A novel MS7 repeat specific for heterochromatin of sex chromosomes in voles of genus Microtas, group arvalis: Genomic organization and chromosomal localization

The tandemly arranged MS4 repeat with monomeric units of 4.1 kb is species- specifically distributed in heterochromatin of sex chromosomes of four comm on vole species of genus Microtus, group arvalis. In this work, we studied the genomic organization of the MS4 homolog in euchromatin of the X chromos ome of M. arvalis. It has been shown by analyzing the phage genomic clones that one MS4 copy makes a part of a monomeric unit exceeding 8.5 kb that al so includes a new MS7 repeat and, possibly, LINE fragments. MS7 is located together with MS4 in heterochromatin of common vole sex chromosomes, but in a substantially lesser amount, Probably, as a result of an evolutionary tr ansition of an original repeat from euchromatin of the X chromosome to hete rochromatin of the Y chromosome, MS4 underwent multiple amplification, and MS7 spread throughout heterochromatin, being surrounded by the MS4 tandem a rrays.