Allopatric origin, secondary contact and subsequent isolation of sympatric rockfishes (Sebastidae: Sebastes) in the north-western Pacific

Understanding how speciation occurs is central to biology. Gene flow between diverging taxa is correlated with geography and other aspects of speciation; therefore, the examination of gene flow during divergence is a potent approach to understanding the nature of speciation. Here, we inferred the speciation process of the sympatric rockfishes Sebastes steindachneri and Sebastes wakiyai in the north-western Pacific and its marginal seas based on genome-wide single nucleotide polymorphism and mitochondrial DNA data. Model-based demographic inference showed that gene flow between the two species was absent in the initial and late stages of divergence and present only in the middle stage. Population expansion occurred before or during the period of gene flow. The estimated timings of the initial divergence and population expansion fell within the Pleistocene, during which the seas currently inhabited by the two species were repeatedly isolated and reconnected. Contemporary isolation was supported by the absence of hybrids and the shared mitochondrial DNA haplotypes. Our results suggest that the two species initially diverged in allopatry, followed by secondary contact and introgression and by the completion of reproductive isolation. Given that complete isolation following secondary contact has rarely been tested or documented in marine organisms, we highlight the importance of careful consideration of alternative divergence scenarios to be tested, which should take into account the geological and environmental settings.