Based on the outbred mouse strain Fzt: Du, which has been obtained
by systematic crossing of
four inbred and four outbred lines, a long-term selection experiment was
carried out for total
protein amount (PA) in the carcass, starting in 1975. An unselected
control line (CO) was kept
under the same management but without continuous protein analysis. The
protein amount of male
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 2·3 g,
which is about 80% above the initial value and corresponds to 9σp
or 12σA . The estimated
realized heritability of protein amount decreased from 0·56 to 0·03
at generation 70, which was due
to an increase in phenotypic variance from 0·065 to 0·24
g2
and a reduction in genetic variance from 0·04 to 0·01 g2.
Half the selection response was obtained after about 18 to 23 generations,
a
half-life of 0·25 to 0·3 Ne. The maximum
selection response was 0·094 g/generation and the
response was 0·01 g/generation at generation 70. The measurements
of body weights at 0, 10, 21,
42 and 63 days throughout the experiment 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 significantly in their
body composition from control mice at 42 days. The direct selection response
was due primarily to
increased general growth. Body weight and protein amount are phenotypically
and genetically
highly correlated (rp=0·82, rA≈1);
however, selection for body weight led to fatter animals,
whereas selection for protein opposed increased fatness (at least until
selection age). This may be
of general importance in animal breeding. The comparatively high selection
response in this
experiment seems due to the heterogeneity of the base population, the relatively
high effective
population size, and the duration of the experiment.
Optimum selection intensities in artificial selection programmes: an experimental evaluation
