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Average Genetic and Heterotic Effects on Growth in Mice Selected for Large 6-Week Body Weight or Rapid Postweaning Gain¹

North Carolina State University, Raleigh 27607

Abstract

Data on body weights at 3 weeks (WK3) and 6 weeks (WK6) and postweaning gain from 3 to 6 weeks (GAIN) were analyzed in eight populations of mice to estimate the differences between populations in average direct genetic effects (ADG), average maternal genetic effects (AMG) and direct heterotic effects (DH). The effects of population, sex, group size and interactions were examined. Females were caged singly or in groups of four. Populations studied were M16, selected for GAIN, H₆, selected for WK6, their respective base population controls (ICR and C₂), reciprocal F₁ crosses between M16 and H₆, and reciprocal F₁ crosses between ICR and C₂. The positive (P<.01) responses to selection in H₆ relative to the C₂ control were 21.7, 33.7 and 46.4% for WK3, WK6 and GAIN, respectively. Corresponding positive (P<.01) responses for these traits in Ml6 were 28.0, 49.5 and 70.1% greater than the ICR control. ADG accounted for the major part of this greater selection response in M16 compared to H₆. AMG were not responsible for any significant selection response difference between M16 and H₆. Differences between the controls (ICR minus C₂), expressed as a percentage of the parental means, were 15.4% (P<01) for WK3 (2.8% for ADG, P<.05 and 12.7% for AMG, P<01), 20.3% (P<01) for WK6 (14.8% for ADG, P<.01 and 5.5% for AMG, P<05) and 25.2% (P<01) for GAIN (26.8% for ADG, P<.01 and —1.7% AMG, P>.05). In the selected populations, the percentage differences (M16 minus H₆) were 20.5% (P<01) for WK3 (22.6% for ADG, P<.01 and —2.1% for AMG, P>.05), 31.3% (P<01) for WK6 (34.2% for ADG, P<01 and —2.9% for AMG P>.05) and 39.8% (P<01) for GAIN (43.3% for ADG, P<01 and —3.5% for AMG P>.05). Direct heterosis effects for WK3 and WK6 were significant in F₁ crosses between the control and F₁ crosses between the selected populations, accounting for about 5% of the midparent value. Sex and population x sex interactions were significant for the three traits, although the differences for WK3 were small. Males tended to show greater heterosis than females. Group size and population x group size interactions were significant for WK6 and GAIN. Females caged in groups of four had higher WK6 than singly caged mice.

¹ Paper No. 4888 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh and Contribution No. 610 of the Animal Research Institute, Ottawa and No. 654-490-7 of the Agricultural University at Wageningen.

² On leave from Department of Animal Husbandry, Agricultural University at Wageningen, the Netherlands.

³ On leave from Animal Research Institute, Agriculture Canada, Ottawa, Ontario, K1A OC6.

⁴ Animal Science Department, North Carolina State University, Raleigh.

⁵ The authors acknowledge the technical assistance of Ms. B. J. Edwards.