American highbush cranberry maintains strong population structure despite naturalization of Eurasian relatives in North America

Premise: The proper classification of taxa is often debated, particularly when organisms lack qualitative diagnostic traits. Highbush cranberry taxa (Viburnum spp.) have been the subject of such disputes since their characterization by 18th- and 19th-century botanists. Despite their allopatric distributions—V. trilobum in North America, V. opulus in Europe, and V. sargentii in Asia—these taxa have received numerous taxonomic treatments as species, subspecies, and varieties due to their morphological similarities. Genetic evidence has shown these taxa to be distinct; however, the human-mediated introduction of V. opulus and V. sargentii into North America may remove their geographic and genetic isolation, with implications for the conservation of V. trilobum. Methods: We used single-nucleotide polymorphisms generated via reduced representation sequencing (DArTseq) to assess the genetic structure and diversity of these taxa, focusing on the impact of V. opulus and V. sargentii introduction into North America. Results: Consistent with prior studies, V. trilobum, V. opulus, and V. sargentii were found to be genetically distinct species. European V. opulus—and, to a lesser extent, V. opulus × V. sargentii hybrids—were found to be widely naturalizing in North America. However, interspecific V. trilobum hybrids were notably rare. All three taxa exhibited low genetic diversity and evidence of clonality. A cryptic subgroup of V. sargentii, originating in Japan, was identified. Conclusions: American V. trilobum shows evidence of continued genetic isolation despite the introduction of Eurasian taxa into North America, suggesting the presence of an unknown reproductive barrier.