in the rpl16 intron was revealed by an analysis of RFLP's and suggests that P. huronensis has originated from multiple source populations. A more exhaustive survey of population level diversity with three ISSR primers revealed higher levels of polymorphic loci and expected heterozygosity in P. huronensis (ca. 40%; 0.116) and P. dilatata (ca. 42%; 0.124) compared to P. hyperborea (ca. 31%; 0.091). Nevertheless, an analysis of molecular variation shows populations of all three species to be highly differentiated-- Fst=0.34 in P. huronensis, Fst =0.41 in P. dilatata, and Fst=0.67 in P. hyperborea. These results are considered in the context of life history, geographic distribution, and colonization patterns of the species.

Key words: Allopolyploid, genetic variation, Orchidaceae