Neuroendocrine regulation of growth hormone and luteinizing hormone in fetal and neonatal pig

The influence of gene-environment interactions on GHR and IGF-1 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-1) and growth hormone receptor (GHR) in brook charr (Salvelinus fontinalis).

Results

Males grew faster than females (μ_♀ = 1.20 ± 0.07 g·d^-1, μ_♂ = 1.46 ± 0.06 g·d^-1) and high-EBV fish faster than low-EBV fish (μ_LOW = 0.97 ± 0.05 g·d^-1, μ_HIGH = 1.58 ± 0.07 g·d^-1; p < 0.05). However, growth was markedly lower in saltwater-reared fish than freshwater sibs (μ_FW = 1.52 ± 0.07 g·d^-1, μ_SW = 1.15 ± 0.06 g·d^-1), yet GHR mRNA transcription level was significantly higher in saltwater than in freshwater (μ_SW = 0.85 ± 0.05, μ_FW = 0.61 ± 0.05). The ratio of actual growth to units in assayed mRNA ('individual transcript efficiency', iTE; g·d^-1·u^-1) also differed among EBV groups (μ_LOW = 2.0 ± 0.24 g·d^-1·u^-1; μ_HIGH = 3.7 ± 0.24 g·d^-1·u^-1) and environments (μ_SW = 2.0 ± 0.25 g·d^-1·u^-1; μ_FW = 3.7 ± 0.25 g·d^-1·u^-1) for GHR. Males had a lower iTE for GHR than females (μ_♂ = 2.4 ± 0.29 g·d^-1·u^-1; μ_♀ = 3.1 ± 0.23 g·d^-1·u^-1). There was no difference in IGF-1 transcription level between environments (p > 0.7) or EBV groups (p > 0.15) but the level of IGF-1 was four times higher in males than females (μ_♂ = 2.4 ± 0.11, μ_♀ = 0.58 ± 0.09; p < 0.0001). We detected significant sexual differences in iTE (μ_♂ = 1.3 ± 0.59 g·d^-1·u^-1; μ_♀ = 3.9 ± 0.47 g·d^-1·u^-1), salinities (μ_SW = 2.3 ± 0.52 g·d^-1·u^-1; μ_FW = 3.7 ± 0.53 g·d^-1·u^-1) and EBV-groups (μ_LOW = 2.4 ± 0.49 g·d^-1·u^-1; μ_HIGH = 3.8 ± 0.49 g·d^-1·u^-1). Interaction between EBV-group and environment was detected for both GHR (p = 0.027) and IGF-1 (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-1 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.