In high-latitude species with high dispersal ability, such as long-distance migratory birds, populations are often assumed to exhibit little genetic structure due to high gene flow or recent postglacial expansion. We sequenced over 120 low-coverage whole genomes from across the breeding range of a long-distance migratory bird, the Veery (Catharus fuscescens), revealing strong evidence for isolation by distance. Additionally, we found distinct genetic structure between boreal, western montane U.S., and southern Appalachian sampling regions. We suggest that population genetic structure in this highly migratory species is detectable with the high resolution afforded by whole-genomic data because, similar to many migratory birds, the Veery exhibits high breeding-site fidelity, which likely limits gene flow. Resolution of isolation by distance across the breeding range was sufficient to assign likely breeding origins of individuals sampled in this species' poorly understood South American nonbreeding range, demonstrating the potential to assess migratory connectivity in this species using genomic data. As the Veery's breeding range extends across both historically glaciated and unglaciated regions in North America, we also evaluated whether contemporary patterns of structure and genetic diversity are consistent with historical population isolation in glacial refugia. We found that patterns of genetic diversity did not support southern montane regions (southern Appalachians or western U.S. mountains) as glacial refugia. Overall, our findings suggest that isolation by distance yields subtle associations between genetic structure and geography across the breeding range of this highly vagile species even in the absence of obvious historical vicariance or contemporary barriers to dispersal.
LAY SUMMARY
Describing how populations in a species differ genetically is important for understanding that species' evolutionary history.
Migratory birds have high dispersal abilities, potentially reducing genetic structure. However, many migratory birds return to the same breeding site year after year, which could reduce gene flow between populations.
We sequenced >120 genomes to detect population genetic differentiation in a common songbird, the Veery (Catharus fuscescens).
We found that genetic similarity between samples decreased with increasing geographic distance (i.e., isolation by distance) and that populations in southern Appalachia were distinct from samples in the rest of the breeding range.
We did not detect differences in genetic diversity patterns between populations, contrary to predictions about putative glacial refugia.
We assigned likely geographic breeding region to birds sampled in the poorly understood wintering range.
Despite the Veery's long-distance migration and high dispersal ability, isolation by distance produces subtle but detectable population structure across its breeding range.
En especies de latitudes altas con elevada capacidad de dispersión, como las aves migratorias de larga distancia, a menudo se asume que las poblaciones exhiben poca estructura genética debido al alto flujo génico o a la reciente expansión post-glaciar. Secuenciamos más de 120 genomas completos de baja cobertura en todo el rango de cría de un ave migratoria de larga distancia, Catharus fuscescens, revelando fuertes evidencias de aislamiento por distancia. Además, encontramos una estructura genética distintiva entre las regiones de muestreo boreales, las montañas del oeste de EEUU y los Montes Apalaches del sur. Sugerimos que la estructura genética poblacional en esta especie altamente migratoria es detectable por la alta resolución que proporcionan los datos genómicos completos porque, al igual que muchas aves migratorias, C. fuscescens muestra una alta fidelidad al sitio de cría, lo que probablemente limita el flujo génico. La resolución del aislamiento por distancia en todo el rango de cría fue suficiente para asignar los probables orígenes de cría de individuos muestreados en el rango no reproductivo -y poco conocido- de esta especie en Sudamérica, demostrando el potencial para evaluar la conectividad migratoria en esta especie utilizando datos genómicos. Dado que el rango de cría de C. fuscescens se extiende por regiones históricamente con y sin glaciares en América del Norte, también evaluamos si los patrones contemporáneos de estructura y diversidad genética son consistentes con el aislamiento poblacional histórico en refugios glaciares. Encontramos que los patrones de diversidad genética no respaldaban a las regiones montañosas del sur (Montes Apalaches del sur o montañas del oeste de EEUU) como refugios glaciares. En general, nuestros hallazgos sugieren que el aislamiento por distancia produce asociaciones sutiles entre la estructura genética y la geografía en todo el rango de cría de esta especie altamente móvil, incluso en ausencia de vicarianzas históricas obvias o de barreras contemporáneas para la dispersión.
