Genotyping-by-sequencing reveals genomic homogeneity among overwintering Pacific Dunlin (Calidris alpina pacifica) aggregations along the Pacific coast of North America

Information on how migratory populations are genetically structured during the overwintering season of the annual cycle can improve our understanding of the strength of migratory connectivity and help identify populations as units for management. Here, we use a genotype-by-sequencing approach to investigate whether population genetic structure exists among overwintering aggregations of the Pacific Dunlin subspecies (Calidris alpina pacifica) sampled at 2 spatial scales (within and among overwintering sites) in the eastern Pacific Flyway. Genome-wide analyses of 874 single nucleotide polymorphisms across 80 sampled individuals revealed no evidence for genetic differentiation among aggregations overwintering at 3 locations within the Fraser River Estuary (FRE) of British Columbia. Similarly, comparisons of aggregations in the FRE and those overwintering in southern sites in California and Mexico indicated no genetic segregation between northern and southern overwintering areas. These results suggest that Pacific Dunlin within the FRE, Sacramento Valley (California), and Guerrero Negro (Mexico) are genetically homogeneous, with no evident genetic structure between sampled sites or regions across the overwintering range. Despite no evidence for differentiation among aggregations, we identified a significant effect of geographical distance between sites on the distribution of individual genotypes in a redundancy analysis. A small proportion of the total genotypic variance (R²⁼ 0.036, P = 0.011) was explained by the combined effect of latitude and longitude, suggesting weak genomic patterns of isolation-by-distance that are consistent with chain-like migratory connectivity between breeding and overwintering areas. Our study represents the first genome-scale investigation of population structure for a Dunlin subspecies and provides essential baseline estimates of genomic diversity and differentiation within the Pacific Dunlin.