Genomic variation in the Black-throated Green Warbler (Setophaga virens) suggests divergence in a disjunct Atlantic Coastal Plain population (S. v. waynei)

Abstract

ABSTRACT We used whole-genome resequencing to estimate genetic distinctiveness in the Black-throated Green Warbler (Setophaga virens)—including S. v. waynei—a putative subspecies that occupies a narrow disjunct breeding range along the Atlantic Coastal Plain. Despite detecting low-global differentiation (FST = 0.027) across the entire species, the principal components analysis of genome-wide differences shows the main axis of variation separates S. v. waynei from all other S. v. virens samples. We also estimated a low-migration rate for S. v. waynei, but found them to be most similar to another disjunct population from the Piedmont of North Carolina, and detected evidence of a historical north-to-south geographic dispersal among the entire species. New World wood warblers (family: Parulidae) can exhibit strong phenotypic differences among species, particularly, in song and plumage; however, within-species variation in these warblers—often designated as subspecies—is much more subtle. The existence of several isolated Black-throated Green Warbler populations across its eastern North American breeding range offers an excellent opportunity to further understand the origin, maintenance, and conservation status of subspecific populations. Our results, combined with previously documented ecological and morphological distinctiveness, support that S. v. waynei be considered a distinct and recognized subspecies worthy of targeted conservation efforts. LAY SUMMARY We used whole-genome resequencing to quantify genetic structure in Black-throated Green Warblers (Setophaga virens) throughout their North American breeding range. We detected low differentiation across all samples but found discrete genetic clustering of S. v. waynei populations that breed along the Atlantic Coastal Plain. We provide evidence of a geographic north-to-south dispersal among the entire species, and found S. v. waynei to be of relatively recent origin within the southern clade. We estimated reduced gene flow in the region separating S. v. waynei from other S. virens populations, but found they were most closely related to a disjunct population from the Piedmont of central North Carolina. We identified two specific regions of the genome that showed elevated differentiation, one on a sex chromosome (Z) and the other on chromosome 6. Our findings provide clarity to the subspecific status of S. virens and bolster conservation efforts for S. v. waynei populations and their habitats. RESUMEN Usamos re-secuenciación del genoma completo para estimar la especificidad genética en Setophaga virens, incluida S. v. waynei, una subespecie putativa que ocupa un estrecho rango de reproducción disyunto a lo largo de la Planicie Costera del Atlántico. A pesar de detectar una diferenciación global baja (FST = 0,027) a lo largo de toda la especie, el análisis de componentes principales de las diferencias en el genoma completo muestra que el eje principal de variación separa a S. v. waynei de todas las demás muestras de S. v. virens. También estimamos una tasa de migración baja para S. v. waynei, pero encontramos que era más similar a otra población disyunta del Piedemonte de Carolina del Norte, y detectamos evidencia de una dispersión geográfica histórica de norte a sur en toda la especie. Las reinitas del Nuevo Mundo (familia: Parulidae) pueden exhibir fuertes diferencias fenotípicas entre especies, particularmente en el canto y el plumaje; sin embargo, la variación intra específica en estas reinitas—a menudo designadas como subespecies—es mucho más sutil. La existencia de varias poblaciones aisladas de S. virens a lo largo de su área de reproducción del este de América del Norte ofrece una excelente oportunidad para comprender mejor el origen, el mantenimiento y el estado de conservación de las poblaciones subespecíficas. Nuestros resultados, combinados con el carácter ecológico y morfológico específico previamente documentado, respaldan que S. v. waynei sea considerada una subespecie distintiva y reconocida que merece esfuerzos de conservación específicos.