EVOLUTION OF THE ARALIA–PANAX COMPLEX (ARALIACEAE) AS INFERRED FROM NUCLEAR RIBOSOMAL ITS SEQUENCES

Abstract

The Aralia–Panax complex consists of three closely related genera of Araliaceae (Aralia L., Panax L. and Sciadodendron Griseb.), widely distributed in temperate, subtropical, and tropical regions of Asia and the Americas. Three major lineages (Panax, Aralia sect. Aralia, and Aralia sect. Dimorphanthus Miq.) are disjunctly distributed in eastern Asia and North America. Comparative sequencing of the internal transcribed spacer (ITS) regions of nuclear ribosomal DNA was carried out in order to examine the phylogenetic and biogeographic relationships of this complex. Results suggest Aralia is paraphyletic, with Panax and the Central American monotypic Sciadodendron nested within. The monophyly of Panax (P. trifolius L. maybe excepted), Aralia sect. Aralia (except A. henryi Harms), and Aralia sect. Humiles Harms is supported. The morphologically isolated A. hispida Vent. is embedded within Aralia sect. Dimorphanthus. The phylogenetic data suggest the need to re-examine the generic delimitation of Aralia. None of the presumed intercontinental species pairs between eastern Asia and eastern North America (e.g. Aralia cordata Thunb. and A. racemosa L., A. spinosa L. and A. stipulata Franch., and Panax ginseng C. A. Mey. and P. quinquefolius L.) have been supported as sister species, and the floristic similarity between eastern Asia and eastern North America seems to have been exaggerated by previous workers. Within Aralia sect. Dimorphanthus, the north temperate series Dimorphanthus was paraphyletic, with the subtropical and tropical series Chinensis J. Wen and series Foliolosae J. Wen nested within. This pattern of differentiation is consistent with the hypothesis of morphological stasis among the closely related disjunct taxa between eastern Asia and eastern North America in series Dimorphanthus. Subtropical/tropical Sciadodendron, and Aralia sect. Pentapanax (Seem.) J. Wen are suggested to have derived from temperate elements, although Araliaceae have a generally subtropical and tropical distribution.