Taxonomy of the Chinese <i>Ligusticum s.l.</i> revisited by molecular data and mericarp morphology

Abstract Ligusticum represents a perplexing taxonomic group within Apiaceae with indistinct circumscription and controversial generic delimitation with its allies, largely as its morphological characteristics vary greatly. We herein performed phylogenetic analyses based on 165 nrDNA ITS sequences and 100 plastomes, complemented by mericarp morphological analyses of 26 Ligusticum s.l. and its allies, to revisit the taxonomy of the Chinese Ligusticum s.l. The morphological diversity of mericarps among Ligusticum s.l. species could serve as a significant foundation for the taxonomic study of this taxon. The phylogenetic analysis revealed that the Chinese Ligusticum s.l. species fell into four clades, namely the Acronema Clade, Selineae, Sinodielsia Clade, and East-Asia Clade. Notably, Ligusticum s.s. was located in Acronema Clade, encompassing only two species. The ITS and plastome trees produced incongruent tree topologies, probably caused by hybridization and incomplete lineage sorting. The circumscription of Chinese Ligusticum should be extremely reduced, and all species of Chinese Ligusticum s.l. involved in this study should be excluded from Ligusticum. According to our study, two new combinations have been proposed. Overall, the taxonomy of the Chinese Ligusticum s.l. is much more complex than expected, and although this study has further deepened our understanding of the taxonomy of this taxon, more investigations are needed to resolve outstanding taxonomic challenges.