Body composition and energetic efficiency in two lines of mice selected for rapid growth rate and their F1 crosses

Summary Correlated responses to selection for increased growth rate were compared in two mouse populations (M16 and H₆) of distinct genetic origin. Traits studied were body composition, feed intake, constituent gains and energetic efficiency. When compared with their respective controls (ICR and C₂) at 6 and 9 weeks of age, body weight increased more in M16 (57%and 69 % of the control mean) than in H₆ (40 % and 34%). The M16 showed correlated responses in fat percent of 2.6% (P <.05), 8.4% (P <.01) and 11.2% (P <.01) at 3, 6 and 9 weeks, respectively, whereas corresponding values in H₆ were –2.4% (P <.05), 3.3% (P <.05) and 2.09 % (P >.05). The correlated responses in fat percent were 2.7 and 4.7 times higher in M16 than H₆ at 6 and 9 weeks. The regression of ln fat weight on ln empty body weight was larger in M16 (P <.05) compared to ICR and larger (P <.01) in H₆ compared to C₂. Both M16 and H₈ exhibited positive correlated responses from 3 to 6 weeks of age in feed intake and gain and efficiency in fat, protein, calories and ash; fat and caloric gain and efficiency exhibited higher correlated responses in M16 than H₆. During the 6- to 9-week interval, the M16 population continued to evince positive correlated responses in gains and efficiencies of fat, protein and calories, whereas H₆ did not. Several possible explanations are presented to account for the differences in correlated responses between the selected populations. Partitioning of correlated response differences between M16 and H₆ into average direct and average maternal genetic effects indicated that average direct genetic effects, favoring M16, were responsible for the major difference between the selected populations. Direct heterosis in F₁ crosses of the selected populations were generally not significant, although there was a tendency for fat percent and fat weight to show heterosis.Paper No. 4929 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh, North Carolina 27607, Animal Research Institute Contribution No. 624 and Agricultural University at Wageningen Contribution No. 654-490-10. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Experiment Station of the Products named, nor criticism of similar ones not mentioned.On leave from Department of Animal Husbandry, Agricultural University, Wageningen, The Netherlands.On leave from Animal Research Institute, Agriculture Canada, Ottawa, Ontario K1A OC6.