Abstract
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.
Highlights
Speciation is usually not instantaneous (Grant, 1981; Levin, 1983, 2002; Wood et al, 2009), but rather involves the grad‐ ual buildup of reproductive isolation between diverging lineages (Coyne & Orr, 2004)
We found an overall pattern of asymmetric hybridization, which could be due to the relative abundances of each species
There is a broad pattern of asymmet‐ ric introgression, with shoal chub alleles tending to introgress into individuals comprised of predominately prairie chub genomic back‐ grounds
Summary
The relatively recent ability to generate genome‐wide data for nonmodel organisms has been accompanied with appropriate computational tools to process these exceptionally large datasets (Buerkle & Lexer, 2008; Gompert & Buerkle, 2009, 2011, 2012; Mandeville, Parchman, McDonald, & Buerkle, 2015) This has enabled evolutionary biologists to ask questions about the nature of reproductive isolation and introgressive hybridiza‐ tion at a genomic scale (Mandeville et al, 2015; Sung, Bell, Nice, & Martin, 2018). This study focused on two small benthic fishes, the shoal chub (Macrhybopsis hyostoma) and prairie chub (Macrhybopsis australis) These species are locally sympatric in the Red River along the border of Texas and Oklahoma upstream of Lake Texoma, an artificial reservoir created by the Denison Dam con‐ structed in 1943 (Figure 1).
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