Long-Term Selection for Rapid Gain in Mice. I. Genetic Analytical at the Limit of Response

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Abstract
A genetic analysis was carried out on a line of mice (G) that had apparently reached a selection limit for rapid postweaning (3- to 6-week) gain. A reverse selection line (GR) was initiated from line G at generation 33, and at generation 34 lines subjected to relaxed selection (GO) and selection with inbreeding (GF) were started. The three lines were maintained for 10, 9 and 9 generations respectively. Selection was continued in line G, and a contemporary unselected control (C) was also maintained. Response to selection in line G from generations 33 to 43 was essentially zero (b =.03 ± .07 g per generation). Realized heritability during this period was .02 ± .05. A linear decline in gain (b = −.75 ± .07 g/generation) and a realized heritability of .24 ± .03 in line GR indicated that additive genetic variation remained in line G. Major genes that might contribute to the rapid response to downward selection were not identified in a backcrossing experiment involving lines GR and GF. Line GO also declined in gain (b = −.32 ± .07 g/generation), and GF decreased (b = −.42 ± .07 g per generation) despite selection for rapid gain. Net reproductive rate, which was much lower in line G than line C, increased in lines GR and GO and decreased in GF. Thus, the selection limit appeared to be due to a negative association between gain and fitness, rather than to exhaustion of additive genetic variance. Inbreeding with selection was not effective in overcoming the selection limit for gain, with crosses of highly inbred subfamilies within GF showing little heterosis for this trait. Copyright © 1981. American Society of Animal Science . Copyright 1981 by American Society of Animal Science.

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