Plastid phylogenomic insights into the evolution of Distylium (Hamamelidaceae)

Background: Most Distylium species are endangered. Distylium species mostly display homoplasy in their flowers and fruits, and are classified primarily based on leaf morphology. However, leaf size, shape, and serration vary tremendously making it difficult to use those characters to identify most species and a significant challenge to address the taxonomy of Distylium. To infer robust relationships and identify variable markers to identify Distylium species, we sequenced most of the Distylium species chloroplast genome. Results: The Distylium chloroplast genome size was 159,041–159,127 bp and encoded 80 protein-coding, 30 transfer RNAs, and 4 ribosomal RNA genes. There was a conserved gene order displayed and a typical quadripartite structure. Phylogenomic analysis based on whole chloroplast genome sequences yielded a highly resolved phylogenetic tree and formed a monophyletic group containing four Distylium clades. A dating analysis suggested that Distylium originated in the Oligocene (34.39 Ma) and diversified within approximately 1 Ma. The evidence shows that Distylium is a rapidly radiating group. Four highly variable markers, such as matK-trnK, ndhC-trnV, ycf1, and trnT-trnL, and 74 polymorphic simple sequence repeats were discovered in the Distylium plastomes. Conclusions: The plastome sequences had sufficient polymorphic information to resolve phylogenetic relationships and identify species accurately.