Characterization of a genomic divergence island between black-and-yellow and gopher Sebastes rockfishes

Islands of high genomic divergence that contain genes of evolutionary significance may form between diverging species. The gopher rockfish, Sebastes carnatus, and black-and-yellow rockfish, S. chrysomelas, are sympatrically distributed temperate marine species inhabiting rocky reefs and kelp forests on the west coast of the United States. Prior studies documented low levels of genetic divergence between the two species, except at a single microsatellite locus that displayed high divergence, Sra.7-2. To better characterize genome wide divergence, we scored 25 additional microsatellite loci. Mean neutral divergence between species (F-ST = 0.01) changed little from prior estimates. Sra.7-2 continued to be an extreme divergence outlier. Five novel microsatellites within +/- 15 kb of Sra.7-2 were characterized. High divergence, consistently low diversity in S. chrysomelas, and linkage disequilibrium were detected at these loci, suggesting the influence of recent selection. However, coalescent modelling of divergence at neutral and Sra.7-2 regions showed that initial divergence at Sra.7-2 was ancient, likely predating divergence at neutral regions. It is therefore unlikely that Sra.7-2 divergence represents solely recent ecological divergence within one species and may represent the action of recurrent selection. Introgressive gene flow (2N(E)m) was much higher (>> 1) at neutral than Sra.7-2 regions (<< 1) despite evidence that two S. carnatus individuals have recently mixed ancestry at the Sra.7-2 region. The Sra.7-2 genomic region is likely one of several regions containing genes involved in initiating and maintaining species integrity. Completion of the final stages of speciation appears to be a slow and ongoing process for these species.