The authors are grateful for assistance in the field and the lab from former REU students, A. Hamilton and A. Pheil, and graduate students, T. Arredondo, M. Grasty, B. Kohrn, and J. Schwoch. The authors thank N. Barton, K. de Lima Berg, and M. Streisfeld for comments on the manuscript. The authors thank M. Sultany, high school science teacher, for volunteering to take microscopy images of petals. The authors are also grateful to M. Morrison at The Nature Conservancy and K. Mergenthaler with the Southern Oregon Land Conservancy for assistance in locating and accessing Ranunculus populations. Specimen data were provided by L. Hardison from the Oregon Flora Project, D. Giblin from the Consortium of Pacific Northwest Herbaria, and M. Link-Perez from the OSU Herbarium. This research was supported by National Science Foundation Macrosystems Biology award NSF-MSB-1340746 to MBC.
Genomes -- research, Microorganisms -- nomenclature
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, whereas putative cases of adaptive introgression had exceptionally wide clines with centers shifted toward 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 m, 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.
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Cruzan, M. B., Thompson, P. G., Diaz, N. A., Hendrickson, E. C., Gerloff, K. R., Kline, K. A., ... & Persinger, J. M. (2021). Weak coupling among barrier loci and waves of neutral and adaptive introgression across an expanding hybrid zone. Evolution.