ABSTRACT:Divergent selection for heat production/loss (kcal·kg −.75 ·d −1 ) , measured in 9-to 11-wk-old male mice, was conducted for 15 generations. Heat loss was measured for 15 h on individual animals placed overnight in direct, gradient-layer calorimeters. Selection for high (MH) and low (ML) heat loss and unselected control (MC) occurred in each of three replicates for a total of nine unique lines. Repeatability of the heat loss measurement was .45 and the CV was 10.5%. Cumulative realized selection differentials, averaged for the three replicates, were 145.1 and −105.0 (kcal·kg −.75 ·d −1 ) and ranged from 136.9 to 149.2 and −107.1 to −101.3 for MH and ML selection, respectively. Cumulative standardized realized selection differentials, averaged for the three replicates, were 10.06 and −9.51 for MH and ML selection, respectively. Direct responses (kcal·kg −.75 ·d −1 ) in heat loss after 15 generations were 44.2 for MH and −27.4 for ML as deviations from MC. Asymmetry of response was evident ( P = .03) by Generation 10. Realized heritability was .28 ± .01 based on divergence of MH and ML selection. For selection for higher and lower heat loss, realized heritabilities were .31 ± .01 and .26 ± .01, respectively.

ABSTRACT:Divergent selection for heat loss (kcal·kg −.75 ·d −1 ) , measured in 9-to 11-wk-old male mice, was conducted for 15 generations. Selection for high (MH) and low (ML) heat loss and unselected control (MC) occurred in each of three replicates for a total of nine unique lines. Feed intake in males was measured during Generations 9 through 15. Body mass at commencement of mating in females and at time of measurement of heat loss in males was recorded. Body fat percentage at 12 wk for animals of Generations 6, 10, and 14 was predicted as a function of electrical conductivity and body mass. Litter size was recorded for all generations, and components of litter size were evaluated at Generation 11 in one replicate and Generation 12 in the other two replicates. Feed intake changed in the same direction as heat loss for the MH and ML selections; at Generation 15, the difference between MH and ML ( P < .002) was 20.6% of the MC mean. Body mass did not change with selection for heat loss. Differences in body fat percentage were not significant in earlier generations, but at Generation 14, MH and ML were significantly ( P < .01) different with MH mice having the lowest fat percentage; MC was intermediate. Selection had a significant (MH vs ML; P < .01) effect on litter size, causing an increase in MH and a decrease in ML. This difference was explained by a difference ( P < .01) in ovulation rate. There was no asymmetry of response in feed intake, fatness, litter size, or number of ovulations.

No abstract

ABSTRACT:Genetic variation for liver mass (LM), body mass (BM), and 1iver:body mass (LM/ BM) was examined for outbred populations of laboratory mice. Liver mass and body mass data were collected on 170 pureline sires a t 12 wk of age, representing four outbred stocks of laboratory mice; 523 of their male and female two-waycross progeny at 9 or 12 wk; and 214 four-way-cross' offspring a t 12, 14, or 16 wk. Genetic differences for LM, BM, and LM/BM were found among the base sire lines and between two-way crosses. Heritabilities and genetic correlations for LM, BM, LM/BM, and LM/MBM (MBM = BM.75) were estimated using offspring-sire regression within and across characteristics. Estimates of heritabilities and genetic correlations were also derived from full-sib covariances in the two-way-cross generation. Heritability estimates pooled over all analyses were .53, .54, .36, and .40 for LM, BM, EM/BM, and LM/MBM, respectively. Body mass was highly genetically correlated (.87) with LM and lowly correlated with LM/BM. Previous research has indicated possible positive relationships between LM/BM and maintenance energy requirements in mature, nonlactating, nonpregnant animals. A selection index was developed for increasing BM but restricting genetic change in LM to zero. Selection using this index would be 40% as efficient in increasing BM as selection on BM alone but may hold maintenance energy requirements at a stable level.