Evolution of a 6-200 Mb long-range repeat cluster in the genus Mus

By fluorescence in situ hybridization, we mapped the location of genes asso ciated with the Sp100-rs cluster, a long-range repeat cluster in chromosome 1 of the house mouse, Mus musculus. Thc cluster comprises between 60 and 2 000 repeats and extends over 6-200 Mb of the M. musculus genome, depending on the source of the cluster. The cluster evolved during the last two milli on years in the genus Mus in the lineage to which M. musculus belongs. The Asiatic mouse species M. caroli is not in this lineage and does not possess the cluster. M. caroli represents the ancestral genomic organization of th e cluster source components Sp100, Csprs and Ifi75: they are located close to each other in the same chromosome band (1D). However, Sp100-rs, the prin cipal gene of the cluster, is not present in the M. caroli genome. It is a chimeric M. musculus gene that arose by fusion of Csprs and the 5 ' part of Sp100. Sp100-rs and Ifi75 are homogeneously distributed throughout the clu ster while Sp100 and Csprs in its original sequence context flank the clust er on opposite sides. Our results suggest a model for the origin and evolut ion of the long-range repeat cluster by duplication, gene fusion and amplif ication.