Rhododendron subg. Hymenanthes (ca. 400 species) comprises 24 taxonomic subsections, 23 of which occur only in southeastern Asia. The combined forces of tectonic uplift and monsoon-driven erosion have shaped the terrain of the Himalaya and the adjacent mountainous Salween-Mekong-Yangtze river system. A nearly perfect overlap exists between this region of high geological relief and that of maximum species-richness in subg. Hymenanthes. New habitat creation resulting from rapid uplift of the Tibetan plateau and ensuing changes in monsoonal patterns, river basin connectivity and glaciation provides a plausible explanation for the occurrence of rapid rhododendron speciation in this area. Paleoclimatic and paleobotanical evidence argues that during the mid-Eocene, when the Indian and Asian plates first collided, rhododendrons were present only at high latitudes. In contrast to the modern ranges of the subsections described above, no members of Rhododendron subsect. Pontica are found in the Himalayan region. Instead, species are distributed in Turkey, eastern North America and the Pacific rim. To study the origin and tempo of speciation of elepidote rhododendrons, we sequenced a 1200 bp intron in RPC1, encoding the largest subunit of RNA polymerase III, from 22 Sino-Himalayan species and all 12 Pontica rhododendrons, and carried out a phylogenetic analysis. The inferred tree strongly supports a vicariance hypothesis of rhododendron distribution. Subsect. Pontica rhododendrons are basal and form two clades: a Eurasian group and a Beringian group. The non-Pontica Asian rhododendrons form a monophyletic assemblage, sister to the Eurasian subsect. Pontica rhododendrons. Clock-like evolution of the RPC1 gene within Rhododendron permits several key nodes in the phylogeny to be dated by fossils or paleogeological events. This suggests a rapid post-Miocene diversification of rhododendron species after the Himalayan orogeny had begun and may signify an adaptive radiation in response to habitat expansion. Our data also suggest that several other recognized elepidote subsections do not constitute natural groups.

Key words: adaptive radiation, intron, phylogeography, Rhododendron, RNA polymerase