Molecular phylogeny and historical biogeography of Caltha (Ranunculaceae) based on analyses of multiple nuclear and plastid sequences

Caltha is a widely distributed genus in the buttercup family (Ranunculaceae) showing interesting distribution patterns in both hemispheres. Evolutionary history of Caltha was examined by means of phylogenetic, molecular dating, and historical biogeographic analyses with a more comprehensive sampling than previous studies. The internal transcribed spacer from the nuclear genome and trnL-F and atpB-rbcL regions from the plastid genome were used and analyzed using parsimony and Bayesian methods. Divergence time was estimated using Bayesian dating analyses with multiple fossil calibrations. Historical biogeography was inferred using the Bayes-DIVA method implemented in RASP. We obtained a well-resolved and well-supported phylogeny within the Caltha lineage. Caltha natans Pall. diverged first from the genus and the other species grouped into two clades. Our expanded sampling scheme revealed a complicated evolutionary pattern in the C. palustris complex. Caltha sinogracilis W. T. Wang was resolved to be a member of the C. palustris complex, rather than closely related to C. scaposa Hook. f. & Thomson. Caltha rubriflora B. L. Burtt & Lauener was also revealed to be not just a red-flower form of C. sinogracilis. The diversification of the genus began at 50.5 mya (95% high posterior density: 37.1-63.9 mya), and its ancestral range was very probably in the Northern Hemisphere. The South American species may derive from western North American ancestors that dispersed along the western American Cordillera during the Cenozoic era. The vicariance model of the Southern Hemisphere species proposed by a previous study was rejected in this study.