History and extent of introgressive hybridization in Puget Sound rockfishes (Sebastes auriculatus, S. caurinus, and S. maliger)

Abstract

Natural hybridization is common in closely related species especially where they invade novel habitat. The patterns of introgressive hybridization are often asymmetrical and are attributed to various influences from selection to gene flow and dispersal. Hybridization has previously been detected in S. auriculatus, S. caurinus, and S. maliger in Puget Sound (the southern Salish Sea) but the details of the history and direction of introgression are incomplete. These Pacific rockfish species are sympatric over most of their geographic range but hybridization has only been detected in Puget Sound. In order to measure interspecific gene flow, we used sequence data from one mitochondrial locus, three nuclear intron loci, and one coding gene to compare interspecific gene flow between collections from the Salish Sea and the Pacific coast. Although ancestral polymorphisms could not be excluded in the analysis of phylogenetic trees, coalescence analysis provided clear evidence for broad-scale, asymmetrical introgression from S. maliger into S. auriculatus and S. caurinus and a much lower incidence of introgression between S. auriculatus and S. caurinus. The absence of F1 hybrids was consistent with historical hybridization events or ongoing, low-level hybridization in the Salish Sea. Although hybrids were found in high frequency, introgressed rockfish in the Salish Sea appear to maintain the morphological characters and coloration of pure parental species morphology. This rockfish hybrid system, with asymmetrical introgression and the maintenance of parental species, may prove useful to study both mechanisms that maintain species boundaries and processes that facilitate speciation.