A phylogenetic analysis of the genus Mertensia (Boraginaceae): Taxonomy, divergence times, and biogeography

This study explores the taxonomy, systematics, and biogeography of Mertensia. Mertensia is a monophyletic genus of herbaceous perennials that comprises 62 species in Asia, Beringia, North America, and circumboreal locales, with a geographic center of diversity in the western United States. The phylogenetic relationships of Mertensia have been uncertain, with placement in various tribes in subfamily Boraginoideae. Moreover, Mertensia has presented considerable taxonomic problems especially centered in broadly circumscribed taxa. Using a phylogenetic approach, we evaluate relationships within Boraginaceae, infer evolutionary lineages, and address problematic taxonomic circumscriptions. We applied DNA sequence data to a broad sampling of Boraginaceae and found strong support for the monophyly of Mertensia and Asperugo as its sister. Mertensia was strongly supported as a member of the tribe Cynoglosseae in Boraginoideae.;Phylogenetic reconstructions using eleven chloroplast markers recovered three deep clades in Mertensia that included: (1) Asian taxa, a Beringian subclade, and a circumboreal subclade; (2) the Beringian M. rivularis; and (3) North American taxa. Although we recovered weak support for several deeper nodes in the North American clade, our results provided moderate to strong support for 12 subclades. Notable results include broadly circumscribed taxa (M. ciliata, M. lanceolata, M. oblongifolia, M. viridis) as polyphyletic; we recommend narrow circumscriptions for each of these taxa.;Divergence times for Mertensia were estimated with molecular dating methods. Our results indicate that the stem lineage of Mertensia split from its nearest extant relative, Asperugo, 26.83–12.22 million years ago (Ma), followed by the first divergence in the crown group at 10.36–5.19 Ma. From geographic reconstructions we infer the ancestral area for Mertensia to have been Asia, or a widespread distribution encompassing parts of Asia, Beringia, and circumboreal locales, with a small probability of a North American ancestry. Initial range expansion of North American lineages occurred in Beringia and the Pacific Northwest 7.70–4.22 Ma, followed by diversification of three deep clades (Pacific Northwest, Southern Rocky Mountain, Central Rocky Mountain). The Southern Rocky Mountain and Central Rocky Mountain clades used the Rocky Mountains for range expansion from the Pacific Northwest, with subsequent range expansions in the American west.