‘Heterosis’ in F1 mice in a cold environment

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Abstract
The effect of hybridity on reproduction in mice has been studied in two environmental temperatures: 21° C. (‘normal’) and −3° C. (unfavourable). Mice of inbred strains A2G and C57BL are maintained as permanent breeding stocks in each of the two temperatures. In each temperature the two strains were crossed, and the reproduction of F1 pairs, to the age of 28 weeks, compared with that of the parent strains.At 21° C. the expected superior fertility of the F1 mice was found: the number of litters produced was not affected, but there were more young produced per pair. The number of young per pair reared to 3 weeks was about twice the mean of the parent strains at the same temperature.At −3° C. the difference was greater. The F1 pairs produced more and larger litters than the parent strains, and deaths between birth and weaning at 3 weeks were few. The number of young per pair reared to 3 weeks was nearly five times the mean of the parent strains. Part of the superiority of the F1 mice, at both temperatures, was due to the fact that they began to breed earlier.Despite the larger litters produced by the F1 pairs, at 21° C. the mean weight of the F2 young at the age of 3 weeks was intermediate between those of the parent strains. At −3° C. it was the same as that of the heavier parent strain, namely, A2G.As a further test of resistance to cold, F1 mice born in the warm environment were transferred to the cold at the age of 22 days and there placed each alone in a cage, with nesting material. In these conditions they had a higher survival rate than the young of either of the parent strains. They also grew faster than the A2G mice.The F1 mice were not only more fertile than the inbred mice, but also more uniform in breeding performance. This difference was especially marked in the less favourable environment.These observations are in conformity with the view that heterosis is a consequence of heterozygosis; and that it depends on an enhanced ability to withstand disturbances of developmental and physiological equilibria.