Genetic and environmental factors determining the morphological maturation of the mouse and its relationship with weight growth.

Data on the morphological development of inbred and Fx hybrid mice reared at 22-8 °C dry bulb, 17-8 °C wet bulb, and 32-2 °C dry bulb, 26-7 °C wet bulb are presented. Animals were weighed at birth, 1 and 2 weeks of age and at each developmental stage. All animals were examined for the freeing of the ear pinnae, eruption of the lower incisor teeth and perforation of the eye membranes and in addition females were examined for the appearance of the first pair of nipples and for perforation of the outer part of the vagina. Within each environment, but especially at 23 °C, there are genotypically characteristic differences in the ages at maturation of each variable. Though the ages of maturation of the different variables are highly correlated within genotypes, early maturation of one variable is not necessarily associated with the early maturation of others. Reciprocal Fx hybrid differences indicate a maternal effect upon the age of maturation which is more marked at 23 than at 32 °C. Comparisons of the ages at maturation in the two environments showed that the only character to mature earlier at 23 than at 32 °C was pinna freeing. The most marked influence of the 32 °C environment in advancing development was on the appearance of nipples and on vaginal opening. In almost all cases, animals reared at 32 °C were lighter at the time of maturation than those at 23 °C. Hence when allowance was made for environmental differences in body weight the association between an environment at 32 °C and early development was further enhanced. Those animals that were relatively heavy when weighed at the weekly interval prior to maturation matured earlier than the lighter ones, especially at 23 °C, indicating that morphological maturation was closely related to overall growth. Both the within, and between, litter variation in the ages and body weights at maturation of a strain tended to be greater in animals exposed to the higher temperature. In bilaterally distributed characters, asymmetrical maturation was more common at 32 than at 23 °C.