Growth and reproduction in mice selected for rapid body weight gain.

EVERAL investigators have reported the results of selecting for measures of growth rate or body size in mice (GOODALE 1938; MACARTHUR 1944, 1949; FALCONER 1953; RAHNEFELD et al. 1963; ROBERTS 1967). Direct response to selection has been substantial in each of these cases. Litter size has been shown to increase in several of the lines, the increase in the experiment reported by RAHNEFELD et al. (1966) being nearly three mice per litter. This suggested the possibility that litter size might be increased more rapidly by indirect selection, for body weight gain, than by direct selection, particularly since results of selection for litter size have indicated quite low heritability for this trait (FALCONER 1960; DALTON and BYWATER 1963). A line selected for rapid post-weaning gain was therefore included in an experiment in which the effects of selecting for litter size and its components were being studied. This permitted a direct comparison of the effects of these two kinds of selection, using the same base population and selecting contemporarily within the same laboratory. Results from this project of selecting for litter size and for its components have been reported (BRADFORD 1968, 1969). The present paper reports the direct and correlated responses to selection in the rapid gain line and lines derived from it. The results suggest a complex relationship between body size and reproduction in the mouse, and provide additional evidence on the nature of the genetic control of litter size.