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The genetic diversity, phylogeography, and population structure of Pacific harbor seals reveal isolation at the southern end of their distribution

Fernández-Martín, Eva María1; Schramm, Yolanda1; Heckel, Gisela2; Abadía-Cardoso, Alicia1

Published Jan 18, 2024 on Dryad. https://doi.org/10.5061/dryad.t76hdr85k

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Abstract

The Pacific harbor seal (Phoca vitulina richardii) occurs in the Mexican Pacific region at the southernmost edge of the subspecies’ distribution, along 700 km of coastline and on nine islands west of the Baja California peninsula. Its abundance corresponds to 3% of its total abundance in the north Pacific Ocean. The species is considered relatively sedentary and highly philopatric, which make it vulnerable to stochastic processes; thus, reproductive and genetic isolation of the Mexican Pacific colonies is expected. This study aimed to genetically characterize the harbor seal in the Mexican Pacific to inform conservation efforts.  We estimated the levels of genetic diversity for five colonies, using a 572-base pair mitochondrial DNA control region fragment and nine microsatellite loci. We examined the population genetic structure and its phylogeographic patterns. We found 15 variable sites that defined 18 mitochondrial DNA haplotypes. Results show one of the lowest levels of diversity reported for the species (overall haplotype diversity h = 0.626 ± SD 0.041; overall nucleotide diversity π = 0.0018 ± SD 0.0013; mean expected heterozygosity HE = 0.537). We found a stronger genetic structure with both markers than in the larger regions of the north Pacific, from Alaska to California. The Pacific harbor seal colonies found in Mexico may have their origins in northern colonies, via a founder event. We found only four haplotypes in common with those observed (451) along the rest of the Pacific colonies, while nine of the 14 haplotypes exclusive to the Mexican Pacific are private.