‘Heterosis’ in F₁mice in a cold environment

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.

Genetic interpretations of the effects of bulk breeding on four populations of beans (Phaseolus vulgaris L.)

“…The interpretation of the data depends critically on the environment in which the experiment was carried out, and lays emphasis on the importance of carrying out selection experiments for yield at the same density as those anticipated for crop growth (DONALD, 1962 ;HINSON & HANSON, 1962 ;HAMBLIN, 1975 ;HAMBLIN, 1977 ;HAMBLIN & EVANS, 1976) . The results also support the general contention (BARNETT & COLEMAN, 1960 ;KNIGHT, 1973) that the interpretation of gene action in hybrid vigour depends on the test environment used .…”

The Growth of Infant Mice at Two Temperatures

“…The new stock, instead of having some undersized young in large litters, had no large litters. A similar absence of large litters at -3°C was recorded by Barnett & Coleman (1960) among very fertile Ft mice produced by crossing strains C57BL and A2G.…”

Inbreeding depression in grazing Blackface sheep

“…Interaction between degree of inbreeding and the severity of the environment, on reproductive performance and survival in mice, has been shown by Barnett and Coleman (1960) and Barnett (1964). The Merino flock was supplied with supplementary feed at all times, the amount being determined by the grazing conditions and the requirements of the sheep.…”