3 | Results

3.1 | Basic characteristics of chloroplast genomes

This study obtained the complete chloroplast genome sequences of six medicinal plants of Polygonatum : P. zanlanscianense ,P. kingianum , P. sibiricum , P. cyrtonema , P. filipes and P. odoratum . The length of the complete chloroplast genome sequences ranged from 154, 578 bp (P. odoratum ) to 155, 807 bp (P. kingianum ), with an average length of 155, 429 bp. ThePolygonatum chloroplast genome has a typical quadripartite structure, including large single-copy (LSC), small single-copy (SSC), and a pair of IR regions. The length of the LSC ranged from 83, 527 bp (P. odoratum ) to 84, 626 bp (P. kingianum ). In the SSC regions, the chloroplast genome of P. sibiricum was the shortest (18, 415 bp) and P. kingianum was the longest (18, 529 bp). The IR lengths ranged from 26, 297 bp for P. odoratum to 26, 415 bp for P. zanlanscianense . GC content was unevenly distributed across the four parts of the chloroplast genome. The total GC content of the chloroplast genome was 37.7% and the GC contents in the LSC, SSC, and IR regions were 35.7–35.8 %, 31.5–31.6 %, and 43 %, respectively (Figure 1, Table 1). Differences were observed in the number of chloroplast genome genes among the six Polygonatum species. P. kingianum andP. filipes encoded 127 and 130 genes, respectively, whereas the other species encoded 131 genes. These genes comprised 84–85 protein-coding genes, 37–38 transfer RNA genes and 6–8 ribosomal RNA genes. The IR regions included seven protein-coding genes (rpl2 ,rpl23 , rps12 , rps19 , rps7 , ndhB , andycf2 ), 11 tRNA genes (trnG-UCC , trnE-UUC ,trnM-CAU , trnH-GUG , trnI-CAU , trnL-CAA ,trnV-GAC , trnI-GAU , trnA-UGC , trnR-ACG , andtrnN-GUU ), and all four rRNA-coding genes. A total of 22 genes in the chloroplast genomes of the six Polygonatum species contained introns ycf3 and clpP each containing two introns.rps12 , trnK-UUA , rps16 , trnG-UCC ,trnT-CGU , atpF , rpoC1 , trnL-UAA ,trnV-UAC , petB , petD , rpl16 , rpl2 ,ndhB , trnI-GAU , trnE-UUC , trnA-UGC , andndhA contained a single intron. trnT-CGU was annotated only in P. kingianum, which lacked rrn4.5, rpoC1and trnV-UAC. ThendhB gene of P. kingianum did not contain introns.The ycf2 and ycf1 genes of P. zanlanscianense also contained two introns, ycf2 gene of P. sibiricum andP. cyrtonema contained one intron, and P. kingianum ,P. filipes and P. odoratum had no introns. Gene and intron losses occurred during the evolution of the six medicinal plants ofPolygonatum (Table 2). TABLE 1 Basic chloroplast characteristics of six medicinal plants ofPolygonatum .