A phylogenetic and biogeographic study of Hamamelis (Hamamelidaceae), an eastern Asian and eastern North American disjunct genus

Hamamelis (Hamamelidaceae) consists of four to six species disjunctly distr ibuted in eastern Asia and eastern North America. Phylogenetic analyses wer e conducted using sequences of the internal transcribed spacer (ITS) region s of the nuclear ribosomal DNA. The North American species of Hamamelis for med a monophyletic group. The previously proposed close relationship betwee n the eastern Asian H. japonica and the eastern North American H. vernalis was not supported by the ITS phylogeny. The lack of direct intercontinental sister species relationship was reported from most other eastern Asian and eastern North American disjunct genera, suggesting the relative antiquity of the disjunct pattern. Fossil evidence supported the Bering land bridge h ypothesis as the explanation for the formation of the disjunct pattern in H amamelis. Although generally treated under the synonymy of H. virginiana, H . macrophylla from southeastern US and H. mexicana from Mexico had an ident ical ITS profile with H. vernalis from the Ozark Plateau, but distinct from H. virginiana, suggesting the need for a taxonomic reexamination. (C) 1998 Elsevier Science Ltd. All rights reserved.