CURRENT RESEARCH

Evolutionary History of Hymenanthes Rhododendrons

Hymenanthes is the subgenus that includes more than 300 species of broadleaf evergreen Rhododendrons, ranging in appearance from the generic (R. macrophyllum) to the exotic big leaf (R. macabeanum) and dwarf alpine (R. forrestii) forms from South Asia.

While the species diversity of these plants is highest in the SW China – Himalayan area, this is not their center of origin. Ancestral area reconstructions using DIVA suggest that both the genus Rhododendron and subgenus Hymenanthes originated in NE Asia.

The sister taxon to Hymenanthes is Rhododendron section Pentanthera, which includes the deciduous azaleas native to the SE USA, R. occidentale (Oregon and California), R. luteum (Turkey) and R. molle (E. China and Japan). Ongoing research seeks to characterize, using molecular systematics, the events that have occurred in speciation and phylogeography since Hymenanthes and Pentanthera diverged.

 

Evolution and Biogeography of Rhododendron Section Vireya

Vireyas are unique among Rhododendrons in many respects:

• Their native location: The jungles and mountains of extreme SE Asia, including the Philippines, Malay Peninsula, Malaysia, Indonesia, the Solomon Islands and, for two species Northern Queensland, Australia. Many vireya species grow as epiphytes on jungle vegetation.
• Their colorful and varied inflorescence types
• The wide variety of pollinators they attract, including bees, birds bats, butterflies and moths
• The large number and great diversity of species on the islands of Borneo and New Guinea
• Their lightweight, winged seeds, which facilitate dispersal

R. crassifolium	R. alticolum	R. tuba	R. superbum

An ongoing phylogenetic study of Section Vireya by Loretta Goetsch and Ben Hall in collaboration with Lyn Craven and Gillian Brown at CSIRO Canberra is now nearly completed. The results show that the direct ancestors of the vireyas were Asian mainland lepidote Rhododendrons. The phylogeny, carried out with RPB2, RPC1 and several regions of plastid DNA, exhibits a clear west to east polarity. The most derived clade encompasses all but one of the euvireya species of New Guinea, Australia and the Solomon Islands. Two of the intermediate clades are, respectively, largely made up of species from Borneo and Sulawesi. These and other results are fascinating to juxtapose with the tectonic history of Malesia. Until 15-20 mya New Guinea, linked to Australia, was far South of its present position and presumably Rhododendron-free. Sulawesi was just being assembled from land segments of diverse origin. Thus the dispersal and speciation events that led to more than 150 Vireya Rhododendron species in New Guinea occurred in a dynamic, constantly changing island archipelago. A focus of current and future research in the Hall lab is to explore how the westward passage of Vireya progenitors through successive island venues may have influenced their evolution. In each of the phyla of potential pollinators, endemism is very common in the Malay Archipelago, raising the possibility that adaptation to newly-encountered pollinators may have played a role in the spectacular radiation of flower types and lifestyles of the Middle-Malesian and Eastern Malesian Vireya species. Our goal is to study the genes involved in these changes.

 

Rhododendron macrophyllum Population Structure

A study of allele frequencies in various R. macrophyllum populations in Washington, Oregon, and British Columbia with California.

These populations are highly polymorphic for each of the three loci so far examined. The spatial distribution for four haplotypes of RPB2-i is shown in Outreach. Further studies are planned, by broadening the sampling still further and scoring SNPs in other genes.