Molecular phylogeny and species delimitation of Stachyuraceae: Advocating a herbarium specimen-based phylogenomic approach in resolving species boundaries
Species concept and delimitation are fundamental to taxonomic and evolutionary studies. Both inadequate informative sites in the molecular data and limited taxon sampling have often led to poor phylogenetic resolution and incorrect species delineation. Recently, the whole chloroplast genome sequences from extensive herbarium specimen samples have been shown to be effective to amend the problem. Stachyuraceae are a small family consisting of only one genusStachyurusof six to 16 species. However, species delimitation inStachyurushas been highly controversial because of few and generally unstable morphological characters used for classification. In this study, we sampled 69 individuals of seven species (each with at least three individuals) covering the entire taxonomic diversity, geographic range, and morphological variation ofStachyurusfrom herbarium specimens for genome-wide plastid gene sequencing to address species delineation in the genus. We obtained high-quality DNAs from specimens using a recently developed DNA reconstruction technique. We first assembled four whole chloroplast genome sequences. Based on the chloroplast genome and one nuclear ribosomal DNA sequence ofStachyurus, we designed primers for multiplex polymerase chain reaction and high throughput sequencing of 44 plastid loci for species ofStachyurus. Data of these chloroplast DNA and nuclear ribosomal DNA internal transcribed spacer sequences were used for phylogenetic analyses. The phylogenetic results showed that the Japanese speciesStachyurus praecoxSiebold & Zucc. was sister to the rest in mainland China, which indicated a typical Sino-Japanese distribution pattern. Based on diagnostic morphological characters, distinct distributional range, and monophyly of each clade, we redefined seven species forStachyurusfollowing an integrative species concept, and revised the taxonomy of the family based on previous reports and specimens, in particular the type specimens. Furthermore, our divergence time estimation results suggested that Stachyuraceae split from its sister group Crossosomataceae from the New World at ca. 54.29 Mya, but extant species of Stachyuraceae started their diversification only recently at ca. 6.85 Mya. Diversification time ofStachyurusin mainland China was estimated to be ca. 4.45 Mya. This research has provided an example of using the herbarium specimen-based phylogenomic approach in resolving species boundaries in a taxonomically difficult genus.
