Macrogeographic variation of nuclear microsatellite loci in an endangered species, Eumetopiias jubatus

Abstract

The Steller sea lion (Eumetopias jubatus) is distributed along the North Pacific Rim from Japan to northern California. The species has experienced a drastic decline from an estimated 240,000-300,000 individuals in the 1960's to an estimated 52,000 individuals in 1994, and is listed as threatened under the United States Endangered Species Act in 1991. Subsequent genetic studies identified two distinct stocks, a western stock, which has declined, and an eastern stock, which has remained stable. The western stock was listed as endangered in 1997, while the eastern stock remains listed as threatened. Six polymorphic microsatellite loci were analyzed for individuals taken from rookeries ranging from the Sea of Okhotsk to northern California. The level of population subdivision observed was compared to that based on mitochondrial DNA (mtDNA). Microsatellite diversity was high but levels of subdivision were not as striking as those observed using mtDNA. Unlike mtDNA, microsatellite loci, revealed no macrogeographic variation among populations. This difference in levels of nuclear and mitochondrial subdivision is reflected in estimates of nuclear gene flow. This pattern may result from high female philopatry combined with high male dispersal. Thus, male-biased gene flow in Steller sea lions may be important in maintaining high levels of genetic diversity. Patterns of microsatellite diversity in Steller sea lions suggest the occurrence of genetic bottlenecks in 15 of 21 rookeries. Despite this result, Steller sea lions exhibit high genetic diversity in both mitochondrial and nuclear genomes. Assessment and conservation of this diversity could be critical in the survival of this species. Twenty-one adult Steller sea lions collected through native harvests in the Pribilof Islands also were examined for assignment of rookery origin. These samples exhibit high levels of nuclear and mitochondrial gene diversity. The samples appear to have originated from the western stock.