Genetic evidence of peripheral isolation and low diversity in marginal populations of the Acropora hyacinthus complex

Zooxanthellate corals are found throughout the tropics, but also extend into subtropical and marginal locations due to the presence of warm ocean currents. The population history of corals in marginal locations is of great interest in relation to changing global climatic conditions, as species edge zones might play an important role in evolutionary innovation. Here, we examine the genetic structure of a widely distributed coral species complex, Acropora hyacinthus, from tropical to high subtropical regions along the Kuroshio Current in Taiwan and Japan. Population genetic analysis of 307 specimens from 18 locations (7 reefal and 11 marginal) identified at least four genetic lineages within the A. hyacinthus complex: HyaA, HyaB, HyaC (dominating reefal locations) and HyaD dominating marginal locations in mainland Japan and Taiwan, except the upper Penghu Islands, which were dominated by HyaC. Crossing experiments suggested semi-incompatibility and hybridization between HyaC and D from reefal locations, implying that the existence of hybridization partners enhances diversification and genetic diversity. An incomplete barrier between the HyaC and HyaD dominations was found along the two straits in the Ryukyu Islands, where Kuroshio Current flows constantly. Despite geographical distance, the genetic composition of populations in mainland Japan was comparable to that in mainland Taiwan, which may reflect a region-specific connectivity around the northern limit of A. hyacinthus in the Pacific. In contrast, populations in the Ryukyu Islands were not significantly different from those of Palau and the Great Barrier Reef. While the precise taxonomic nature of the lineages found around the Kuroshio Current remains to be elucidated, these results indicate that, despite the presence of four lineages in the Kuroshio triangle, low genetic diversity populations of the two main lines might be isolating and differentiating in the marginal region.