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Oklahoma State University, Stillwater 74074
Abstract
THE objectives of this study were to determine direct and correlated selection responses between two lines of albino mice (HML and LML) which had undergone seven generations of selection for heavy and light hindleg muscle system weight, respectively. A genetic control (CL) consisting of the mean performance of two unselected control lines was utilized to compare the magnitude of the responses obtained in each selection line.
In each selection line each generation, prospective sires (21 to 24) were mated to two females each and sacrificed at 84-days of age and the muscle systems from both hindlegs of each sire were dissected and weighed. Selection was based on the combined weight of the muscle systems dissected from both hindlegs. Correlated responses were measured in reproductive performance; litter weight at 12-days of age; individual weights at 21, 42 and 84 days of age; ADG from 21 to 42 days of age and the ratio of hindleg muscle weight to 84-day weight.
After seven generations of two-way selection, total hindleg muscle weight was 2.83± 0.07, 2.54±0.03 and 2.10±0.07 g in HML, CL and LML, respectively. The estimate of realized h2 for hindleg muscle weight obtained from the regression of selection response on cumulative selection differential was 0.24± 0.06, 0.70±0.17 and 0.41±0.06 for HML, LML and total divergence, respectively.
Correlated responses in reproductive performance demonstrated significant (P<.05) divergence between the selection lines for number of live offspring per litter in favor of HML. Maternal performance as measured by 12-day litter weight and 21-day weaning weight indicated that selection for hindleg muscle weight had resulted in no significant change in maternal performance in HML but had significantly decreased maternal performance in LML. Weights at 21, 42, 56 and 84 days of age and ADG from 21 to 42 days of age exhibited asymmetrical correlated responses with the larger response being obtained in LML for all traits.
Estimates of rG between hindleg muscle weight and the live performance traits as calculated from selection responses were 0.51-!0.20, 0.77-!-0.11, 0.59±0.20 and 0.77±0.11 for weights at 21, 42, 56, and 84 days, respectively; between muscle weight and ADG was 0.48--0.16; between muscle weight and hindleg weight was 0.98±0.01 and between muscle weight and percent muscle was 0.35±0.27. The ratio of hindleg muscle weight to 84-day weight was significantly (P < .O1) larger in HML than in LML after generation 3. However, selection for increased hindleg muscle weight was apparently ineffective in increasing the proportion of muscle as the ratio in the CL tended to fluctuate above and below HML. Conversely, selection for light muscle weight resulted in a significant (P<.Ol) decrease in percent muscle.
As a result of the considerable genetic variation that existed in this population for hindleg muscle system weight, selection was effective in both increasing and decreasing hindleg muscle weight. However, selection was more effective in decreasing hindleg muscle weight. Corresponding correlated responses occurred for all live performance traits measured suggesting that hindleg muscle system weight and measures of live performance were related genetically.
1 Journal Article 2443 of the Agricultural Experiment Station, Oklahoma State University, Stillwater.
2 Present address: Louisiana Animal Breeders Coop, LSU Box Bd, Baton Rouge, La. 70803.
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