Comparative chloroplast genomics of 34 species in subtribe Swertiinae (Gentianaceae) with implications for its phylogeny.

BACKGROUND: Subtribe Swertiinae, a medicinally significant and highly speciose Subtribe of family Gentianaceae. Despite previous extensive studies based on both morphology and molecular data, intergeneric and infrageneric relationships within subtribe Swertiinae remain controversial. METHODS: Here, we employed four newly generated Swertia chloroplast genomes with thirty other published genomes to elucidate their genomic characteristics. RESULTS: The 34 chloroplast genomes were small and ranged in size from 149,036 to 154,365 bp, each comprising two inverted repeat regions (size range 25,069-26,126 bp) that separated large single-copy (80,432-84,153 bp) and small single-copy (17,887-18,47 bp) regions, and all the chloroplast genomes showed similar gene orders, contents, and structures. These chloroplast genomes contained 129-134 genes each, including 84-89 protein-coding genes, 37 tRNAs, and 8 rRNAs. The chloroplast genomes of subtribe Swertiinae appeared to have lost some genes, such as rpl33, rpl2 and ycf15 genes. Comparative analyses revealed that two mutation hotspot regions (accD-psaI and ycf1) could serve as effective molecular markers for further phylogenetic analyses and species identification in subtribe Swertiinae. Positive selection analyses showed that two genes (ccsA and psbB) had high Ka/Ks ratios, indicating that chloroplast genes may have undergone positive selection in their evolutionary history. Phylogenetic analysis showed that the 34 subtribe Swertiinae species formed a monophyletic clade, with Veratrilla, Gentianopsis and Pterygocalyx located at the base of the phylogenetic tree. Some genera of this subtribe, however, were not monophyletic, including Swertia, Gentianopsis, Lomatogonium, Halenia, Veratrilla and Gentianopsis. In addition, our molecular phylogeny was consistent with taxonomic classification of subtribe Swertiinae in the Roate group and Tubular group. The results of molecular dating showed that the divergence between subtrib Gentianinae and subtrib Swertiinae was estimated to occur in 33.68 Ma. Roate group and Tubular group in subtribe Swertiinae approximately diverged in 25.17 Ma. CONCLUSION: Overall, our study highlighted the taxonomic utility of chloroplast genomes in subtribe Swertiinae, and the genetic markers identified here will facilitate future studies on the evolution, conservation, population genetics, and phylogeography of subtribe Swertiinae species.