Origin of the chromosome 1 HSR of the house mouse detected by CGH

The polymorphic Sp100-rs repeat cluster in chromosome band 1D of the house mouse, Mus musculus, makes up as much as 0.1-5% of the haploid genome. 'High-copy' versions of this long-range repeat cluster are cytogenetically apparent as DAPI-negative chromomycin-A3-positive homogeneously staining regions (HSRs). The cluster is a relatively recent acquisition in the genus Mus; the related species M. caroli possesses neither the Sp100-rs cluster nor even the Sp100-rs gene. Except for chromosomes with high-copy clusters, no major rearrangements are visible in chromosomes 1 from M. musculus and M. caroli: they have the same order of G-bands, DAPI-bands and chromomycin A3-bands. Comparative genomic hybridization (CGH) visualizes the cluster in M. musculus and detects a single region of sequence homology to the cluster in M. caroli chromosome band 1D. This indicates that the M. musculus cluster has evolved in situ from sequences originally present in the same chromosome band.