Hall Lab

University of Washington


The major current interests of our lab are to better understand speciation, evolution and biogeographic history in genus Rhododendron. These interests encompass mainly rhododendrons found in three regions:

- China and the Himalayas, center of diversity of Rhododendron subgenus Hymenanthes, of which 250 species are known. Both in Sino-Himalaya and Malesia, Rhododendron diversification and tectonic/volcanic mountain building are closely associated.

- Malesia, where epiphytic rhododendrons of section Vireya rapidly spread from the Asian mainland across Sumatra, Borneo, and New Guinea, spawning more than 300 distinctive species.

- The region between the Cascade Mountains and the Pacific Ocean: Here, R. macrophyllum (Pacific Rhododendron) and R. occidentale (Oregon azalea) are the two most important Rhododendron species. Hall lab studies of R. macrophyllum have uncovered an amazing degree of genetic polymorphism at all loci tested.

Evolutionary History of Rhododendron subgenus Hymenanthes

Hymenanthes is a 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 this subgenus is highest in SW China, in the Himalayan area, this is not their center of origin. Ancestral area reconstructions suggest that both the genus Rhododendron and subgenus Hymenanthes originated in NE Asia.

Current research seeks to understand 1) the genomic architecture of subgenus Hymenanthes, using next-generation sequencing and linkage mapping, and 2) the origin and dispersal of subgenus Hymenanthes, using molecular systematics and restriction-site associated DNA sequencing.

*Hypothesized dispersal of Rhododendron subgenus Hymenanthes.

Evolution and Biogeography of Rhododendron section Vireya

Vireyas are unique among rhododendrons in many respects:

They are tropical in distribution, occurring in the jungles and mountains of SE Asia, including Indonesia, the Malay Peninsula, Malaysia, the Philippines, the Solomon Islands and Australia; and many species grow as epiphytes on jungle vegetation.
They have colorful and varied inflorescence types.
They attract a wide variety of pollinators, including bees, birds, bats, butterflies and moths.
There are a large number and great diversity of species on the islands of Borneo and New Guinea.
They have lightweight, winged seeds, which facilitate dispersal.

(1)                                        (2)                                        (3)

(1) Rhododendron acrophilum, butterfly pollinated
(2) Rhododendron tuba, moth pollinated
(3) Rhododendron blackii, bird pollinated

Previously published research by our lab and others using phylogenetic reconstructions based on RNA polymerase genes (RPB2-D, RPB2-I, RPC1) and several regions of plastid DNA (Brown et al. 2006, Goetsch et al. 2011) showed that the ancestors of vireyas were Asian mainland lepidote rhododendrons, and that vireyas exhibited a clear west to east dispersal. The most derived clade encompasses species from 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.

*Map of SE Asia with numbered clades that correspond to groupings on phylogeny below, figure modified from Goetsch et al. 2011.

*Phylogeny using RPB2D, RPB2I, RPC1, figure modified from Goetsch et al. 2011.

A focus of current and future research in our lab is to explore how the eastward 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 central Malesian and eastern Malesian vireya species. Our goal is to use genome-wide molecular data to develop a well resolved and densely sampled phylogenetic reconstruction to identify dispersal events between islands and to identify genes involved in floral changes.

Characterizing Flower Symmetry Genes in Rhododendron

Flowering plants exhibit diverse forms of floral symmetry that represent many independent changes throughout their evolutionary history. This phenomenon is also observed within the genus Rhododendron, which displays a variety of radially and bilaterally symmetric flowers. A number of transitions between between radial and bilateral symmetry have been associated with duplication of the TCP transcription factor CYCLOIDEA (CYC) in flowering plants. Therefore, we hypothesize that duplications in CYC play a role in the symmetry changes that have occurred in Rhododendron. We have sampled CYC sequences from across the major subgenera in Rhododendron and have identified at least 2 CYC duplications throughout the genus. Future research will pinpoint timing of these duplications and examine the function of CYC duplicates in this group.

Rhododendron macrophyllum Population Structure

Much of the Pacific Northwest was laden with ice at the last glacial maximum, presenting the need for large scale migrations or refugial survival for many plant species. Rhododendron macrophyllum is an evergreen rhododendron that is distributed from British Columbia, Canada, to Monterey Bay, California, USA. It is found from sea level to high mountain peaks, although populations persisting today have disparate positions, and populations are not continuous. Despite a wide range, little morphological variation occurs.

To observe genome wide patterns of diversification in this species that has a history complicated by glacial cycling in the Pacific Northwest, we sampled approximately 2% of the genome from ~100 samples using restriction-site associated DNA sequencing.

Taken together with previous studies of single markers, there appears to be strong homogeneity across populations of Rhododendron macrophyllum, suggesting a recently constricted larger ancestral population. Ecological niche modeling using climate models from 20,000 years ago support this wider range, across Western North America, and down the ridge of the Cascades, south to Mexico.