Plastid Phylogenomics Provide Evidence to Accept Two New Members of Ligusticopsis (Apiaceae, Angiosperms)

Peucedanum nanum and P. violaceum are recognized as members of the genus Peucedanum because of their dorsally compressed mericarps with slightly prominent dorsal ribs and narrowly winged lateral ribs. However, these species are not similar to other Peucedanum taxa but resemble Ligusticopsis in overall morphology. To check the taxonomic positions of P. nanum and P. violaceum, we sequenced their complete plastid genome (plastome) sequences and, together with eleven previously published Ligusticopsis plastomes, performed comprehensively comparative analyses. The thirteen plastomes were highly conserved and similar in structure, size, GC content, gene content and order, IR borders, and the patterns of codon bias, RNA editing, and simple sequence repeats (SSRs). Nevertheless, twelve mutation hotspots (matK, ndhC, rps15, rps8, ycf2, ccsA-ndhD, petN-psbM, psbA-trnK, rps2-rpoC2, rps4-trnT, trnH-psbA, and ycf2-trnL) were selected. Moreover, both the phylogenetic analyses based on plastomes and on nuclear ribosomal DNA internal transcribed spacer (ITS) sequences robustly supported that P. nanum and P. violaceum nested in Ligusticopsis, and this was further confirmed by the morphological evidence. Hence, transferring P. nanum and P. violaceum into Ligusticopsis genus is reasonable and convincing, and two new combinations are presented.