Altered expression of growth hormone/insulin-like growth factor I axis hormones in domesticated fish

There are genetically based differences in growth and behavior between domestic and wild (W) trout and salmon, although the molecular underpinnings of the physiological alterations have not been identified. To test for genetically based alterations in the GH/IGF-I axis, which is thought to mediate some of the differences in growth and behavior, we measured circulating concentrations of GH, IGF-I, and thyroid hormone (T(3)), as well as mRNA levels for GH, IGF-I, and GH receptor, from multiple tissues and from fish reared under different environments. Both age-matched and size-matched individuals were examined to overcome difficulties examining strains with inherently different growth rates (and, thus, body size at age). A principal components analysis detected four factors that explained over 70% of the variation in the data; of these, a factor composed of mRNA expression of GH receptor in the liver, IGF-I in the liver, and circulating IGF-I was most strongly correlated with genotype. W coho salmon families responded to environmental alteration with a lower level of plasma IGF-I detected in the seminatural (reduced food) environment relative to the culture environment, whereas no environmental response was detected in the domestic families. The results suggest that genetically based differences in hormone expression and regulation, particularly for IGF-I, are present in response to anthropogenic selection pressures in salmon and trout. In addition, although rearing environment alone can alter relative hormone expression, domestication appears to have reduced the physiological response to environment relative to W fish.