Extensive chloroplast haplotype variation indicates Pleistocene hybridization and radiation of North American Arabis drummondii, A. x divaricarpa, and A-holboellii (Brassicaceae)

Arabis drummondii, A. holboellii and their hybrid A. x divaricarpa are widespread perennials of open habitats in North America. A phylogenetic analysis based on noncoding chloroplast DNA sequences (trnL intron and trnL/F intergenic spacer) resolved A. drummondii as a monophyletic taxon, but found A. holboellii to bear chloroplast haplotypes from highly diverged evolutionary lineages. This raised the question of a possible polyphyletic origin of A. holboellii. Arabis x divaricarpa was found to be of recent and polytopic origin, a result consistent with its presumed hybrid origin. One hundred and three chloroplast haplotypes were detected within 719 Arabis accessions investigated. The majority of chloroplast-types were estimated to have arisen prior to the Wisconsin glaciation. Phylogeographical analysis using nested clade analysis, suggested for A. holboellii (i) past fragmentation events, partitioning genetic variation in several instances between the Sierra Nevada, the Southern Rocky Mountains and the Colorado Plateau on the one hand and the Central to Northern Rockies of the United States and adjacent Cascades on the other; and for both parental species (ii) recolonization of major areas formerly covered by the Wisconsin glaciation by three haplotypes; and (iii) restricted gene flow indicating isolation by distance in areas south of the last glacial maximum. Arabis x divaricarpa is closely codistributed with its parental species and resampled their haplotypes. The highest genetic diversity was found in the Rocky Mountains from Idaho and Montana south to Utah and Colorado. This area was further hypothesized to have played a major role in the origin of both parental species and probably represented an important glacial refugium. However, evidence for glacial refugia was also found in arctic and boreal regions of Alaska and near the Great Lakes. In comparison to nuclear ribosomal internal transcribed spacer data, chloroplast DNA divergence was very high and evidently predated the origin of A. drummondii and possibly A. holboellii. Divergence of major chloroplast lineages dates back to the middle of the Pleistocene at least. Extensive hybridization is the most likely evolutionary factor working on A. holboellii to explain the revealed discrepancy in nuclear DNA and chloroplast DNA diversification.