Phylogeny and Evolution of Phemeranthus (Montiaceae) in North American Xeric Habitats

The genus Phemeranthus (Montiaceae; fameflowers, rockpinks, sunbrights) comprises ca. 25 species of succulent, terete-leaved herbaceous perennials, mostly found in xeric rock outcrops and sand barrens. Phemeranthus' center of diversity is in northern Mexico and the southwestern United States, but several species occur in glade and flat-rock ecosystems in the midwestern and southeastern United States. DNA sequences of chloroplast and low-copy nuclear regions were used to infer the phylogenetic relationships of Phemeranthus species. Phemeranthus (excluding P. aurantiacus) is monophyletic and likely sister to the remainder of Montiaceae. The genus contains two geographically structured and morphologically distinguishable clades: a southern clade centered in Mexico and a northern clade distributed primarily in the United States. Dramatic range disjunctions within each clade suggest broad-scale movements early in the genus' diversification, while the current distribution indicates an origin in the southwestern United States and northern Mexico followed by northward and eastward expansion. Discordance between the chloroplast phylogeny and morphological species boundaries and between chloroplast and nuclear gene trees was further explored using multi-locus species-tree reconstruction methods. The results indicate that hybridization has played an important role in the evolution of this xerophytic genus. Finally, in a greenhouse-based experiment, seeds of the widespread species P. parviflorus collected from natural populations along a latitudinal gradient were chilled for varying periods prior to germination. The differential responses of seed germination to chilling duration for the sampled populations suggest the presence of local adaptation or at least of adaptive phenotypic plasticity, an important consideration for the use of this species in ecological restoration and green-roof projects.