An understanding of the physiological and genetic changes which determine the response to selection is critical for both evolutionary theory and to assess the application of new molecular techniques to commercial animal breeding. We investigated an aspect of physiology, growth hormone (GH) metabolism, which might a priori have been expected to play a large part in the response of mouse lines selected for high or low body weight. Disruption of endogenous GH or addition of exogenous GH had similar proportionate effects on body weight in both lines of mice (although differences in body composition arose) suggesting that neither the production of GH nor receptor sensitivity to GH had been altered as a result of selection. This supports a 'pleiotropic model' of the response to selection: that many genes with diverse metabolic roles all contribute to the divergent phenotype. This result has significant commercial implications as it suggests that artificial selection, transgenic technology and environmental manipulation may be synergistic rather than antagonistic strategies.

The effects of exogenous GH on growth and body composition were investigated in lines of mice selected for high or low body weight (P-lines) or high or low body fat (F-lines). Mice from all lines were given daily injections of recombinant bovine GH or a placebo for 21 days from 4 weeks old. They were killed and various organ weights measured. There was no consistent effect of GH on organ weights. In all lines of mice the rate of weight gain and final weight increased in response to GH. In both lines selected for body fat, GH treatment decreased fat content. The low body weight mice also became less fat, but in the high body weight mice GH treatment increased fat percentage. The results indicate that the differences in growth rate and body composition observed in these lines are not due to differences in responsiveness to GH.