Components of heterosis for growth traits and litter size in line crosses of mice after long-term selection

Strains selected for 54 generations for large 8-week weight (N8, S8), for s mall weight (N6, S6), for high control strains (N9, 3-5-week gain and low 8 -week weight (NI) as well as unselected randomly mate S9) were crossed, and F-1, F-2 and both back crosses created. N strains are derived from a NMRI base, S strains from a synthetic four-way cross among inbred lines. Heteros is for 8-week weight (8-ww) was some 6%, but 30% for 3-5-week gain and it w as on average negative (22%) for 5-8-week gain. When epistatic effects were taken into account, it appeared that heterosis for 8-ww was largely due to beneficial effects of non-parental gene combinations, i.e. additive x addi tive epistatic effects were negative. These non-parental combinations neutr alized the mostly negative dominance effects. The latter were mostly positi ve for early gain but in all 9 line crosses negative for late gain. The dig enic effects of the non-parental origin were not large enough to balance th e negative dominance effects for late gain, so that heterosis was negative. It appears therefore that early and late gain are affected by different se ts of genes. Maternal additive effects on 8-ww and early gain are smaller t han direct additive effects but mostly of similar sign. For late gain there appears to Se no connection with direct additive effects. Direct litter he terosis was almost 50% but much of this was due to positively actin recombi national gene combinations, while direct litter dominance was mostly negati ve. Maternal additive effects on litter size differed depending on whether they were estimated from dams with crossbred or with purebred litter. Heter osis was small in crosses between control strains. Heterosis caused by reco mbinant gene pairs should be amenable to improvement by selection.