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. 'Hig h-copy' versions of this long-range repeat cluster are cytogenetically appa rent as DAPI-negative chromomycin-A3-positive homogeneously staining region s (HSRs). The cluster is a relatively recent acquisition in the genus Mus; the related species M. caroli possesses neither the Sp100-rs cluster nor ev en the Sp100-rs gene. Except for chromosomes with high-copy clusters, no ma jor rearrangements are visible in chromosomes 1 from M. musculus and M. car oli: they have the same order of G-bands, DAPI-bands and chromomycin A3-ban ds. Comparative genomic hybridization (CGH) visualizes the cluster in M. mu sculus and detects a single region of sequence homology to the cluster in M . caroli chromosome band 1D. This indicates that the M. musculus cluster ha s evolved in situ from sequences originally present in the same chromosome band.