The Japanese large abalone species, Ezo, Kuro, Madaka, and Megai abalones, have recently evolved following the expansion of their North American ancestor species to the Japanese Archipelago.Although these abalones are genetically similar, they exhibit distinct ecological characteristics.Kuro, Madaka, and Megai, whose distribution areas overlap along the southern coast, occupy different habitat depths.In contrast, Ezo and Kuro are believed to represent cold-and warm-water adapted subspecies, with distributions in the north and south, respectively.The evolution of abalone species, which are related to vertical and horizontal heterogeneity in coastal environments, serves as a crucial model for understanding mechanisms driving marine biodiversity.Population genomics analysis of Kuro, Madaka, and Megai revealed three distinct clades corresponding to each species, suggesting that they are reproductively isolated in natural environments, despite the fact that their crossbreeding produces fertile offspring under laboratory conditions.We also found that their speciation events have been accompanied by continuous gene flow and that genomic introgression from Kuro to Madaka occurs in certain geographic regions, suggesting that speciation is ongoing with incomplete ecological isolation probably due to differences in the habitat depth.Ezo and Kuro formed two distinct genetic clusters but exhibited genomic admixture along boundary coastal regions.Furthermore, we found a significant relationship between genetic structure of the two subspecies and coastal environmental factors, such as mean seawater temperature in autumn.These findings highlight that speciation of abalones is a continuous process driven by unstable ecological barriers, emphasizing the need for conservation strategies that account for such continuity.
Shotaro Hirase (Sat,) studied this question.