It is commonly believed that plants found in the Japan Sea region evolved from similar species on the Pacific side. Notable examples include Camellia japonica and Camellia rusticana, which were thought to have originated in this manner. Yet, recent research indicates that their roots go back to the Miocene period, a time when the Japanese archipelago fractured from the mainland.
The variety and spread of plants have been influenced over time by geological shifts and climate changes, resulting in cycles of migration, extinction, and adaptation to new habitats. The genus Camellia, which consists of over 100 species primarily found in East Asia, represents a significant warm-temperate tree within the Sino-Japanese Floristic Region.
Japan is home to four species of the Camellia genus, with Camellia japonica and Camellia rusticana being the most prominent. C. japonica has a wide distribution ranging from Aomori Prefecture in the cooler northern regions to subtropical Taiwan and the coastal areas of China, indicating its strong ability to adapt to various climates. In contrast, C. rusticana is a plant that thrives in areas with heavy snowfall and is classified as a Japan Sea element. It was long assumed that these plants evolved from closely related species found on the Pacific side, with the idea that C. rusticana adapted as an offshoot of C. japonica to better survive in snowy conditions. This assumption had not been rigorously tested until now. The aim of this study was to investigate the evolutionary lineage of these two species through genetic analysis and ecological niche modeling.
Phylogenetic assessments showed distinct differences between C. japonica, C. rusticana, and C. chekiangoleosa, a related species from the mainland. The analysis suggests that their last common ancestor diverged about 10 million years ago in the Late Miocene, which aligns with the time when the Japanese archipelago broke away from the continent. This evidence points to the two species branching off simultaneously due to geographical separation, refuting the previous theory that C. rusticana stemmed from C. japonica due to glacial changes in the Quaternary period.
Populations of C. japonica fall into three principal groups: northern Japan, southern Japan (inclusive of mainland China and Korea), and the Ryukyu-Taiwan region. Among these, the northern section is especially unique. Investigating their evolutionary pathways revealed that the southern population split from the northern population around 3.3 million years ago. Following this, the Ryukyu-Taiwan population diverged from the southern group 1.8 million years ago, and the continental group (including Korea) separated from the southern population just 27,000 years ago. These results illustrate that C. japonica initially migrated from the continent to the Japanese archipelago, then adapted and eventually re-colonized the continent (reverse colonization). This provides compelling evidence that islands can be sources of genetic diversity rather than evolutionary dead ends.
This research not only clarifies the evolutionary background of Camellia but also demonstrates how the formation of the Japanese archipelago has directed plant evolution. “By uncovering the history of camellias, we can potentially gain new perspectives on the evolutionary pathways of other unique Japanese plants,” states Dr. Harue Abe. Additionally, this research isn’t solely focused on past events; it also offers insights that may help predict future shifts in plant distribution. “With ongoing global warming, understanding how Camellia species might change their geographical ranges is increasingly critical. Our findings are vital for anticipating these changes,” she emphasizes.
