Behavioural and neuroanatomical divergence between two sublines of C57BL/6J inbred mice

Male mice of strains C57BL/6J and DBA/2J differ reproducibly in a number of behaviours displayed in an open-field. In particular, C57BL/6J mice show a higher rearing-up frequency than do DBA/2J animals. Recently, a marked drop occurred in the frequency with which this behaviour is displayed by the C57BL/6J//Nmg (N) subline, that has been separated from the original C57BL/6J line over 62 generations. Comparison of our animals with the C57BL/6J//Kun(K) subline, separated from the Jackson parent over at least 40 generations, showed a significant strain difference for rearing. Since both a positive additive-genetic correlation between rearing and the size of the intra- and infrapyramidal mossy fibre terminal field (iipMF) and a correlated response in the size of the iipMF to selection for rearing have been found previously, we expected to find smaller iipMF in N, as compared with K. After processing for Timm's stain, this predicted difference was indeed found. Skin grafting demonstrated that the two sublines were still completely histocompatible, excluding a possible genetic contamination of N. This provides very strong support for the hypothesis that both the behavioural and the neuroanatomical differences between these sublines are caused by a single spontaneous mutation in the N line and strengthens the idea of a functional relationship between the structural and the behavioural variable.