Results from 40 generations of divergent selection for litter size in mice

Abstract

After 21 generations of selection upward (H-line) and downward (L-line) for litter size at birth in mice, a new high line (B) with a similar selection history as the H-line was introduced to the experiment. A synthetic high line (X) was formed by crossing the two high lines H and B. Selection was conducted in the three high lines and the one low line for total number born based on the mother's performance. An unselected control line (K) was maintained throughout the experiment. Litter size was standardised to a maximum of 8 pups at birth in all lines. Average litter size at the start of the experiment was 10.8 pups. When the H- and B-lines plateaued at generations 23–26, the performance was 15.6 and 16.1 pups per generation. The synthetic line, X, had its highest litter size, 17.9 pups, in generation 26. The lowest generation average in the low line was 4.3 pups in generation 28. Realised heritability from generations 1–21 was 0.15 in the high line and 0.17 in the low line. Average realised selection differentials per generation were 2.46, 2.66 and 2.78 in the H, B and X high lines in generations 22–41, while in the low line it was only −0.39. The actual litter size, however, decreased over this period in all three lines selected for large litters and increased in the low line. Average litter size in generations 22–41 was 1.6 mice higher in the synthetic line compared to the two founder lines. Realised heritabilities were zero in the H-, B- and L-lines, and 0.02 in the X-line. The results for the correlated fertility traits number of live born pups and litter size per exposed females as well as for the development of sex ratio are given in the text. Sex ratio showed 46.3 percent males in the low line, significantly lower than the other lines. The lack of response to selection is expected to be caused by a combination of reduction of genetic variation and presence of recessive alleles at low frequencies.