Green turtles (Chelonia mydas) exhibit a global distribution that spans tropical and subtropical regions. Japan serves as the northernmost breeding site and foraging habitat for this species. Previous studies have clarified that there is a distinct distribution boundary of foraging habitats in Japanese waters depending on the turtles' natal origins: The coastal seagrass beds south of Japan's Ryukyu Islands are utilized by populations from low latitudes and the northernmost nesting sites. In contrast, the coastal waters surrounding Japan's main islands are almost exclusively used by the population from the northernmost nesting sites. Since the coastal waters around Japan's main islands are temperate, the water temperature seasonally varies. Especially in winter, it substantially drops compared to the tropical and subtropical waters. In addition, the types of food resources for green turtles also differ in these two waters due to the differences in environments. Thus, the distribution boundary suggests differences in adaptation between turtles that can live in this temperate water and those that cannot, such as tolerance to low water temperatures in winter. However, the differences in adaptive traits in individuals from low latitudes and northernmost natal origins have yet to be clarified due to the challenges in understanding the in-water ecology of such highly mobile marine animals. In addition, seawater temperatures along the coast of Japan's main islands have been increasing due to climate change, which may cause changes in their future distribution. Understanding the genetic signatures related to adaptation can provide insights into how species respond to this global challenge and may be useful for future monitoring.
This study seeks adaptive traits from identifying regions that underwent natural selection using population genomic methods. Whole-genome sequencing data from 17 individuals from the northernmost nesting populations (Ogasawara, Japan) and the low-latitude nesting populations (Southeast Asia and the western Pacific islands) were collected, and their variants were analyzed. The regions under selective pressures were estimated for each population. Based on the genetic loci related to the regions extracted, the adaptive traits of the Northernmost population will be estimated and discussed.