Comparative molecular-cytogenetic analysis of the X-chromosome organization in voles of the subgenus Microtus

Comparative study of the X-chromosomes of five species belonging to the sub genus Microtus was performed to clarify the peculiarities and mechanism of chromosome reorganization in mammalian evolution. M. kirgisorum, M. transca spicus, M. arvalis, M. rossiaemeridionalis form the "arvalis" group of spec ies, while M. agrestis belongs to the "agrestis" group. With the aid of X-c hromosome microdissection and DOP-PCR, four overlapping paints specific to different regions of the M. rossiaemeridionalis X-chromosomes were generate d and hybridized on metaphase chromosomes of all species. As a result, homo logous regions on the X-chromosomes of these species were identified and in versions that distinguish the X-chromosomes were described. FISH with two r egion-specific paints revealed signals not only in the regions of chromosom e dissection, but also narrow signals in the other regions of X-chromosomes of M. rossiaemeridionalis and other species. We suppose that small cluster s of repeats are responsible for the appearance of these signals. It is nec essary to note that the "additional" signals were detected in the same regi ons that were involved in the formation of inversion. This might be due to either breakage of repeated cluster in the ancestral X-chromosome with inve rsion and subsequent transfer of its part to distant regions, or involvemen t of two clusters of homologous repeats localized in different regions in t he formation inversion. We suppose that the appearance of small clusters of repeats in euchromatic chromosome regions could be responsible for the emergence of "hot spots" of chromosome rearrangements in mammalian evolution.