The influence of gene-environment interactions on GHR and IGF-I expression and their association with growth in brook charr, Salvelinus fontinalis (Mitchill)

Background: Quantitative reaction norm theory proposes that genotype-by-environment interaction (GxE) results from inter-individual differences of expression in adaptive suites of genes in distinct environments. However, environmental norms for actual gene suites are poorly documented. In this study, we investigated the effects of GxE interactions on levels of gene transcription and growth by documenting the impact of rearing environment (freshwater vs. saltwater), sex and genotypic (low vs. high estimated breeding value EBV) effects on the transcription level of insulin-like growth factor (IGF-I) and growth hormone receptor (GHR) in brook charr (Salvelinus fontinalis). Results: Males grew faster than females (mu female = 1.20 +/- 0.07 g center dot d(-1), mu male = 1.46 +/- 0.06 g center dot d(-1)) and high-EBV fish faster than low-EBV fish (mu(LOW) = 0.97 +/- 0.05 g center dot d(-1), mu(HIGH) = 1.58 +/- 0.07 g center dot d(-1); p < 0.05). However, growth was markedly lower in saltwater-reared fish than freshwater sibs (mu(FW) = 1.52 +/- 0.07 g center dot d(-1), mu(SW) = 1.15 +/- 0.06 g center dot d(-1)), yet GHR mRNA transcription level was significantly higher in saltwater than in freshwater (mu(SW) = 0.85 +/- 0.05, mu(FW) = 0.61 +/- 0.05). The ratio of actual growth to units in assayed mRNA ('individual transcript efficiency', iTE; g center dot d(-1) center dot u(-1)) also differed among EBV groups (mu(LOW) = 2.0 +/- 0.24 g center dot d(-1) center dot u(-1); mu(HIGH) = 3.7 +/- 0.24 g center dot d(-1) center dot u(-1)) and environments (mu(SW) = 2.0 +/- 0.25 g center dot d(-1) center dot u(-1); mu(FW) = 3.7 +/- 0.25 g center dot d(-1) center dot u(-1)) for GHR. Males had a lower iTE for GHR than females (mu male = 2.4 +/- 0.29 g center dot d(-1) center dot u(-1); mu female = 3.1 +/- 0.23 g center dot d(-1) center dot u(-1)). There was no difference in IGF-I transcription level between environments (p > 0.7) or EBV groups (p > 0.15) but the level of IGF-I was four times higher in males than females (mu male = 2.4 +/- 0.11, mu female = 0.58 +/- 0.09; p < 0.0001). We detected significant sexual differences in iTE (mu male = 1.3 +/- 0.59 g center dot d(-1) center dot u(-1); mu female = 3.9 +/- 0.47 g center dot d(-1) center dot u(-1)), salinities (mu(SW) = 2.3 +/- 0.52 g center dot d(-1) center dot u(-1); mu(FW) = 3.7 +/- 0.53 g center dot d(-1) center dot u(-1)) and EBV-groups (mu(LOW) = 2.4 +/- 0.49 g center dot d(-1) center dot u(-1), mu(HIGH) = 3.8 +/- 0.49 g center dot d(-1) center dot u(-1)). Interaction between EBV-group and environment was detected for both GHR (p = 0.027) and IGF-I (p = 0.019), and for iTE in the two genes (p < 0.0001; p < 0.05, respectively), where increased divergence in levels of GHR and IGF-I transcription occurred among EBV-groups in the saltwater environment. Conclusion: Our results show that both environment and sex have major impacts on the expression of mRNA for two key genes involved in the physiological pathway for growth. We also demonstrate for the first time, at least in fish, genotype-by-environment interaction at the level of individual gene transcription. This work contributes significantly to ongoing efforts towards documenting environmentally and sexually induced variance of gene activity and understanding the resulting phenotypes.