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Molecular analysis of hybridization between native westslope cutthroat trout (oncorhynchus clarki lewisi) and introduced rainbow trout (O.mykiss) in southeastern British Columbia Rubidge, Emily


Westslope cutthroat trout (Oncorhynchus clarki lewisi, WCT) and introduced rainbow trout (0. mykiss, RBT) readily hybridize and introgression has occurred in many drainages across the historic range of WCT. In British Columbia, the upper Kootenay River drainage is the heart of WCT distribution and is thought to be a refuge for genetically pure populations. In this study, I assess the extent and distribution of WCT x RBT hybridization in the upper Kootenay River drainage, examine the genotypic structure of hybridizing populations using population genetic analyses, and examine the potential for differential selection between pure WCT and hybrid individuals using cohort analysis. Caudal fin clips were collected from 981 fish at 23 sample sites in 12 different streams in the upper Kootenay River drainage. I used 4 diagnostic nuclear loci to determine the extent of hybridization at each sample site. Fourteen percent (142/981) of individuals were identified as hybrids, 3.4% (33/981) were identified as pure RBT, and the remaining individuals were identified as pure WCT. Mitochondrial DNA analysis indicated that hybrid matings occur between males and females of both species. Although pure RBT were absent from the majority of sites (20/23), I found evidence of hybridization at 78% (18/23) of samples sites and the percentage of heterospecific alleles (% I) ranged from 0.7-97.1%. Only 22% (5/23) of sample sites showed no evidence of hybridization. The majority of hybrid individuals were genotypically classified as WCT backcrosses (59%) and post-F₂ individuals (24%). The skewed ratio of pure WCT to RBT (27:1), and the rarity of F₁ individuals (4 of 142 hybrids), suggests that the spread of RBT alleles is facilitated by hybrids straying to neighbouring populations. Spatial analysis showed clustering among hybridized sites and decreasing hybridization with increasing distance from Koocanusa Reservoir, suggesting that the reservoir acts as a RBT source. I found little evidence of differential selection between pure WCT and hybrid individuals. My results suggest that hybridization is relatively recent in the upper Kootenay River drainage and that it is increasing in magnitude and distribution. In the absence of timely management intervention, the genetic integrity of WCT populations in the heart of their Canadian range may be lost.

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