GENETIC-VARIATION AT MINISATELLITE DNA LOCI AMONG NORTH PACIFIC POPULATIONS OF STEELHEAD AND RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Genetic variation at minisatellite DNA, or variable number tandem repe at (VNTR), loci is widely studied in the context of animal breeding an d pedigree analyses, but comparatively little information exists on th e levels of variation at such loci in natural populations. I examined allelic variability at two VNTR loci (Ssa1 and T34) by Southern hybrid ization analyses within and between populations of steelhead and rainb ow trout, the sea-run and freshwater resident life-history forms of On corhynchus mykiss, from eight populations tributary to the northeast P acific Ocean. Single-locus expected heterozygosities ranged from an av erage of 61% (Ssa1) to 80% (T34) in the eight populations, and no sign ificant departures from Hardy-Weinberg expected genotype frequencies w ere detected. Eighteen putative allelic fragments were resolved in the 267 steelhead and rainbow trout examined at Ssa1 [molecular weight ra nge, 3.6-9.5 kilobase pairs (kbp)], and 26 alleles were resolved at T3 4 (1.7-9.4 kbp). At Ssa1, however, one allele accounted for 58% of all alleles scored and at T34 three alleles accounted for 72% of those sc ored. Allele frequencies at both loci were stable within two populatio ns sampled over successive years, but varied significantly between pop ulations within watersheds, and large frequency differences were detec ted between major geographic areas (e.g., Alaska versus British Columb ia). Neighbor-joining analyses of genetic distances among populations accompanied by bootstrap analysis provided strong support (>70%) for c lustering of populations by geographic region, as well as for a major genetic distinction (100% bootstrap support) between interior Fraser R iver populations of rainbow trout and coastal steelhead trout.