Population Genetics of Steller Sea Lions
As a result of the dramatic decline of the Steller sea lion in the 1970’s, research into the genomic diversity of Eumetopias jubatus has become a hotspot for scientific study. Variation between individuals within populations suggested the species be divided into two separate stocks. Three specific approaches have been used to study genetic diversity and its implications in the sea lion population: mitochondrial DNA (mtDNA), microsatellites and amplified fragment length polymorphism (AFLP) analysis. AFLP is a current popular method used to organize the Steller sea lion into stocks. Methods such as microsatellites or mitochondrial DNA (mtDNA) have been deemed less reliable . Disease susceptibility in Steller sea lion pups shows the effects that a lack of genetic diversity has within the populations . Direct evidence of the link between genetic variability and population size would be useful in determining future population sizes.
One of the original genetic markers studied within this species besides alleles was mitochondrial DNA (mtDNA). Using mitochondrial DNA diversity, the Steller sea lion species can be organized into separate populations, or stocks . Extensive variation between individuals within populations across the globe suggested the species be divided into two separate stocks: the Western and the Eastern stocks. The Western stock consisted of populations between the Commander Islands in Russia, the Aleutian Islands and the Gulf of Alaska. Populations around Southern Alaska and Oregon comprised the Eastern stock. MtDNA evidence also contributed to the hypothesis that stocks were formed by separation of several glacial bodies .
Genetic Susceptibility to Disease
Exploring the role that genetically-dependant diseases have within the sea lion species can significantly increase our knowledge of how certain populations will respond to different diseases in terms of size. Placed in the same taxonomic family as Steller sea lions, the New Zealand sea lion is seen as a model for understanding the correlation between genetic variability, population size and disease. Recent studies investigated the correlation between genetic variability and hookworm disease susceptibility in the New Zealand sea lion . Allelic diversity and heterozygosity were found to be higher than expected for a threatened species. Homozygosity at a particular allele increases the probability of having rare alleles contribute negatively towards fighting disease after contracting hookworms .
Mitochondrial DNA as a Genetic Tool
A more recent study using mtDNA claimed the separation of the two original Western and Eastern stocks were supported by new mtDNA data. Several regions within the Western stock displayed mtDNA diversity significant enough to be classified as a separate Asian stock . The stocks are now redefined: the Western stock between Prince William Sound and the Commander Islands; the Asian stock includes Kamchatka Peninsula, the Kuril Islands and the Sea of Okhotsk; the Eastern stock remains unchanged- Southern Alaska through California . The creation of separate subspecies was deemed necessary for the Eastern stock, Eumetopias jubatus monteriensis, as well as the Western and Asian stocks, Eumetopias jubatus jubatus, with the hope that the Asian stock will be placed in its own subspecies pending further research .
Redefining Stocks: Microsatellite DNA
Attempts to understand the genetic diversity and eventually the population trajectory of the Steller sea lion resulted in different methods for discovering gene flow, heterozygosity and allelic diversity. Genetic markers such as microsatellite DNA are considered especially useful. Certain microsatellite studies strongly support the split between the Western and Eastern stocks, but weakly support the Western and Asian stocks . Microsatellite data recognizes and supports data from previous mtDNA studies  . It also assists in developing separate stocks, focusing conservation needs and providing more information on sea lion phylogeography.
Recent research in the sea lion genome has shifted towards using genetic markers such as amplified fragment length polymorphism (AFLP) markers as well as mtDNA and microsatellite loci   . Across different populations, Steller sea lion genetic diversity was contrasted via three different genetic markers: mtDNA, microsatellite loci and AFLPs. Individuals and rookeries sampled for AFLP diversity at a variety of loci show little genetic diversity within the populations. Western and Asian stocks display increased genetic diversity through AFLPs and microsatellites, while the Eastern stock genetic diversity is shown highest in mtDNA . AFLP diversity presents a better understanding of how the Stellar sea lion populations have evolved through the evolutionary timeline, and may predict future population sizes .
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