Date of Award
Honors College Thesis
Mac H. Alford
Most taxonomic treatments currently recognize two to three species of native yams in eastern North America: Dioscorea villosa, D. floridana, and sometimes D. quaternata, a segregate of D. villosa. Earlier authors (e.g., J. K. Small) had recognized as many as five species (with D. hirticaulis and D. glauca also as segregates of D. villosa). Key morphological features in distinguishing these putative species are rhizome morphology (long and cord-like vs. thick and contorted), number of first leaves (1–3 vs. 4–7), and habitat (sandy, rocky, swampy). Unfortunately, these critical features are rarely collected and preserved on herbarium sheets, given the length and twining nature of these perennial vines. Instead, herbarium material often consists of the terminal part of the vine, usually less than 0.5 m, and reproductive parts of a single sex. To assess species boundaries, then, representatives of the putative species were collected and assessed for genetic variation. Unique haplotypes corresponding to the morphological units would support the hypothesis of separate species; common haplotypes would be ambiguous, that is, would neither support nor refute the hypothesis of separate species, but would provide baseline data for future studies. Dioscorea floridana and a broadly circumscribed D.villosa were recovered as genetically distinct, but no variation was found in the D. villosa complex. Given these data, boundaries within the D. villosa complex remain ambiguous. Combined with data from other studies, these DNA data were then used to infer relationships of the native U.S. species to other species in the genus as well to the species in Dioscorea section Stenophora.
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Collins, Tori A., "Species Delimitation and Phylogenetic Relationships of the Wild Yams (Dioscorea) Native to Eastern North America" (2015). Honors Theses. 318.