Resolution of population structure in a species with high gene flow: microsatellite variation in the eulachon (Osmeridae : Thaleichthys pacificus)

Five microsatellite loci were used to examine genetic variation within and among putative populations of the eulachon, Thaleichthys pacific-us (Pisces : Osmeridae), over the entire range of the species. A previous mitochondria l DNA study, while revealing a high degree of genetic variation within the species, did not resolve the level of population sub-division expected for this anadromous fish. Two microsatellite loci were developed from eulachon DNA and, in addition to three microsatellite loci from the rainbow smelt, O smerus mordax, were employed as a class of "higher resolution" markers in a n attempt to further resolve the population structure of eulachon. The leve l of genetic variation observed at these loci was surprisingly low (heteroz ygosity ranged from 4% to 64%; number of alleles ranged from three to ten; maximum size range of alleles was 16 base pairs), yet revealed the greater power of microsatellites over mitochondrial DNA for resolving population su b-division within eulachon. More pairwise population comparisons were signi ficant with the microsatellite data, and the microsatellite F-ST value was twice the value observed with mtDNA (mtDNA F-ST = 0.023; microsatellite F-S T = 0.045). Despite this greater sensitivity, it was difficult to define di stinct demographic units in eulachon, a species which is currently the focu s of conservation concern. Eulachon highlight the challenges of examining p opulation structure in species with inferred high gene flow.