Long-Term Selection for Rapid Gain in Mice II. Correlated Changes in Reproduction1

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Abstract
The reproductive performance of a strain of mice (G) selected for rapid growth to an apparent plateau, and of lines derived from it by reverse selection, relaxed selection and selection with inbreeding, was analyzed. Fertility, ovulation rate, embryo survival and combinations of these traits were evaluated. Results indicated large amounts of residual genetic variation for both growth and reproduction in the plateaued line. It is proposed that much of this variation is due to pleiotropic loci with additive effects on growth and dominant effects of opposite sign on reproduction, principally fertility and embryo survival. A model in which the relative values of genotypes A1 A1, A1 A2 and A2 A2 are 2, 1 and 0 for growth, and 0, 2 and 2 for reproduction, is consistent with the results of the genetic analysis for growth and with the following observations: (1) lowered fertility in line G, with marked oscillations in fertility between consecutive generations persisting for many generations; (2) rapid restoration of fertility as growth rate declined in response to reverse selection; (3) improvement in fertility and reduction in growth rate in a relaxed selection line, both occurring more slowly than in the reverse line; (4) decline in both reproductive rate and gain in sibmated lines selected for rapid gain, and (5) substantial heterosis for fertility and litter size, but little for gain, from crossing of the inbred lines. The model is consistent with the conclusions of primarily additive genetic variance for growth rate and of a high degree of dominance for genes affecting fertility and litter size. The results represent a clear example of a negative genetic correlation with fitness generated by long-term directional selection for a quantitative trait, resulting in a cessation of response to selection in a population with large residual additive genetic variance. Copyright © 1981. American Society of Animal Science . Copyright 1981 by American Society of Animal Science.

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