Heterosis among lines of mice selected for body weight : 2. Reproduction.

To examine the effect of selection for body weight on levels of heterosis for reproductive traits, crosses were made between three groups of six lines of mice, one group unselected (controls) and the other two selected for high (large lines) and low (small lines) 6-week body weight, respectively. The coefficient of inbreeding of each line was about 0.60. In a comparison of purebred and crossbred progeny, both out of purebred mothers, there was on average 4% heterosis for number born, 3% for percentage weaned and 8% for numbers weaned. In a comparison of purebred and crossbred mothers, each mated to males of an unrelated strain and dissected on the 17th day of gestation, crossbreds had on average 1.6 more live embryos, which was 22% of the purebred mean. This comprised an increase of 0.6 corpora lutea, of 0.4 in survival to implantation and of 0.6 in subsequent survival to 17 days. The heterosis was similar whether the mothers had parents of the same or different size.