Advancements in DNA sequencing have revolutionized ecology by facilitating analyses of genome-wide variation in non-model organisms. Genomics can address evolutionary history, ecological interactions, and adaptation in rare taxa at risk of local extinction. The Neosho Smallmouth Bass (Micropterus dolomieu velox), one of two Smallmouth Bass subspecies, is endemic and range-restricted to tributaries of the Arkansas River Basin. To enhance angling opportunities, the more widely-distributed Northern Smallmouth Bass (M. d. dolomieu) is often used to supplement natural populations. Hybridization in the Neosho range may swamp out unique genetic diversity, ultimately decreasing evolutionary fitness. Neutral genetic markers (15 microsatellites) distinguish the subspecies. However, to understand the long-term effects of hybridization, patterns within the central Interior Highlands should be further resolved with a broader view of the genome, incorporating markers under selection. We therefore sought to assess:1) population structure, 2) local adaptation, and 3) the extent and origins of hybridization between the Smallmouth subspecies in the central Interior Highlands.
We conducted double-digest restriction enzyme-associated DNA sequencing to genotype 127,429 single nucleotide polymorphisms (SNPs) for 64 Neosho Smallmouth, 24 Northern Smallmouth, and 4 Spotted Bass (outgroup) distributed across the central Interior Highlands. After rigorous filtering, we retained 50,828 SNPs for population genomic analysis.
Results/Conclusions
We found strong differentiation between the Neosho Smallmouth Bass and Northern Smallmouth Bass in the central Interior Highlands. Within the Neosho Smallmouth Bass range, we found complex patterns of population structure, identifying two genomically distinct populations: the upper to middle Arkansas River Basin and the Boston Mountains of the lower Arkansas River. We discovered 19 SNPs that are likely under selection. We also detected extensive admixture in all but two Neosho populations. Honey and Sycamore Creek in the upper Arkansas, along with Lee and Mulberry Creek in the Boston Mountains, were found to be unadmixed. Populations in the upper Arkansas River tributaries were admixed with alleles from the White River drainage (which lies within the Northern range), while populations in the middle Arkansas tributaries were admixed with Northern hatchery strain alleles. Our results demonstrate that the Smallmouth Bass subspecies are historically independent lineages that have hybridized, possibly due to human introductions. The endemic Neosho subspecies may be at risk of losing genetic integrity through hybridization with the Northern subspecies. These findings warrant further investigation of the fitness consequences of this admixture and suggest the need for stricter conservation policy in this ecoregion.