MOLECULAR PHYLOGENETICS OF LIGUSTICUM (APIACEAE) BASED ON NR DNA ITS SEQUENCES: RAMPANT POLYPHYLY, PLACEMENT OF THE CHINESE ENDEMIC SPECIES, AND A MUCH-REDUCED CIRCUMSCRIPTION OF THE GENUS

摘要

Premise of research. Earlier molecular systematic studies revealed that the genus Ligusticum is polyphyletic, with its members allied with representatives of several other genera in five clades of Apiaceae subfamily Apioideae. These studies included only a small sampling of Ligusticum species and its putative allies, including those from the Hengduan Mountains of southwestern China, a major distribution center for the genus. The proper circumscription of the genus, the placement and phylogenetic relationships among those species hitherto accommodated in Ligusticum, and the relationships among the Chinese endemic species have been elusive. Methodology. Internal transcribed spacer (ITS) sequences were obtained from 103 accessions of Ligusticum and its putative allies, representing the most comprehensive sampling of Ligusticum to date (46 species). These data, along with published ITS sequences from selected Apioideae (for a total of 2911 accessions), were subjected to a maximum likelihood analysis. Pivotal results. Besides the five clades to which Ligusticum has been referred previously (Acronema clade, Conioselinum chinense clade, Pyramidoptereae, Selineae, and Sinodielsia clade), two members allied with the East-Asia (Physospermopsis) clade. Most Chinese endemic Ligusticum species (plus members of other polyphyletic genera, such as Cortiella, Ligusticopsis, and Oreocome) fell within the Chinese Ligusticum clade of tribe Selineae. All North American species of Ligusticum except L. scoticum (the generic type) occur within the North American Ligusticum clade (the previously recognized Conioselinum chinense clade). Excluding the monotypic Para-ligusticum and Rupiphila, many Eurasian genera putatively allied with Ligusticum s.lat. (such as Cnidium, Hymenidium, Ligusticopsis, Pachypleurum, Selinum, and Tilingia) are polyphyletic, and none ally with Ligusticum s.s. Conclusions. Ligusticum s.s., placed in the Acronema clade, is reduced to comprise only two species: L. scoticum (with subspecies scoticum and hultenii) and L. holopetalum. Relationships among those other species traditionally accommodated in Ligusticum, especially those of the Chinese Ligusticum clade, are far more complex than previously realized and will require further study utilizing the framework phylogeny presented herein before their taxonomy can be revised to reflect evolutionary relationships.