Weak coupling among barrier loci and waves of neutral and adaptive introgression across an expanding hybrid zone

Hybridization can serve as an evolutionary stimulus, but we have little understanding of introgression at early stages of hybrid zone formation. We analyze reproductive isolation and introgression between a range-limited and a widespread species. Reproductive barriers are estimated based on differences in flowering time, ecogeographic distributions, and seed set from crosses. We find an asymmetrical mating barrier due to cytonuclear incompatibility that is consistent with observed clusters of coincident and concordant tension zone clines (barrier loci) for mtDNA haplotypes and nuclear SNPs. These groups of concordant clines are spread across the hybrid zone, resulting in weak coupling among barrier loci and extensive introgression. Neutral clines had nearly equal introgression into both species' ranges, while putative cases of adaptive introgression had exceptionally wide clines with centers shifted towards one species. Analyses of cline shape indicate that secondary contact was initiated within the last 800 generations with the per-generation dispersal between 200 and 400 meters, and provide some of the first estimates of the strength of selection required to account for observed levels of adaptive introgression. The weak species boundary between these species appears to be in early stages of dissolution, and ultimately will precipitate genetic swamping of the range-limited species.Missing values in all datasets. Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: NSF-MSB-1340746Greenhouse crosses, seed counts, germination, flowering data. GPSoccurances and bioclimatic data for environmental niche modeling. Herbarium records for phenology/flowering overlap. Genotyping by Sequencing SNP data analyzed for cline width and center. Data Files Morphology. Petals (Petal Datcsv) Leaves (Leaf trichome data.csv) Greenhouse crosses. Seed set (filled, aborted, unfertilized) (Ranunc_Greenhouse_SeedCounts.csv) Germination (Germination data.csv) Flowering (Greenhouse flowering data.csv) Field seed set. Seed set (filled, aborted, unfertilized) (Field seed set datcsv) Reproductive isolation. ENM occurrences bioclim data (Ranunc_enm_bioclime_datcsv) Herbarium records (Ranunc_Flowering.csv) Genetic data SNPs (50) fitting cline models (SNP 50 loci fitting cline models.csv) Clines center and width (HZAR Output Summary.csv) Readme file with data descriptions