Long-term selection for protein amount over 70 generations in mice

Based on the outbred mouse strain Fzt: Du, which has been obtained by syste matic crossing of four inbred and four outbred lines, a long-term selection experiment was carried out for total protein amount (PA) in the carcass, s tarting in 1975. An unselected control line (CO) was kept under the same ma nagement but without continuous protein analysis. The protein amount of mal e carcasses at 42 days of age (P42) increased from 2.9 g in generation 0 to 5.2 g at generation 70, representing 97 % of a theoretical selection limit . The total selection response amounts to 23 g, which is about 80 % above t he initial value and corresponds to 9 sigma(P) or 12 sigma(A). The estimate d realized heritability of protein amount decreased from 0.56 to 0.03 at ge neration 70, which was due to an increase in phenotypic variance from 0.065 to 0.24 g(2) and a reduction in genetic variance from 0.04 to 0.01 g(2). H alf the selection response was obtained after about 18 to 23 generations, a half-life of 0.25 to 0.3 N-e. The maximum selection response was 0.094 g/g eneration and the response was 0.01 g/generation at generation 70. The meas urements of body weights at 0, 10, 21, 42 and 63 days throughout the experi ment showed a strong correlated effect for all weights. The PA mice are one of the heaviest lines of mice ever reported, and do not differ significant ly in their body composition from control mice at 42 days. The direct selec tion response was due primarily to increased general growth. Body weight an d protein amount are phenotypically and genetically highly correlated (r(P) = 0.82, r(A)approximate to 1); however, selection for body weight led to f atter animals, whereas selection for protein opposed increased fatness (at least until selection age). This may be of general importance in animal bre eding. The comparatively high selection response in this experiment seems d ue to the heterogeneity of the base population, the relatively high effecti ve population size, and the duration of the experiment.