The growth in weight and tail length of inbred and hybrid mice reared at two different temperatures I. Growth in weight II. Tail length

Abstract

Four inbred strains of mice and theF1hybrids between them have been reared from 3 to 16 weeks of age at 70 °F (d.b.), 61 °F (w.b.) (control-reared) and 90 °F (d.b.), 85 °F (w.b.) (heat-reared), using a split-litter experimental design. It is shown that growth in body weight is in part determined by environmental temperature. The weight of young heat-reared mice usually increases more rapidly than that of their control-reared litter-mates but at later ages, particularly during the maturation period, the latter typically grow more rapidly. The magnitude and direction of the environmentally determined responses is dependent upon the genotype of the animals. The growth of the F1hybrids in the two environments is more similar than the growth of the inbreds from which they were derived, and the differences in the responses of the various hybrids are smaller than the corresponding inbred differences. To some extent, however, the response of a hybrid can be predicted from the responses of its inbred parents. The variability in weight of heat-reared animals is usually smaller than that of control-reared ones. The variabilities of the two environmental forms show the greatest difference soon after the animals are separated, but they become more similar with increasing age; in some genotypes the variation between the heat-reared animals may ultimately exceed that of the control-reared ones. There is a correspondence between the growth rate and the variance and it is concluded that both reflect the optimality of the environment: a rapid growth rate and a low variation indicating a favourable temperature. Heterozygotes in any one environment usually have a smaller variability than homozygotes and this is indicative of their greater homeostasis, but the similarity of one of the inbreds to the hybrids in weight variance behaviour and weight responses shows that a marked reduction in somatic fitness is not an inevitable consequence of inbreeding.