Molecular phylogeny and biogeography ofAstilbe(Saxifragaceae) in Asia and eastern North America

Abstract

Phylogenetic analyses were conducted for Astilbe (Saxifragaceae), an Asian/eastern North American disjunct genus, using sequences of nuclear ribosomal internal transcribed spacer (ITS) and plastid matK, trnL-trnF and psbA-trnH regions. The monophyly of Astilbe is well supported by both ITS and plastid sequences. Topological incongruence was detected between the plastid and the ITS trees, particularly concerning the placement of the single North American species, A. biternata, which may be most probably explained by its origin involving hybridization and/or allopolyploidy with plastid capture. In Astilbe, all species with hermaphroditic flowers constitute a well-supported clade; dioecious species form a basal grade to the hermaphroditic clade. Astilbe was estimated to have split with Saxifragopsis from western North America at 20.69 Ma (95% HPD: 12.14–30.22 Ma) in the early Miocene. This intercontinental disjunction between Astilbe and Saxifragopsis most likely occurred via the Bering land bridge. The major clade of Astilbe (all species of the genus excluding A. platyphylla) was inferred to have a continental Asian origin. At least three subsequent migrations or dispersals were hypothesized to explain the expansion of Astilbe into North America, Japan and tropical Asian islands. The intercontinental disjunct lineage in Astilbe invokes a hybridization event either in eastern Asia or in North America. This disjunction in Astilbe may be explained by a Beringian migration around 3.54 Ma (95% high posterior density: 1.29–6.18 Ma) in the late Tertiary, although long-distance dispersal from eastern Asia to North America is also likely. The biogeographical connection between continental Asia, Taiwan, the Philippines and other tropical Asian islands in Astilbe provides evidence for the close floristic affinity between temperate or alpine south-western China and tropical Asia.