Genetic swamping of endemic Neosho Bass by nonnative Smallmouth Bass in the Illinois River basin, Oklahoma

Objective Impoundments serve as significant sources of aquatic invasive species, notably nonnative black basses Micropterus spp., which often hybridize with native stream-dwelling congeners. Recently, the fluvial endemic Neosho Bass M. velox was elevated to species from its former status as a subspecies of the widely ­distributed Smallmouth Bass M. dolomieu. In the 1990s, nonnative Smallmouth Bass were introduced into the Illinois River basin, Oklahoma, which has led to introgressive hybridization with native Neosho Bass that ­inhabit tributaries of the upper Illinois River basin. To inform management and conservation efforts for the newly elevated Neosho Bass, the objective of this study was to provide an updated characterization of the spatial extent and directionality of introgressive hybridization. Methods We paired longitudinal field sampling of impoundment tributaries with molecular techniques. We diagnosed Neosho Bass, Smallmouth Bass, and hybrids using a single nucleotide polymorphism panel and several complementary analytical approaches. To assess directionality of hybridization via maternal ancestry, we also sequenced a subset of specimens at a region of the mitochondrial cytochrome b gene and built a haplotype network that included reference sequences from previous studies. Results Our findings illuminate ongoing genetic swamping of Neosho Bass, with higher hybridization rates observed closer to impoundments and in larger tributaries. Upstream reaches of smaller tributaries retained nonhybrid Neosho Bass, but the main-stem Illinois River—the largest river within the native range of Neosho Bass—contained only 11% nonhybrid Neosho Bass. Mitochondrial sequencing revealed that 66.7% of F1 hybrids possessed Smallmouth Bass maternal ancestry, suggesting asymmetry in initial hybridization. We also recovered incidences of complete cytonuclear mismatch, providing additional evidence of multigenerational backcrossing in both directions. Conclusions Our results reveal ongoing and widespread swamping of native Neosho Bass in the Illinois River, which heightens conservation concern for this recently elevated species. Although some upstream headwaters contained nonhybrid Neosho Bass, to what extent those populations will experience future invasion and introgression remains unclear and warrants further investigation.