Multiple glacial refugia lead to genetic structuring and the potential for reproductive isolation in a herbaceous plant.

PREMISE OF THE STUDY Glacial cycles have influenced the genetic structure of many species. In addition to facilitating genetic divergence, isolation in multiple glacial refugia may have contributed to the development of genetic incompatibility and reproductive isolation. We examined the phylogeography of Campanulastrum americanum, a monocarpic herbaceous plant that exhibits strong intraspecific reproductive isolation, to determine whether the current genetic structure reflects a history of multiple glacial refugia. METHODS Chloroplast loci and nuclear RAD sequencing were used to characterize the range-wide phylogeography of C. americanum, in order to determine locations of potential glacial refugia and recolonization routes. Potential locations of refugia during the Last Glacial Maximum were also identified using ecological niche modeling. KEY RESULTS Together, the chloroplast and nuclear phylogenies found support for three geographically structured, genetically divergent lineages, among which gene flow appears to be restricted. The distribution of these lineages indicates that C. americanum survived the Last Glacial Maximum in at least three refugia located in the Appalachians and on the Atlantic and Gulf coasts. The ecological niche model also supported the existence of multiple refugia. CONCLUSIONS The isolation of populations of C. americanum in multiple refugia has led to a degree of phylogeographic structure greater than that found in most previously studied plants in eastern North America, which may be attributable to its short generation time. Reproductively isolated populations of C. americanum belong to divergent lineages, which suggests that survival in multiple glacial refugia contributed to the development of reproductive isolation in this species.