Effect of reproduction on the consistency of the between-line type divergence in laboratory mice selected on Basal metabolic rate.

Artificial selection experiments are an effective tool for testing evolutionary hypotheses, because they allow one to separate genetic and environmental variances of the phenotype. However, it is unclear whether trait divergence typically selected early in life persists over an animal's life and altered physiological states, such as reproduction. Here we analyzed the long-term consistency of the between-line type divergence in basal metabolic rate (BMR) selected at 12 wk of age in laboratory mice. We measured BMR in nonreproducing and reproducing females at the age of 22 wk and then at 27 wk of age. Our results show that within both the reproducing group and the control group, the between-line type separation in BMR is consistently retained over time and reproductive status. Metabolically active internal organs (heart, liver, kidneys, and small intestine) also consistently differed in size between the two line types with no significant long-term effect of reproduction. The observed consistency of the between-line type divergence in BMR suggests the existence of the persistent effect of the selection on metabolic traits applied early in life. Moreover, BMR variation achieved by means of artificial selection is considerably higher than that found in natural/unmanipulated populations. The latter may therefore be characterized by insufficient variance to statistically resolve correlations involving BMR.