Comparative genome analysis revealed gene inversions, boundary expansions and contractions, and gene loss in the Stemona sessilifolia (Miq.) Miq. chloroplast genome

Stemona sessilifolia (Miq.) Miq., commonly known as Baibu, is one of the most popular herbal medicines in Asia. In the Chinese Pharmacopoeia, Baibu has multiple authentic sources and there are many similar herbs sold as Baibu in herbal medicine markets. The existence of counterfeits of Baibu brings challenges to its identification. To assist in its accurate identification, we sequenced and analyzed the complete chloroplast genome of S. sessilifolia using next-generation sequencing technology. The genome was found to be 154,037 bp in length, possessing a typical quadripartite structure consisting of a pair of inverted repeats (IRs: 27,090 bp) separated by a large single copy (LSC: 81,949 bp) and a small single copy (SSC: 17,908 bp). A total of 112 unique genes were identified, including 80 protein-coding, 28 transfer RNA and four ribosomal RNA genes. In addition, 45 tandem, 27 forward, 23 palindromic and 104 simple sequence repeats were detected in the genome by repeated analysis. Compared with its counterfeits (Asparagus officinalis and Carludovica palmata) we found that IR expansion and SSC contraction events of S. sessilifolia resulted in two copies of the rpl22 gene in the IR regions and a partial duplication of the ndhF gene in the SSC region. An approximately 3-kb-long inversion was also identified in the LSC region, leading to the petA and cemA genes being presented in the complementary strand of the chloroplast DNA molecule. Comparative analysis revealed some highly variable regions, including trnF-GAA_ndhJ, atpB_rbcL, rps15_ycf1, trnG-UCC_trnR-UCU, ndhF_rpl32, accD_psaI, rps2_rpoC2, trnS-GCU_trnG-UCC, trnT-UGU_trnL-UAA and rps16_trnQ-UUG. Finally, gene loss events were investigated in the context of phylogenetic relationships. In summary, the complete plastome of S. sessilifolia will provide valuable information for the distinction between Baibu and its counterfeits and assist in elucidating the evolution of S. sessilifolia.