Complete chloroplast genomes of two medicinal Swertia species: the comparative evolutionary analysis of Swertia genus in the Gentianaceae family

Abstract

The complete chloroplast genome of Swertia kouitchensis has been sequenced and assembled, compared with that of S. bimaculata to determine the evolutionary relationships among species of the Swertia in the Gentianaceae family. Swertia kouitchensis and S. bimaculata are from the Gentianaceae family. The complete chloroplast genome of S. kouitchensis was newly assembled, annotated, and analyzed by Illumina Hiseq 2500 platform. The chloroplast genomes of the two species encoded a total of 133, 134 genes, which included 88-89 protein-coding genes, 37 transfer RNA (tRNA) genes, and 8 ribosomal RNA genes. One intron was contained in each of the eight protein-coding genes and eight tRNA-coding genes, whereas two introns were found in two genes (ycf3 and clpP). The most abundant codon of the two species was for isoleucine, and the least abundant codon was for cysteine. The number of microsatellite repeat sequences was twenty-eight and thirty-two identified in the chloroplast genomes of S. kouitchensis and S. bimaculata, respectively. A total of 1127 repeat sequences were identified in all the 23 Swertia chloroplast genomes, and they fell into four categories. Furthermore, five divergence hotspot regions can be applied to discriminate these 23 Swertia species through genomes comparison. One pair of genus-specific DNA barcodes primer has been accurately identified. Therefore, the diverse regions cloned by a specific primer may become an effective and powerful molecular marker for the identification of Swertia genus. Moreover, four genes (ccsA, ndhK, rpoC1, and rps12) were positive selective pressure. The phylogenetic tree showed that the 23 Swertia species were clustered into a large clade including four evident subbranches, whereas the two species of S. kouitchensis and S. bimaculata were separately clustered into the diverse but correlated species group.

Keywords: Codon usage; DNA barcodes primer; Gene characteristics; Hypervariable region; Phylogenetic analysis; SSR and repeat analysis; Selective pressure; Swertia bimaculata; Swertia kouitchensis.