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Genetic structure in populations of steelhead trout in British Columbia Parkinson, Eric A.


Rainbow trout (Salmo gairdneri) collected from 63 locations in British Columbia were studied using electrophoresis to examine genetic structuring of populations. Most collections were from anadromous populations, in which gene flow is possible between widely separated locations. Repeated sampling of a number of locations provided an estimate of the intrapopulation variability of allozyme frequencies. Although 20-30 enzyme loci were initially screened for a small number of populations, only four enzyme systems (LDH-4, SOD, MDH-3, IDH-3,4) were consistently variable. The AGP-1 locus was scored when possible because it was highly variable in two populations. Each of these systems has been shown to have a clear genetic basis by other workers. In addition, the ME and SDH systems were used to distinguish cutthroat and rainbow trout fry which often cannot be easily distinguished morphologically. The variability in allozyme frequencies within streams was highly significant for each of the four enzyme systems studied. This variability was not clearly associated with differences between locations, year classes or life history stages. Genetic drift, a result of the small numbers of adults spawning each year can account for all of the within stream variability. A hierarchical ANOVA was used to determine the size of area in which genetic structuring is present in Salmo gairdneri. Significant differences, implying genetic structuring, were found in three out of four systems between streams adjacent to each other along a large river or ocean coastline. With one exception gene frequencies averaged over larger areas were uniform and similar to previously measured frequencies in areas of the northwestern United States. The one exception to this large scale uniformity is a sharp discontinuity in LDH-4 and SOD frequencies in tributaries of the Fraser River at the boundary between the coastal and interior regions. A similar discontinuity on the Columbia River is considered to represent the boundary between two large groups of populations isolated during the last glaciation. Evidence against this contention includes the lack of coincidence of the LDH-4/S0D discontinuity in the more northerly Dean and Skeena rivers, the presence of noncoincident discontinuity in AGP-1 and PEP frequencies in the Fraser and Columbia rivers, respectively, and evidence of physiological effects, implying selection, in the variation at the LDH-4 locus. A single non-anadromous population transplanted from an interior lake to a coastal lake approximately 40 years ago is fixed for the SOD(IOO) allozyme variant and shows no obvious change in the frequency of the LDH-4(100) allozyme and therefore provides no evidence of selection acting on this locus.

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