Population structure and gene flow in the Louisiana Iris species complex
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Identifying the distribution of genetic variation and gene flow is important for understanding how evolutionary dynamics have shaped the genetic structuring of populations undergoing divergence. Natural selection and genetic drift play a role in shaping the distribution of population structure and gene flow throughout the ranges of species and can drive the divergence of taxa. With the advent of next generation DNA sequencing techniques, it is now possible to explore population structure and gene flow at a genomic level throughout the range of such ecologically divergent taxa. The Louisiana Irises (Iris, series Hexagonae) comprise a group of three or more ecologically and reproductively divergent lineages that occasionally produce hybrids in nature, giving an opportunity to explore the process of speciation as it happens. Here we sampled populations of Louisiana Iris spp. in an attempt to characterize population structure and gene flow throughout their respective ranges. We discovered evidence for gene flow in some parts of the range and tested several standing hypotheses of nominal taxonomy accepted by Louisiana Iris enthusiasts. We also quantified introgression in a newly discovered hybrid zone between Iris hexagona and I. brevicaulis using a Bayesian Genomic Cline analysis. In addition, we tested the hypothesis that a purportedly hybrid species, I. nelsonii, indeed shows ancestry from two or more of the hypothesized parental species. We discovered that a relatively small proportion of the loci we sampled in the hybrid zone are experiencing extreme patterns of introgression. We found evidence that population structure appears to be more complex than previous taxonomic designations suggest, with more variation within the I. brevicaulis lineage than between other species in the group. It was also discovered that I. nelsonii appeared to share ancestry with only one of the purported parent species, I. fulva, at the sampled loci. This study provides a foundation for future exploration of evolutionary dynamics affecting these taxa and sets the stage for understanding the standing distribution of genomic variation in the clade.