Family Tree Secrets: Island Tree Populations

Tsukuba, Japan: It is often assumed that the island’s plant and animal populations are simply the simple and fragile cousins ​​of those on the mainland. But now, researchers in Japan have discovered that island populations may be much tougher and more complex than previously thought.

In a recently published study, a research group led by the University of Tsukuba has revealed that Siebold’s beech populations on the northernmost island, near crenatathey are older and genetically more diverse than expected.

Island and mainland populations often differ as a result of the islands’ geographic isolation, which is often assumed to restrict the genetic diversity of their populations. However, several studies on land plants have shown that island populations have considerable genetic diversity despite their remoteness, indicating that the processes underlying their diversity are more complex than previously thought.

“Although many island populations have been around for thousands of years or more, the origins of some of them are still unknown,” says Professor Yoshiaki Tsuda, lead author of the study. “This includes the northernmost island populations of Japan of the native species F. crenate.”

The research group investigated populations of F. crenate on the island of Okushiri in the Sea of ​​Japan, which is believed to have separated from the mainland in the Middle Pleistocene (the ice age, which occurred between 2.58 million and 11,700 years ago) and has remained separated ever since. The northward expansion of this species began on the continent approximately 6,000 years ago, after the Last Glacial Maximum (LGM). The researchers studied the genetics of the populations on the island and those in nearby regions, finding that the island populations had high genetic diversity and may not have arisen from a single colonization event.

Populations on Okushiri Island had a comparable number of private alleles (genetic sequences that are present in a single population and essentially absent in other populations) to populations studied in nearby Hokkaido, pointing to the existence of relict populations on Okushiri Island. A relict is a population of organisms that was more widespread or more diverse in the past in a restricted area.

These results, added to the paleoecological and vegetation studies, as well as the geology of the island, indicate that F. crenate persisted in cryptic refugia (places where climatically sensitive species can survive regardless of regional climate incompatibility) on the island.

“Our evidence indicates that populations of this species already existed on Okushiri Island before the LGM and persisted there for longer than previously thought,” explains Professor Tsuda. The results of this study contribute to a growing body of evidence that island plant populations are more genetically diverse than previously estimated, which has implications for conservation research and management of island species, and the study of gene flow between island and mainland populations.

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This study was supported by JSPS KAKENHI (JP17K07852 and JP20K06152) and the Core-to-Core Program (Asia-Africa Science Platforms: JPJSCCB20220007) of the Japan Society for the Promotion of Science and the 27th Grant Program of the Pro Natura Fund of the Fundación Pro Natura Japan.

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The article, “Possible northern persistence of Siebold’s beech, near crenataat its northernmost range limit on an island in the Sea of ​​Japan: Okushiri Island, Hokkaido,” was published in Frontiers in plant science in DOI: 10.3389/fpls.2022.990927

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Associate Professor TSUDA Yoshiaki
College of Environmental and Life Sciences, University of Tsukuba

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Faculty of Life and Environmental Sciences
Sugadaira Research Station, Mountain Science Center