A Dispersal-Dependent Zone of Introgressive Hybridization between Weakfish, Cynoscion regalis, and Sand Seatrout, C. arenarius, (Sciaenidae) in the Florida Atlantic

Abstract

Five diagnostic codominant nuclear DNA markers and a diagnostic mitochondrial DNA marker were used to survey weakfish (Cynoscion regalis) and sand seatrout (C. arenarius), with particular focus on heretofore uncharacterized juvenile populations along the Florida (FL) Atlantic coast. Geographic and reproductive ranges of weakfish and sand seatrout were shown to overlap on the Atlantic coast along north and central FL. An active bidirectional zone of introgressive hybridization exists between these taxa, centered in the St Johns River, FL. Strong patterns of Hardy-Weinberg, linkage, and cytonuclear disequilibrium and a bimodal hybrid index distribution were observed for juvenile cohorts in the zone center, coupled with narrow (∼240 km) concordant clines. Parental forms had disparate habitat preferences; hybrid forms occurred predominantly in intermediate habitats. All genetic data were consistent with the hypothesis that the C. arenarius-C. regalis hybrid zone is maintained by a dynamic equilibrium between continued interspecific gene flow and one or more opposing forces. Cytonuclear analyses indicated that parental forms mate assortatively in the zone but that mate recognition was imperfect. Ethological mating dynamics are likely stabilized by some form of endogenous or exogenous postfertilization selection against hybrids such that parental taxa will likely continue to evolve independently.