Setophaga townsendi x S. occidentalis (1 Viewer)

S. Wang, S. Rohwer, D. E. Irwin. Genomic insights of a moving Setophaga warbler hybrid zone. 27th IOC, Vancouver 2018, Oral Presentation.

Abstract:

Hybrid zones, where diverging populations interbreed, are natural arena for understanding the evolution of reproductive isolation underlying speciation. The rapid speciation of Setophaga warblers is an interesting speciation puzzle. We can understand such rapid speciation in hybrid zones between young Setophaga sister pairs. One of the hybrid zones between S. townsendi and S. occidentalis in the Pacific Northwest was thought to be moving over time, based on the shift of plumage hybrid index at the same sites over time. We found more signatures of movements in this hybrid zone: 1) the plumage cline have shifted 10km over 20 years; 2) there is a latitudinal gradient of genomic heterozygosity reduction from the current hybrid zone location to the postulated historical hybrid zone location; 3) there was asymmetry of linkage disequilibrium cline. However, when we looked at the genome-wide cline, there was no sign of shift between sampling periods. This pattern might be due to the fusion of gene pools between species in such rapidly moving and extensively hybridizing taxa. Thus a genome-wide association study (GWAS) was conducted to detect genetic regions underlying plumage variations in the genome. We found that although the plumage-associated genetic regions demonstrated more differentiation between parental populations than the rest of the genome, such differentiation decays over time. This study sheds light on population genetic cause and consequences of a moving hybrid zone, which is emergently needed for understanding populations, species, and speciation in the rapidly changing world.

Silu Wang, Sievert Rohwer, Kira Delmore & Darren E. Irwin. Cross‐decades stability of an avian hybrid zone. Journal of Evolutionary BiologyFirst Published: 20 August 2019 https://doi.org/10.1111/jeb.13524

Abstract:

Hybrid zones are particularly valuable for understanding the evolution of partial reproductive isolation between differentiated populations. An increasing number of hybrid zones have been inferred to move over time, but in most such cases zone movement has not been tested with long‐term genomic data. The hybrid zone between Townsend's Warblers (Setophaga townsendi) and Hermit Warblers (S. occidentalis) in the Washington Cascades was previously inferred to be moving from northern S. townsendi southward toward S. occidentalis, based on plumage and behavioral patterns as well as a 2000‐km genetic wake of hermit mitochondrial DNA (mtDNA) in coastal Townsend's Warblers. We directly tested whether hybrid zone position has changed over 2‐3 decades by tracking plumage, mtDNA, and nuclear genomic variation across the hybrid zone over two sampling periods (1987‐94 and 2015‐16). Surprisingly, there was no significant movement in genomic or plumage cline centers between the two time periods. Plumage cline widths were narrower than expected by neutral diffusion, consistent with a “tension zone” model, in which selection against hybrids is balanced by movement of parental forms into the zone. Our results indicate that this hybrid zone is either stable in its location or moving at a rate that is not detectable over 2‐3 decades. Despite considerable gene flow, the stable clines in multiple phenotypic and genotypic characters over decades suggest evolutionary stability of this young pair of sister species, allowing divergence to continue. We propose a novel biogeographic scenario to explain these patterns: rather than the hybrid zone having moved thousands of kilometers to its current position, inland Townsend's met coastal Hermit Warbler populations along a broad front of the British Columbia and Alaska coast and hybridization led to replacement of the Hermit Warbler plumage with Townsend's Warbler plumage patterns along this coastline. Hence hybrid zones along British Columbia and Alaska moved only a short distance from the inland to the coast, whereas the Hermit Warbler phenotype appears stable in Washington and further south. This case provides an example of the complex biogeographic processes that have led to the distribution of current phenotypes within and among closely related species.