The genetic diversity of a population is the foundation of its adaptability to environmental challenges. The South American fur seal is a widely distributed pinniped in the south cone of South America. However, a large gap in the Pacific coast separates two distinct evolutionary units for the species: the Peruvian and the Southern Pacific/Atlantic populations. Throughout the Pacific, one of the main breeding colonies is located in Guafo Island, in the southern Chilean Patagonia. As the closest reproductive population to the isolated Peruvian group, Guafo’s colony may potentially facilitate gene flow, contribute with new alleles and increase genetic variability to Peruvian populations’, connecting the entire Pacific’s distribution of the species. In this study, Guafo’s Island South American fur seal population was characterized by the identification and genotyping of species-specific microsatellite markers. As a result, we confirm that Guafo’s colony is a diverse group with mild evidence of genetic structure. Although a couple of family groups among seasons were observed, results indicate that half-siblings are rare and suggest that polygyny in this species is more relaxed than previously thought. Additionally, three full-sibling pairs were genetically identified within the 2017 season, which is the first genetic support that describes the presence of twins for the species. These attributes suggest that the colony at Guafo is a panmictic large group, and could serve as a potential genetic source for other isolated populations.

The fieldwork was conducted in Guafo Island. Animals were sampled weekly during the breeding seasons (December through March) from 2014 to 2017. A 2 mm skin biopsy was collected from the distal portion of the third finger of the hind flipper and preserved in 95% ethanol.

DNA was extracted from 128 animal samples using a commercial kit. Sixteen loci (NCBI accession number: MK934158-73) were amplified through conventional PCR using fluorescent-labeled primers. Genotyping was performed in an ABI 3500 genetic analyzer, and electropherograms were analyzed in Geneious Prime software (Kearse et al., 2012).

Missing values are coded as 0.