Postnatal maternal effects on growth and fat deposition in mice selected for large and small size.

A crossfostering experiment involving lines of mice selected for large (L) and small (S) 6-week body weight was designed to determine correlated responses in direct genetic and postnatal maternal genetic effects and postnatal litter size effects on fat deposition at 6 weeks of age. The gonadal fat pad was used as an index of adiposity. The L line exceeded the S line in both direct genetic and postnatal maternal genetic effects on weight and percentage (of body weight) of the gonadal fat pad. Postnatal maternal genetic effects were about one-third as large as direct genetic effects. A prenatal line x postnatal line interaction for weight and percentage of gonadal fat was caused by the inability of S line dams to provide sufficient milk to maintain normal development of L line young. Further evidence supporting this hypothesis was the high mortality rate among L line young reared by S line dams when compared with the mortality in all other subgroups. Increasing postnatal litter size reduced weight and percentage of gonadal fat, but this factor was of less importance than direct genetic effects. In general, the relative importance of direct genetic, postnatal maternal genetic and postnatal litter size effects was similar for metric measures of growth (body weight, body length and tail length) and for adiposity (gonadal fat pad weight and percentage). Rate of gonadal fat pad development relative to body weight was higher in line L than in line S. At a constant body weight, however, line L mice had less fat than line S mice.