Dated phylogeography of western North American subalpine marshmarigolds (Caltha spp., Ranunculaceae): Miocene–Pliocene divergence of hexaploids, multiple origins of allododecaploids during the Pleistocene, and repeated recolonization of Last Glacial Maximum glaciated regions

Aim: In order to understand how a montane polyploid species complex responded to Cenozoic mountain uplift and climate change, we reconstructed the biogeographic history of the subalpine marshmarigold polyploid complex, including Caltha biflora, Caltha chionophila and Caltha leptosepala. Phylogenies at multiple taxonomic levels were used to estimate the timing of species divergence, allopolyploid formation and migration patterns, and to identify Last Glacial Maximum (LGM) refugia and recolonization routes. Location: Western North America. Methods: A fossil-calibrated chronogram was estimated for the eudicot order Ranunculales that was in turn used to set age priors on genus-level Caltha phylogenies. Nuclear ribosomal and chloroplast DNA sequence data were collected from subalpine marshmarigolds, including 161 specimens from across the geographical range of the complex. The datasets were analysed under a strict or relaxed molecular clock and a structured coalescent model of evolution estimated under a Bayesian framework. Results: Hexaploids C. biflora and Caltha chionophila diverged in the Upper Miocene to Lower Pleistocene (chloroplast and nuclear dataset estimates overlapping at the Miocene-Pliocene boundary), and upon secondary contact formed allododecaploid C. leptosepala bidirectionally and at least three times. The hexaploids persisted to the south of LGM icesheets and recolonized LGM glaciated regions in multiple waves, mainly from the C. biflora clade in the Cascades and Coast Ranges. Main conclusions: This study of a widespread western North American plant lineage shows a complex response to past geological and climatic changes, with multiple refugia, allopolyploidization events, and migrations during the Pleistocene. Bidirectional allododecaploid formation has resulted in cryptic lineages, some of which (C. leptosepala in the Cascades and Coast Ranges) have been more successful at reclaiming glaciated regions than others (C. leptosepala in the Northern Rockies).