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Contributions of genetic variations and phenotypic plasticity to variation in high pH tolerance in Rainbow Trout Northrup, Sara Lynn


High pH is physiologically stressful for Rainbow Trout, causing poor survival when fish are stocked into high pH lakes. To assess the relative contributions of genetic variation and phenotypic plasticity in high pH tolerance in Rainbow Trout, I examined high pH (pH 9.5) tolerance in three Rainbow Trout strains (Blackwater River, Eagle Lake and Fraser Valley Domestics) under four different rearing conditions: 1) near-neutral hatchery conditions (pH 7.2) from fertilization; 2) pH 8.5 from fertilization; 3) pH 8.8 from fertilization; and 4) near-neutral hatchery conditions from fertilization followed by acclimation to pH 8.8 for one month prior to testing (at fry and yearling). In general, I found that either rearing or acclimating fish to elevated pH improved high pH tolerance. Variation among strains was observed only at the fry life stage. I performed a genome wide association study to identify genetic variation that may be associated with differences in pH tolerance among strains. The results suggest that pH tolerance is likely controlled polygenically. To assess mechanisms underlying phenotypic plasticity in high pH tolerance, gill gene expression of fish reared under control conditions and those acclimated to pH 8.8 were compared using RNA-Seq. There were 140 genes that were significantly differentially expressed in response to high pH, but the most dramatic results were the strong interaction effects between pH and strain suggesting that each strain compensates for high pH conditions in different ways. Finally, the variation among strains and rearing treatments observed within the laboratory was tested in natural lakes. In general, short-term net pen trials were consistent with laboratory results showing higher pH tolerance in fish reared at or acclimated to elevated pH levels. Long-term survival trials indicate that the large differences in survival in natural lakes between strains mask subtler effects of prior exposure to high pH and require further investigation. My data suggest that it is the remarkable plasticity of Rainbow Trout rather than a specific strain or genotype which has the greatest effect on high pH tolerance, and that modifications of hatchery practices could be used to improve survival of stocked Rainbow Trout in high pH lakes.

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