Limited hybridization and introgression despite stocking among endemic Interior Highlands black basses (Centrarchidae: Micropterus)

Aim Smallmouth Bass (Micropterus dolomieu; SMB) are globally popular among
anglers and have been widely introduced (i.e., stocked) for population
management and sportfishing. Importantly, stocking was prevalent before
cryptic diversity within the SMB complex was known, which now includes
three newly elevated species: Neosho Bass (M. velox; NB), Little River
Bass (M. sp. cf. dolomieu Little River; LRB), and Ouachita Bass (M. sp.
cf. dolomieu Ouachita River; OB). We sought to quantify population
structure and hybridization and introgression in these three recently
described species. Location Species-level diversity, particularly in the
basin-restricted LRB and OB in the Ouachita Mountains within the Central
Interior Highlands (CIH), North America, suggests the presence of distinct
genetic variation that could be eroded by
introgression.   Methods We estimated interspecific
introgression and intraspecific population differentiation in the
Smallmouth Bass species complex (SMB-C) using 472 specimens comprising
SMB, NB, LRB, and OB, including the naturally sympatric Spotted Bass (M.
punctulatus; SPB). Genomic samples were genotyped on a SNP panel of 192
loci designed to detect allele-sharing on multiple hierarchical
levels.  Results We found low range-wide hybridization between
species in the SMB-C and SPB (mostly SMB-C backcrosses), and interspecific
heterozygosity varied, indicating differential introgression. Range-wide
hybridization between species in the CIH and SMB was similar overall (but
mostly F2 and CIH backcrosses) and was observed in streams with known SMB
stocking in connected reservoirs. Interspecific heterozygosity in SMB
hybrids was also generally lower, indicating later-generation backcrosses.
We found strong population structure in the Ouachita Mountains (LRB and
OB).  Main Conclusions Despite isolated incidences of natural
(SPB) and human-mediated (SMB) introgression, genomic identity appears
intact in endemic LRB and OB, suggesting potential ecological or
behavioral isolating mechanisms preventing cross-species reproduction. Our
findings reveal that genetic variation remains in cryptic,
basin-restricted species in the Ouachita Mountains ecoregion that may be
managed for long-term conservation.