Variation among mouse ribosomal RNA genes within and between chromosomes

We compared nucleotide sequences of the transcription-initiation region of five mouse ribosomal DNA (rDNA) clones. Two clones were isolated from Japanese wild mice, Mus musculus molossinus, and the other three from BALB/c mice originating in M. m. domesticus. Two BALB/c clones that are derived from the same chromosome are very similar, suggesting the occurrence of intrachromosomal homogenization of rDNA repeats. However, the other clone of BALB/c was approximately as different from the former two clones as from the rDNA clones of M. m. molossinus. These results suggest that, compared with intrachromosomal homogenization, homogenization among rDNA repeats on nonhomologous chromosomes occurs relatively infrequently after the mouse subspecies separation.