What Role Has Hybridization Played in the Replacement of Native Roanoke Bass with Invasive Rock Bass?

North American sunfishes (Centrarchidae) are frequently introduced beyond their native ranges, where they may negatively impact and eventually replace native taxa through a combination of competition and hybridization. Congeneric centrarchids often lack reproductive isolating barriers and appear particularly susceptible to hybridization, though the roles of hybridization and introgression in species replacement are poorly understood. We studied hybridization among native Roanoke Bass Ambloplites cavifrons and introduced Rock Bass A. rupestris in Atlantic Slope river basins of Virginia. We sampled Ambloplites spp. from sites of known and potential species sympatry and reference sites where species were allopatric. Based on variation at microsatellite nuclear DNA (nDNA) markers and the cytochrome-b mitochondrial DNA (mtDNA) gene, individuals were classified as A. cavifrons, A. rupestris, or hybrid, and populations were examined for genetic introgression. We detected A. rupestris and/or hybrids in seven of nine historical A. cavifrons populations, indicating multiple introduction events and a significant decline in the range of A. cavifrons. Although inferred hybrids were uncommon (15 of 417 sampled individuals), the majority had F-2 or backcross origins, indicating that F-1 hybrids are fertile and bidirectional introgression is possible. However, individuals with mtDNA from one species but with nDNA that is all or predominantly from the other were extremely rare, suggesting that cytonuclear incompatibilities reduced hybrid fitness. Moreover, introgression did not seem to persist over time, even in locations long occupied by both species. As such, we posit that these species readily hybridize in sympatry, but advanced hybrids have reduced fitness, inhibiting significant introgression or the development of hybrid swarms. Thus, we hypothesize that A. rupestris has replaced A. cavifrons primarily through high propagule pressure and competitive displacement, rather than by introgressive hybridization. Regardless the mechanism, prevention of further invasion of A. rupestris is key to the long-term persistence of A. cavifrons.