DENTON, AMY L.* AND BENJAMIN D. HALL. Department of Botany & Plant Sciences, University of California, Riverside CA 92521 and Departments of Botany and Genetics, University of Washington, Seattle WA 92521. - Adaptive radiation of elepidote rhododendrons in the post-Miocene Himalaya: evidence from RNA polymerase III intron sequence data.
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