Data from: Asymmetric introgression between fishes in the Red River basin of Texas is associated with variation in water quality

When ecologically divergent taxa encounter one another, hybrid zones can
form when reproductive isolation is incomplete. The location of such
hybrid zones can be influenced by environmental variables, and an
ecological context can provide unique insights into the mechanisms by
which species diverge and are maintained. Two ecologically differentiated
species of small benthic fishes, the endemic and imperiled prairie chub,
Macrhybopsis australis, and the shoal chub, Macrhybopsis hyostoma, are
locally sympatric within the upper Red River Basin of Texas. We integrated
population genomic data and environmental data to investigate species
divergence and the maintenance of species boundaries in these two species.
We found evidence of advanced-generation asymmetric hybridization and
introgression, with shoal chub alleles introgressing more frequently into
prairie chubs than the reciprocal. Using a Bayesian Genomic Cline
framework, patterns of genomic introgression were revealed to be quite
heterogeneous, yet shoal chub alleles were found to have likely
selectively introgressed across species boundaries significantly more
often than prairie chub alleles, potentially explaining some of the
observed asymmetry in hybridization. These patterns were remarkably
consistent across two sampled geographic regions of hybridization. Several
environmental variables were found to significantly predict individual
admixture, suggesting ecological isolation might maintain species
boundaries.