Natural Hybridization and Introgression between Berberis trifoliolata and Berberis swaseyi in the Edwards Plateau
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Hybridization and hybrid zones can give us a means by which to discover the ecological and genomic interactions that occur between closely related species, providing an understanding of the mechanisms by which biodiversity arises and is maintained through reproductive isolation even in the face of introgression. Using next generation DNA sequencing techniques, we are now able to ask such questions at a genomic level. Here I used two species in the genus Berberis, Berberis trifoliolata and Berberis swaseyi to determine if hybridization is occurring, identify hybrid classes, examine the genomic architecture of introgression, and examine the asymmetry of introgression if any. Both species are native to the Edwards Plateau region of central Texas, and hybridization between the two species has been hypothesized because of morphological intermediacy. Admixture proportions were calculated in order to determine the degree to which hybridization is occurring as well as to describe substructure in the more widespread and common B. trifoliolata. Using the Admixture Class model in Entropy, it was found that the hybrids are either early hybrids (F1 and F2) or late generation hybrids that appear to have selfed instead of crossing with others. The Bayesian Genomic Cline model was used to quantify variation in introgression in hybrid individuals. The exceptional loci revealed that introgression tended to more readily occur from B. trifoliolata towards B. swaseyi with 305 exceptional loci introgressing towards B. swaseyi, and 229 introgressing towards B. trifoliolata. The results of all tests showed evidence of hybridization occurring with later generation hybrids largely absent, indicating that perhaps they were less fit. Small amounts of introgression were detected such that loci from B. swaseyi largely appeared to have a selective advantage over those of B. trifoliolata. This study provides a starting point to ask more questions about this system and the effects of reproductive isolation in closely related plant species.