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, Ohio and Pennsylvania, indicating apomictic reproduction is common in these groups. While the triploid clusters were distinct from each other, they did have strong associations with sympatric diploid species, and also with the western triploid species C. reticulata. The triploid Celtis pumila demonstrates remarkable phenotypic plasticity, with the same genotype variously presenting as dwarf shrubs in xeric, exposed sites, and subcanopy tree in mesic forests.

This archive contains complete details on the methods used to collect the data, the raw data, and all the code necessary to reproduce the analyses in our manuscript.

Note that Data Dryad required me to include a separate README file and a data file. These are taken from the Zenodo/Github archive. You can ignore the single files: all the information, including these two files, is found in the archive.

README.md contains metadata for the included files. data_prep.R contains the code used to prepare the raw data, and the code to generate the figures are embedded in the text of celtis.Rmd.