Selection for fertility in mice - the selection plateau and how to overcome it.

A long-term experiment for increasing the traits first day litter size (LS1) and litter weight (LW1) was conducted with two populations for 33 generations. The selection plateau was reached in population DU-C (selection and estrus synchronization (h(2) = 0.02±0.01); in population DU-K (selection) the plateau (h(2) = 0.05±0.2) was nearly reached. Selection progress per generation was in LS1 b = 0.11±0.02; b = 0.12±0.04 (1st to 18th generation DU-K, DU-C) and b = 0.10±0.03; b = 0.07±0.05 (19th to 33rd generation, DU-K, DU-C) in LW1 b = 0.16±0.04 g; 0.19±0.07 g (DU-K, DU-C) b = 0.20±0.09 g; 0.001±0.09 g (DU-K, DU-C). Reverse and relaxe selection as well as systematic inbreeding was applied for 10 generations. Reverse selection yielded h(2) = 0.28±0.11 (R-DU-K) and h(2) = 0.17±0.05 (R-DU-C) and showed that there was still additive genetic variance. Relaxe selection did not cause alterations in the selection parameters, whereas inbreeding lead to inbred depressions (b = LS1 = -0.42±0.15; -0.45±0.12; b = LW1 = -1.13±0.20; -0.82±0.18 I-DU-K, I-DU-C). The plateau was based upon the heterozygote advantage. Several methods for overcoming the plateau were applied. A new selective useful variance could be created by crossing the plateau populations (h(2) = 0.14±0.04). A short-term progress in overcoming the plateau (1st to 3rd generation) could be obtained by litter size standardization (LS = 388). Tandem selection (selection for body weight - BW42) as well as crossing of inbred strains were not suitable for overcoming the selection plateau. Altering the environmental conditions as a possibility for overcoming the plateau has been discussed.