Complete chloroplast genome of Salvia plebeia: organization, specific barcode and phylogenetic analysis

Salvia plebeia has been in use as traditional Chinese medicine (TCM) for more than 500 years. In this study, the complete chloroplast (cp) genome of S. plebeia was sequenced, assembled and compared to those of other five published Salvia cp genomes. It was found that the cp genome structure of S. plebeia was well conserved and had a total size of 151 062 bp. Four parameters were used to display the usage conditions of the codons of the amino acids in Salvia genus. Although the number of protein-coding genes in each species was the same, the total number of codons was different. Except for amino acids Trp and Met whose Relative Synonymous Codon Usage (RSCU) value of one condon was equal to 1, the remaining 19 amino acids had 1-3 preferred codons. The preferred codon names of each amino acid were coincident. The period size for the tandem repeats of six species ranged from 9 to 410 bp. Salvia cp genomes mainly possessed tandem repeats with a copy number less than or equal to 3. The sequence length of tandem repeats of the six species ranged from 25 to 824 bp. Highly viarable regions including four intergenic spacers and six partial genes were discovered as potential specific barcodes for Salvia species through cp genome-wide comparison. Finally, we performed phylogenetic analyses based on the complete cp genome and coding sequences respectively. These results provide information to help construct the cp genome library for Salvia, which may support studies of phylogenetics, DNA barcoding, population and transplastomics.