Abstract
A long-term selection experiment for large litter size in mice was conducted, starting with Line L at average litter size of 8.3 ± 2.2. At Generation 30 with litter size of 14.0 ± 2.7, a selection plateau appeared. The nature and causes of the plateau were investigated. From Generation 48 of Line L, divergent selection for litter size was applied upward (Line LH) and downward (Line LL). After 12 generations, average litter size in LH was 13.9 ± 2.6 and in LL 10.2± 4.7. Realized heritabilities were zero in LH and 0.20±0.04 in LL. The response to reverse selection was attributed to the continued segregation of recessive alleles at the plateau. The role of increased prenatal mortality as a cause of the plateau was investigated. Prenatal mortality ( M) in Line L increased with number of implantations ( I) as M = 0.11 + 0.0003 × I 3. This exponential increase could eventually balance as increasing number of implantations and thus contribute to a plateau for litter size. The relevance of this study for the understanding of genetic effects on litter size in pigs is indicated.
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