MICROSATELLITE ANALYSIS OF GENETIC POPULATION-STRUCTURE IN AN ENDANGERED SALMONID - THE COASTAL CUTTHROAT TROUT (ONCORHYNCHUS-CLARKI-CLARKI)

The genetic population structure of coastal cutthroat trout (Oncorhync hus clarki clarki) in Washington state was investigated by analysis of variation in allele frequencies at six highly polymorphic microsatell ite loci for 13 anadromous populations, along with one outgroup popula tion from the Yellowstone subspecies (O. clarki bouvieri) (mean hetero zygosity = 67%; average number of alleles per locus = 24). Tests for g enetic differentiation revealed highly significant differences in geno typic frequencies for pairwise comparisons between all populations wit hin geographical regions and overall population subdivision was substa ntial (F-ST = 0.121, R-ST = 0.093), with 44.6% and 55.4% of the among- population diversity being attributable to differences between streams (F-SR = 0.054) and between regions (F-RT = 0.067), respectively. Anal ysis of genetic distances and geographical distances did not support a simple model of isolation by distance for these populations. With one exception, neighbour-joining dendrograms from the Cavalli-Sforza and Edwards' chord distances and maximum likelihood algorithms clustered p opulations by physiogeographic region, although overall bootstrap supp ort was relatively low (53%). Our results suggest that coastal cutthro at trout populations are ultimately structured genetically at the leve l of individual streams. It appears that the dynamic balance between g ene flow and genetic drift in the subspecies favours a high degree of genetic differentiation and population subdivision with the simultaneo us maintenance of high heterozygosity levels within local populations. Results are discussed in terms of coastal cutthroat trout ecology alo ng with implications for the designation of evolutionarily significant units pursuant to the US Endangered Species Act of 1973 and analogous conservation units.