Rhododendron is the largest woody plant in the northern hemisphere, and the largest genus of seed plants in my country. There are more than 1,000 species around the world, including about 590 species in China. Rapid radiation evolution and frequent natural hybridization events make the classification and phylogenetic research of azalea genus challenging. Hengduanshan and Himalaya are one of the global centers of distribution and differentiation of plant species of the genus Azalea. They have more than 320 species of this genus, of which about two-thirds are endemic to the region. Therefore, the diversified history of the Rhododendron in Hengduan Mountains and Himalayas has attracted much attention.
Recently, Ku Lianming's team/Li Dezhu team cooperated with Richard Milne, Ph.D., University of Edinburgh, UK, and Peter Hollingsworth, professor of Edinburgh Royal Botanical Garden, to conduct extensive sampling of species of the azalea genus, and obtained shallow genomic data of 161 species in all 8 subgenus of the azalea genus, including about 45% of the species in the Hengduan Mountains and Himalayas. Based on chloroplast genomic data, the phylogenetic relationship and diversity history of the Rhododendron genus were explored through phylogenetic reconstruction, differentiation time estimation and species diversity rate.
study shows that the chloroplast genome can better solve the phylogenetic relationship between subgenus and most species of the Rhododendron genus. Compared with the phylogenetic comparison of the phylogenetic inferred based on chloroplast genome and nuclear genome data in the early stage, the study found that the genus Azalea includes 13 stable single-line branches; the strong conflict between the phylogenetic and plastid phylogenetic suggests that deep branches may have reticular evolutionary events such as hybridization/gene introversion. The foliar genus azalea (R. subg. Therorhodion) differentiated during the Eocene (about 56 Mya). After the historical changes of the Eocene and Oligocene (about 32 Mya), it rapidly diversified in a relatively short period (within about 6 Mya) in the early Miocene (23.8 ~ 17.6 Mya) and formed 10 branches of the core genus (12 branches in total). The diversification history of the two subgenus (representing more than 90% of the species in the region) with Hengduan Mountains and Himalayas as the center of differentiation is different. The azalea subgenus (R. subg. Rhododendron) originated in the early Miocene (20.1 Mya), and the diversification rate increased significantly after the subsequent (about 16.6 Mya), and after that (about 13.7 Mya) differentiated into two branches that adapt to high altitude (Clade RH) and low altitude (Clade RL), which coincides with the continued geological tectonic events in the mid-Middle Miocene (about 17 ~ 14 Mya) and the uplift to near the present altitude. The continuous active orogenic movement , complex terrain, heterogeneous habitats and diffusion barriers caused by factors such as the cooling climate and the intensification of Asian summer monsoons have promoted the formation of parallel radiation species in these two branches. Another subgenus evergreen azalea subgenus (R. subg. Hymenanthes) with the Hengduan Mountains and Himalayas as the differentiation centers originated later in the early Miocene (19.5 Mya). Its crown group began to differentiate in the late Miocene (about 10 Mya), and the diversification rate began slowly. However, after the Pliocene (about 5 Mya), the rapid radiation differentiation occurred into species after diffusing to the Hengduan Mountains-Hamalayas in the morning, the net diversification rate was nearly 5 times that of the azalea subgenus. This may be caused by the spread of the subgenus Evergreen Rhododendron in Hengduan Mountains and Himalayas. At the same time, the global climate cooling and monsoon strengthening of Pliocene on may directly affect its differentiation rate. Although some branches of the evergreen azalea subgenus contain only species that are adapted to high altitude, most branches have species that are adapted to low altitude and high altitude at the same time; compared with the azalea subgenus, their altitude preference is more malleable, indicating that the subgenus has strong ecological plasticity in the diversification process. This study provides important research cases for understanding the history and adaptive evolution of species diversity formation in the Hengduan Mountain-Himalayan region.
related research results are published on Annals of Botany under the title of Resolution, conflict and rate shifts: insights from a densely sampled plastic phylogeny for Rhododendron (Ericaceae).The research work has been supported by Strategic Pioneer Science and Technology Special Project (Class B), National Natural Science Foundation of China , major projects of Yunnan Province Basic Research Program, and Yunnan Province Postdoctoral Targeted Training Funding Project.
Figure 1. Azalea phylogenetic tree constructed based on chloroplast genome
Figure 2. Azalea differentiation time and diversification rate based on chloroplast genome
Source: Kunming Institute of Botany, Chinese Academy of Sciences
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