The genetic diversity of triploid Celtis pumila and its diploid relatives C. occidentalis and C. laevigata (Cannabaceae)

The genus Celtis in eastern North America shows puzzling patterns of variation. While three species are generally recognized, many authors have suggested hybridization may be blurring the boundaries among them. Suspected hybridization between C. occidentalis and C. pumila has hampered conservation planning for the latter, which is a Threatened species in Canada. Using microsatellite markers and flow cytometry, we assessed the relationship between genetic diversity, ploidy, and morphology in this group. We confirmed the presence of two diploid species, C. occidentalis and C. laevigata, and that they do hybridize where they co-occur in southern Missouri and Illinois. We found two triploid genetic groups. These groups had distinct geographic ranges, but were morphologically very similar, corresponding to C. pumila. Furthermore, the triploid groups were characterized by a small number of heterozygous multi-locus genotypes. A single genotype dominated populations across Ontario, Michigan, Oh...