A combined parsimony analysis of cpDNA trnK, nrDNA ITS/5.8S, and morphology reveals that the genus Strophostyles is monophyletic. In contrast to the conventional view of the geographic relationships of eastern North America, Strophostyles is most closely related to neotropical genera. Its sister is the South American genus Dolichopsis, which is endemic to the Chaco, a region characterized by having an annual frost interval. Strophostyles is apomorphically diagnosed by divergent stipules, persistent secondary floral bracts, calyces with four acute to sometimes attenuate lobes, and seed testa often with a cellular coat. The relationship with Dolichopsis is supported in part by a shared keel petal morphology involving a gibbous ventral margin proximal to the rostrum. Phylogenetic analysis of ITS/5.8S sequences and morphometric analysis of quantitative traits suggest that the three traditionally recognized species of Strophostyles can be recognized under the phylogenetic species concept. Strophostyles umbellata is the most genetically variable at the ITS locus and geographically centered in southern Appalachia. Strophostyles helvola shows the least amount of intraspecific genetic variation at this locus, suggesting a recent and rapid range expansion throughout eastern North America. Nucleotide sequence variation is intermediate in Strophostyles leiosperma, a species distributed primarily in central North America.
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