Nuclear DNA-based phylogenetic analysis of Neocinnamomum species

Neocinnamomum plants are evergreen shrubs or small trees belonging to the Neocinnamomeae tribe of the Lauraceae family. Their seeds are rich in fatty acids, and their leaves are often used in traditional Chinese medicine. Presently, only a few studies have been performed on Neocinnamomum plants; therefore, the genome-based phylogeny among Neocinnamomum species has not been determined, which limits the germplasm innovation of this genus. In this study, by the Illumina (next-generation sequencing) and third-generation sequencing technologies, the whole genomes of seven Neocinnamomum species samples were sequenced, their nuclear DNA (nrDNA) sequences were assembled and characterized, and their phylogeny was reconstructed. The results revealed four hypervariable regions (i.e. transcribed spacer regions) in the nrDNA sequences, among which the highest degree of variation was observed in the external transcribed spacer (ETS) region localized behind the 26S gene. A total of 27 insertions/deletions and 184 single-nucleotide polymorphisms, both localized mainly in the ETS and internal transcribed spacer regions, were identified. Phylogenetic trees were constructed based on the nrDNA sequences using the maximum likelihood (ML) and Bayesian inference (BI) methods with Caryodaphnopsis henryi as the outgroup. The ML tree divided the seven Neocinnamomum species into four clades. Clade I consisted of Neocinnamomum caudatum var. macrocarpum and Neocinnamomum caudatum, clade II included Neocinnamomum delavayi and Neocinnamomum mekongense, clade III included Neocinnamomum fargesii and a branch species of N. delavayi and clade IV included Neocinnamomum lecomtei, constituting a monophyletic and basal group. The BI tree shared the same topological structure as the ML tree, and all the support values of the BI tree were one except for that of one Neocinnamomum species (0.98). The results of this study provide new evidence regarding the phylogenetic evolution of the Neocinnamomum plants.