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Physiological, behavioural, and fitness consequences of introgression of domestic genotypes into wild salmonid populations Tymchuk, Wendy Elizabeth Vandersteen

Abstract

Within the natural environment, growth rates of an animal can be maintained at an adaptive level determined by cost-benefit tradeoffs. Due to an altered selection regime, fish selectively bred for the aquaculture industry may not be as adapted to the natural environment as wild fish. To increase understanding of the genetic changes underlying selection for enhanced growth which results in phenotypic differentiation of domestic (farmed) from wild Pacific salmon, multiple generations of pure and hybrid families were generated for coho salmon (Oncorhynchus kisutch) and rainbow trout (Oncorhynchus mykiss), including pure domestic (D), pure native (W), F₁ and F₂ hybrids, F₁ x wild (B₁), and B₁ x wild (B₂) backcross genotypes. The family groups were reared in: (1) culture conditions, (2) semi-natural conditions with either competition, risk of predation, or both, and (3) nature. Under culture and most semi-natural environments there was a significant genotype effect on growth performance (mass and length), with a strong linear relationship between the proportion of domestic alleles within the genotype and size. This rank remained the same for both species. Behavioural differences were observed among the families, with fast-growing domestic families showing a reduced anti-predator response relative to slow-growing wild families. Expression of the phenotypic differences in the hybrids and backcrosses, as well as results from a joint-scale analysis on line means, suggests that additive genetic effects contribute significantly to the divergence between the fast- and slow-growing strains. Survival of the fry in a semi-natural environment with competition fit an additive model of gene action with the domestic fish having the highest survival and the wild fish the lowest, but under risk of predation outbreeding depression was suggested by low survival of the B₂ lines. Evidence of a tradeoff in growth and survival under risk of predation along with observations of genetically-determined behavioural differences among the strains may provide some explanation for the observed differences in survival among the strains. Endocrine analysis indicates that an increase in circulating thyroid hormone and insulin-like growth factor I, correlated with the proportion of domestic alleles in the genome, may explain some of the phenotypic differences observed between domestic and wild strains of salmonids. This information is relevant to improving our evolutionary understanding of the interaction among genomes, and the influence of environment, during hybridization events. Results from these experiments indicate that alteration of phenotype likely plays a prominent role in the reduced fitness experienced by progeny produced after three generations of introgression, supporting theory that disruption of genotypes selected for adaptation to local conditions may be a primary cause of outbreeding depression in species such as salmon.

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