Polymorphic state of C-Bands in the Japanese house mice, mus musculus

We quantified C-band size variations of the No. 2 chromosomes in the Japanese house mice, Mus musculus, including wild-caught mice and their F1 offsprings produced by crosses between them. The sizes of C-bands were expressed by their relative lengths as a positively stained region / a negatively stained region by Cbanding in each of the No. 2 homologue. In 11 of the 16 mice examined by one-way ANOVA, the relative lengths of the C-bands were statistically polymorphic in the No. 2 homologue with variable C-band sizes at p=0.05. On the other hand, the remaining five individuals carried slightly different relative lengths without significance, which is considered as a homomorphism. Usually, heterochromatin is thought to be evolutionally neutral without strong selections, and it is considered that the polymorphic state is a frequent phenomenon of the Japanese house mice without any cytogenetically deleterious effects. In addition, the mean of the difference between the relative lengths in the No. 2 homologue ranged from 0.046 to 0.210 in the 11 individuals showing polymorphic state and from 0.032 to 0.044 in the five showing homomorphic state. Thus, a mean index of approximately 0.045 is considered to be the boundary dividing polymorphic and homomorphic states.