Asymmetrical Gene Flow Between an Endemic and Widespread Shrub Species




Mottet, Avery

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Patterns of hybridization and gene flow among divergent taxa provide unique opportunities to investigate the barriers which isolate and maintain distinct species. Genetic and demographic structure of divergent populations can influence the impact of hybridization, and consequences to interspecific gene flow can be neutral, destructive or creative. The endemic Texas Barberry, Mahonia swaseyi, and Agarita, Mahonia trifoliolata, are locally sympatric within Central Texas where apparent phenotypically intermediate hybrids occur. Population genomic and morphological data were integrated to quantify differences among and variation within these two species. We found evidence of early generation hybrids and asymmetries with respect to later-generation hybridization, with directionality towards M. trifoliolata. A heterogeneous introgression pattern across the genome was revealed using a Bayesian Genomic Cline framework, however M. swaseyi alleles were found to introgress across species boundaries significantly more often than those of M. trifoliolata. Hybrids were determined to be more similar to M. trifoliolata across most of the morphological traits measured in this study. The phenotypic variation observed between intermediates and M. trifoliolata and M. swaseyi supports the genomic results of asymmetric introgression between M. trifoliolata and M. swaseyi. For rare plant species such as M. swaseyi, hybridization with a widespread congener can constitute a serious risk to the maintenance of species integrity. However, the findings from this study do not support evidence of M. swaseyi being at risk of genetic swamping, and reproductive isolation appears to be maintaining genetic integrity of the two species despite natural hybridization.



population genetics, hybridization, asymmetrical gene flow


Mottet, A. (2022). Asymmetrical gene flow between an endemic and widespread shrub species (Unpublished thesis). Texas State University, San Marcos, Texas.


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