Systematics, biogeography, and character evolution of Deutzia (Hydrangeaceae) inferred from nuclear and chloroplast DNA sequences

- Our study represents the first molecular phylogenetic analysis of Deutzia.
- We propose to merge sections Mesodeutzia and Deutzia.
- The induplicate petals, 2-dentate filaments, and polystemonous androecia are the ancestral states.
- We suggest a Northeast Asian origin for Deutzia and subsequent spread to Mexico and SW China.
- The uplift in the Qinghai-Tibetan Plateau and monsoon regimes are important factors for Deutzia diversification.

The genus Deutzia (Hydrangeaceae), containing ca. 60 species circumscribed in three sections, is disjunctly distributed in eastern Asia and Central America (Mexico). Although the genus is well delimited, its subdivisions into sections and series have not been the subject of an explicit test of monophyly based on molecular data. A comprehensive examination of the evolutionary relationships within the genus is thus still lacking. We present a fossil-calibrated, molecular phylogeny of Deutzia based on two nuclear ribosomal DNA (ITS and 26S) and three chloroplast DNA regions (matK, rbcL, and trnL-F intergenic spacer). Within this framework, we examine character evolution in petal arrangement, filament shape, and the number of stamens, and infer the ancestral area and biogeographic history of the genus. Our molecular phylogeny suggests that Deutzia is monophyletic. Two major clades are recovered: one composed of the species of sect. Neodeutzia from Mexico, and the other containing all remaining Deutzia species of sections Mesodeutzia and Deutzia from SW China and Northeast Asia. The latter two Asian sections were each revealed to be polyphyletic. The induplicate petals, 2-dentate filaments, and polystemonous androecia are inferred to be ancestral character states. Biogeographic reconstructions suggest a Northeast Asian origin for the genus and subsequent spread to Mexico during the Oligocene and to SW China during the Miocene. Based on our results, a new infrageneric classification of Deutzia inferred from molecular phylogeny is required. We propose to merge sections Mesodeutzia and Deutzia to ensure the monophyly at the sectional level. Cooling trends during the Oligocene resulted in isolation, separating eastern Asian and Mexican taxa, while the warm period during the middle Miocene stimulated the diversification from Northeast Asia to SW China. The uplift in the Qinghai-Tibetan Plateau and monsoon regimes are important in promoting high species diversification of Deutzia in SW China.