Variation in plasma concentration of insulin-like growth factor-1 and its covariation with liveweight in mice.

Three experiments were undertaken to examine the degree and causes of variation in plasma concentrations of insulin-like growth factor-1 (IGF-1) in mice. The relationship between IGF-1 concentrations and liveweight was also examined. In all three experiments, a number of non-genetic factors were found to contribute significantly to the variation in IGF-1 concentrations, the most important of these being sex and litter size. In one experiment, where pups from 16 litters were cross-fostered to avoid the confounding of maternal and direct genetic effects, a heritability of 0.40 +/- 0.27 was estimated for plasma IGF-1 concentration at 35 days of age. To examine further the existence of genetic variation in plasma concentrations of IGF-1 and the genetic covariation between plasma IGF-1 levels and other body traits, a selection experiment with mice has been initiated. Moderate to strong phenotypic correlations between IGF-1 concentrations and weight at an early age have been found in all three experiments.