Phylogeographic and phylogenetic analysis for <i>Tripterygium</i> species delimitation

Abstract

<i>Tripterygium wilfordii</i> (Celastraceae) is a traditional Chinese medicine; and the dried root and rhizome constitute the main officinal parts. <i>Tripterygium wilfordii</i> has been identified as a potential candidate for the treatment of systemic lupus erythematosus, rheumatoid arthritis, nephritis, asthma, leprosy, and cancer. The phylogenetic relationships within the <i>Tripterygium</i> genus are ambiguous; thus, our aim is to clarify the relationships within this genus using phylogeographic and phylogenetic analyses. Here, we first sequenced three plastid DNA regions (i.e., <i>psb</i>A<i>-trn</i>H, <i>rpl32-trn</i>L, and <i>trn</i>L-<i>trn</i>F) and found that <i>Tripterygium hypoglaucum</i> and <i>T. wilfordii</i> were clustered together based on the strength of the topology in the phylogenetic analysis: <i>T</i>. <i>hypoglaucum</i> is polyphyletic, and <i>T. wilfordii</i> is paraphyletic. A spatial analysis of molecular variance showed that the best group value is 4, and the groups were almost consistent with the topology of in the phylogenetic analysis. The Mantel analyses of <i>Tripterygium</i> using IBD web showed statistically significant relationships between genetic and geographical distance distributions (<i>r</i> = .3479, <i>p</i> < .0001). The molecular dating using Fossil calibration indicated that the divergence in <i>Tripterygium</i> was approximately 8.13 Ma. Furthermore, we also analyzed four DNA regions (i.e., ITS2, <i>psb</i>A-<i>trn</i>H, <i>mat</i>K, and <i>rbc</i>L) that were obtained from the NCBI nucleotide database; these results showed that <i>T. wilfordii</i> and <i>T. hypoglaucum</i> clustered together, while <i>Tripterygium regelii</i> represented a separate cluster. <i>Tripterygium hypoglaucum</i> and <i>T. wilfordii</i> were never distinct lineages, and the species circumscriptions are artificial. We propose that <i>T. wilfordii</i> and <i>T. hypoglaucum</i> are conspecific, while <i>T. regelii</i> likely constitutes a separate species.