The genus Microtus consists of 65 extant species, making it one of the rodentia genera with the highest number of species. The extreme karyotype diversification in Microtus has made them an ideal species group for comparative cytogenetics and cytotaxonomy. Conventional comparative cytogenetic studies in Microtus have been based mainly on chromosomal banding patterns; the number of Microtus species examined by molecular cytogenetics-cross-species chromosome painting-is limited. In this study, we used whole chromosome painting probes of the field vole Microtus agrestis to detect regions of homology in the karyotypes of eight Microtus species. For almost all investigated species, species-specific associations of conserved chromosomal segments were revealed. Analysis of data obtained here and previously published data allowed us to propose that the ancestral Microtus species had a 2n = 54 karyotype, including two associations of field vole chromosomal segments (MAG 1/17 and 2/8). Further mapping of the chromosome rearrangements onto a molecular phylogenetic tree allows the reconstruction of a karyotype evolution pathway in the Microtus genus.

Pairing of X and Y chromosomes at meiotic prophase in 14 species of the subfamily Microtinae (Clethrionomys rufocanus, C. rutilus, C. glareolus, Arvicola terrestris, Microtus guentheri, M. socialis, M. afghanus, M. bucharicus, M. oeconomus, M. arvalis, M. rossiaemeridionalis, M. kirgisorum, M. transcaspicus, M. (Pitymys) majori) was analysed in relation to their taxonomic position and variation in the morphology of their sex chromosomes. The sex chromosomes formed a synaptonemal complex (SC) at pachytene in all Clethrionomys species, Arvicola terrestris, and M. oeconomus, while they did not pair at all in M. (Pitymys) majori, Microtus socialis, M. guentheri, M. afghanus, M. bucharicus, M. arvalis, M. rossiaemeridionalis, M. kirgisorum, and M. transcaspicus. The X chromosome of these species varied in centromere position independently of pairing pattern. Insertion of heterochromatin of different size and location was found in some, but not in all species with asynaptic sex chromosomes. It is suggested that the sex chromosomes lost their ability to pair at male meiosis in the common ancestor of palearctic species of the genus Microtus. This event was not caused by a gross chromosomal rearrangement.

Karyotypes of Calomyscus from different regions of Turkmenistan, Iran, and Azerbaijan were studied using chromosome banding (G- and C-banding) and analyses of meiosis in laboratory hybrids. Extensive variation in the diploid number and the number of autosomal arms (FNa) was revealed (2n = 30, FNa = 44; 2n = 32, FNa = 42; 2n = 44, FNa = 46; 2n = 44, FNa = 58; 2n = 37, FNa = 44; 2n = 50, FNa = 50; 2n = 52, FNa = 56). Centric and tandem fusions and heterochromatin changes were identified as the major modes of karyotype evolution in this group. Natural hybrids between individuals with different karyotypes were recorded, and regular chromosome pairing in meiosis was observed in laboratory hybrids. Fluorescence in situ hybridization with a 353-bp BspRI complex tandem repeat indicated that chromosomal repatterning occurred recently within the genus. There is no unequivocal evidence suggesting the role of chromosomal change in the speciation of the populations of Calomyscus examined.

ABSTRACT. Dentition and baculum morphology in social voles of the subgenus Sumeriomys from the Southern Europe, Caucasus and Central Asia were examined. The specimens investigated come from 20 different sites, majority of which are the type localities for the nominal species-level taxa. Also results of the study of the karyotype, including differentially stained chromosomes for social voles from different parts of their geographic range and their hybrids are discussed. The synaptoneme complexes (SCs) for study of the chromosome pairing in different forms of voles was investigated. For Microtus (S.) schidlovskii Argyropulo, 1933 and M. (S.) paradoxus Ognev et Heptner, 1928 a species status is confirmed by our research. Two new subspecies of M (S). socialis Pallas, 1773 are described: M. s. aristovi Golenishchev ssp. nov. and M. s. zaitsevi Golenishchev ssp. nov. The identification keys based on the baculum structure for the subgenus Sumeriomys are presented.

We examined the karyotypes of 212 specimens of the house mouse obtained from 44 localities in central and eastern Europe, and several regions of Asia. The Robertsonian chromosome fusion 5.12 was found in a population of Mus musculus musculus in Czechoslovakia. Two large HSRs on chromosome 1 were ascertained in four female mice from western Siberia. In most of the localities under study, the mice possessed a normal karyotype with 40 acrocentric chromosomes.