Genetic comparison of introduced and native populations of common buckthorn (Rhamnus cathartica), a woody shrub introduced into North America from Europe

Introduced from Europe to North America in the early 19th century as an ornamental shrub and for medicinal purposes, common buckthorn (Rhamnus catharticaL.) has since spread and naturalized throughout regions of the United States and Canada. The purpose of this study was to investigate levels of genetic variation and population differentiation inR.catharticain its introduced range in North America compared with its native range in Europe to better understand patterns of spread. By analyzing introduced and native populations using microsatellite markers, we found that introduced populations generally exhibited similar or slightly lower levels of genetic variation compared with native populations, consistent with a slight bottleneck effect. Introduced populations contained many different genotypes, indicating genetic admixture, rather than one or few genotypes. In a few cases, populations had been misidentified in the field and were glossy buckthorn (Frangula alnusMill.; syn.Rhamnus frangulaL.). Overall, there was no substantial genetic differentiation detected between native and introduced populations ofR. cathartica. Invasive spread in this species is likely due to its past horticultural history as well as adaptive biological traits such as competitive behavior, potential allelopathy, and seed dispersal via birds.