We first studied several rare cases of over-diploid spermatocyte emergence using advanced immunocytochemical methods and a cross-species approach in subterranean rodents Ellobius tancrei (Blasius, 1884), E. alaicus Vorontsov et al., 1969, E. talpinus (Pallas, 1770), and Nannospalax leucodon (Nordmann, 1840) (all belong to the order Rodentia). The tetraploid spermatocytes exhibited specific features during meiotic prophase I, including symmetric and asymmetric chromosome quadrivalents with partner-switching, extended asynapsis, altered recombination patterns, and variable chromatin inactivation. These anomalies suggest that meiotic checkpoints, which are potentially triggered by failed synapsis or incomplete sex chromosome silencing, may act to prevent progression of polyploid spermatocytes. However, the quadrivalents assembled shelterin complexes at chromosome ends, as observed in E. talpinus, and these ends were connected to the nuclear envelope through the linker of nucleoskeleton and cytoskeleton (LINC) complex, as observed in E. alaicus, similarly to normal spermatocytes.

The authors’ and source data on the geographical distribution of the Sicista sibling species of the caucasica group (S. caucasica, S. kluchorica, S. kazbegica, and S. armenica) are summarized. For the first time, based on an analysis of our own collections obtained in the period from 1979 to 1990 and in 2010, environmental features (biotopic confinement, abundance, reproduction strategy, and food specializations) are compared for several geographically replacing Sicista sibling species of the caucasica group (S. caucasica, S. kluchorica, S. kazbegica, and S. armenica), inhabiting different sectors of the middle mountains and highlands of the Caucasus, considered in a comparative aspect in connection with the differentiation of the group.

The Caucasian pine vole Microtus daghestanicus was characterized by high karyotypic and molecular genetic variability. However, in the eastern part of the Greater Caucasus, the populations of this species were poorly studied. To reveal the genetic characteristics of these populations and their phylogenetic relationships with other conspecific populations, a sample of Caucasian pine voles from the south of the Chechen Republic (Daikhokh Mount) was analyzed. Immunocytochemical analysis of the synaptonemal complexes at meiotic prophase I showed the belonging of the studied population to the most widespread 54-chromosomal form of M. daghestanicus. Analysis of the complete sequence of the mitochondrial cytochrome b gene showed that on the phylogenetic tree, specimens from the Chechen Republic and the previously studied Caucasian pine vole from North Georgia grouped into one compact cluster. It indicates that they belong to a distinct genetic form. Within the studied sample from the Chechen Republic, variation of the BRCA1 and XIST nuclear genes was revealed that probably reflects complex history of this population associated with alternating events of its isolation from neighboring populations and restoration of contacts between them.

Speciation is not always accompanied by morphological changes; numerous cryptic closely related species were revealed using genetic methods. In natural populations of Ellobius tancrei (2n = 54-30) and E. alaicus (2n = 52-48) of the Pamir-Alay and Tien Shan, the chromosomal variability due to Robertsonian translocations has been revealed. Here, by comprehensive genetic analysis (karyological analyses as well as sequencing of mitochondrial genes, cytb and COI, and nuclear genes, XIST and IRBP) of E. alaicus and E. tancrei samples from the Inner Tien Shan, the Alay Valley, and the Pamir-Alay, we demonstrated fast and independent diversification of these species. We described an incompletely consistent polymorphism of the mitochondrial and nuclear markers, which arose presumably because of habitat fragmentation in the highlands, rapid karyotype changes, and hybridization of different intraspecific varieties and species. The most intriguing results are a low level of genetic distances calculated from mitochondrial and nuclear genes between some phylogenetic lines of E. tancrei and E. alaicus, as well significant species-specific chromosome variability in both species. The chromosomal rearrangements are what most clearly define species specificity and provide further diversification. The "mosaicism" and inconsistency in polymorphism patterns are evidence of rapid speciation in these mammals.

The position of the Sicista strandi population from the southwestern periphery of its range (the mountainous Caucasus, the vicinity of the Ekiptsoko cordon, Zol’sky district of Kabardino-Balkaria, the adjacent territory with terra typica of the species) in the species structure was investigated on the basis of a comparative analysis of the nucleotide sequences of a fragment of the gene of nu-clear DNA in this population and in representatives of the species from a number of localities from the Russian Plain for the first time. It was established that according to this molecular feature, the studied Caucasian population belongs to the southern grouping of the species, covering, in addition to the Caucasus, samples from the basins of the Lower (Rostov region) and Middle Don (Lugansk region) and presumably from the Lower Volga region, and differs from samples from the northern parts of the range in the Central Chernozem Zone (Kursk and Belgorod region). In the S. strandi population from the southwestern periphery of the range, a number of environmental and biological characteristics (biotopic confinement, population composition, abundance, diurnal, seasonal activity, reproduction, and nutrition) considered in a comparative aspect in connection with the genetic differentiation of the species were studied for the first time.

Priority data on chromosomal (routine and C-banding) and molecular (cyt b and p53)-study of individuals (n = 19) belonging to common vole sibling species captured in three localities of Olenii Wildlife Park (Krasnoe district, Lipetsk oblast) are presented; no such data were collected for this part of the Central Black Earth Region before. Based on the two genetic markers, all individuals captured in Olenii Wildlife Park, which is located in the northwestern part of Lipetsk oblast, have been identified as the 'arvalis' chromosomal form of M. arvalis (MAA). No representatives of other M. arvalis s. l. sibling species, including recombinants, were found in the samples studied. The geographical location of the captured M. arvalis individuals belonging to the arvalis' form was assessed with respect to the known distribution boundaries and sites of hybridization of 46-chromosome M. arvalis s. l. forms in the Central Black Earth Region. It is shown that the individuals studied were captured within the distribution range of the 'arvalis' form of M. arvalis at considerable distances from the sites of hybridization of the 46-chromosome M. arvalis s. l. forms discovered earlier in the southern part of Lipetsk oblast and in the southeastern part of Kursk oblast. No M. rossiaemeridionalis individuals were identified in the studied sample from Olenii Wildlife Park. It is shown that the M. arvalis samples from Olenii Wildlife Park identified as the 'arvalis' form are confined to open meadow biotopes. The data obtained on the taxonomic status of M. arvalis s. l. individuals from Olenii Wildlife Park are consistent with the existing data on the geographical distribution and biotopical preferences of M. arvalis s. l. sibling species and karyomorphs in the Central Black Earth Region.

Restricted mobility, sociality, and high inbreeding-characteristic for subterranean mammals-lead to rapid changes in their genome structure. Up to now, the Alay mole vole Ellobius alaicus was a data-deficient species; its spatial and phylogenetic relationships with a sibling species, E. tancrei, were not clarified. We carried out a genetic analysis including differential G-banding of chromosomes and mitochondrial (cytb) and nuclear gene (XIST and IRBP) sequencing. The phylogenetic reconstruction based on cytb represented the expected phylogenetic relationships of two species. Using the XIST, we revealed two new lineages among E. alaicus from the Alay Valley (Southern Kyrgyzstan). Analysis of IRBP demonstrated presence of the specific genotype in most of E. alaicus specimens, but also revealed the haplotype, typical for E. tancrei, in some Alay mole voles. The results may be explained as persistence of ancestral gene polymorphism in E. alaicus or limited interspecific hybridization with E. tancrei. Several chromosomal forms were revealed in E. alaicus in the Alay Valley. We propose that 'mosaic' genetic polymorphism might appear in E. alaicus due to fragmentation of their habitats in highlands of the Alay Valley, Tien Shan, and Pamir-Alay as well as due to hybridization with E. tancrei or persistence of ancestral polymorphisms.

Nonhomologous chromosome interactions take place in both somatic and meiotic cells. Prior to this study, we had discovered special contacts through the SYCP3 (synaptonemal complex protein 3) filament between the short arms of nonhomologous acrocentrics at the pachytene stage in the Alay mole vole, and these contacts demonstrate several patterns from proximity to the complete fusion stage. Here, we investigated the nonhomologous chromosome contacts in meiotic prophase I. It turned out that such contacts do not introduce changes into the classic distribution of DNA double-strand breaks. It is noteworthy that not all meiotic contacts were localized in the H3k9me3-positive heterochromatic environment. Both in the mid zygotene and in the early–mid diplotene, three types of contacts (proximity, touching, and anchoring/tethering) were observed, whereas fusion seems to be characteristic only for pachytene. The number of contacts in the mid pachytene is significantly higher than that in the zygotene, and the distance between centromeres in nonhomologous contacts is also the smallest in mid pachytene for all types of contacts. Thus, this work provides a new insight into the behavior of meiotic contacts during prophase I and points to avenues of further research.

Priority data are presented on the chromosomal (routine, C-banding) and molecular (cyt b, p53) marking of several (n = 19) individuals of common vole sibling species from three previously not studied localities in the Central Black Earth region, at the territory of the Deer Natural Park (Lipetsk Region, Krasninsky District). All individuals caught on the territory of the Deer Natural Park in the northwestern part of the Lipetsk Region were identified by both genetic markers as M. arvalis form “arvalis” (MAA). No representatives of other M. arvalis s. l. sibling species, including recombinants, were found in our samples. The geographical location of the M. arvalis form “arvalis” found by us was estimated with respect to the distribution boundaries and hybridization sites of the 46-chromosome forms of M. arvalis s. l. in the Central Black Earth region. It is shown that the studied individuals have been caught within the range of the Microtus form “arvalis” and are largely removed from the hybridization sites of the 46-chromosomal forms M. arvalis s. l., discovered earlier in the southern Lipetsk region and the southeastern Kursk Region. No M. rossiaemeridionalis in the examined sample from the Deer Natural Park were found. The correspondence between the samples studied and identified as the M. arvalis form “arvalis” from the Deer Natural Park and native meadow biotopes is shown. The data of determining the taxonomic status of M. arvalis s. l. individuals from the Deer Natural Park are consistent with our perceptions of the nature of the geographical distribution and biotopic correspondence of M. arvalis s. l. sibling species and the chromosomal forms on the territory of the Central Black Earth region.

To clarify genetic differences between subspecies of the house mouse Mus musculus, their distribution, and hybridization, we first conducted a comparative analysis of variability of nucleotide sequences of fragments of the nuclear gene Brca1, exon 11 (2331 bp), and mitochondrial gene Cox1 (1260 bp) in 40 house mice from West and East Europe, Transcaucasia, Siberia, and Central and South Asia. Brca1 genotypes were divided into five main groups, which differed in a number of fixed substitutions. Genotypes of each group are characteristic for the certain geographical region and the following subspecies: M. m. musculus, M. m. domesticus, M. m. castaneus, and M. m. wagneri together with M. m. gansuensis; a fifth group corresponds to an unidentified subspecies or a distinct genetic form of M. musculus from India (Sikkim State). Besides the homozygous specimens, we revealed mice, which were heterozygous for all diagnostic loci simultaneously; these specimens were determined as hybrid. Hybrid mice were mainly found in the zones of contact of subspecies, but in some cases, quite far from one of the parent subspecies (possibly, due to transportation). In two hybrid mice (from Bakhtiari Province of Iran and Transbaikalia of Russia), unique Brca1 haplotypes were detected. It cannot be ruled out that, at least partly, they may be characteristic of the M. m. bactrianus and M. m. gansuensis subspecies, respectively. Thus, the results of the study showed that the nuclear Brca1 gene is a promising molecular genetic marker for the analysis of variability, differentiation, and hybridization of house mice as well for subspecific identification of M. musculus specimens. Despite more rapid evolution of the Cox1 gene, it is not well suited for discrimination of M.m. musculus, M. m. wagneri, M. m. gansuensis specimens and Transcaucasian representatives of M.m. domesticus due to introgression and long-term maintenance of foreign mitochondrial DNA in populations. However, Cox1 gene analysis (along with the diagnostics of animals by nuclear DNA) may be useful for estimation of population differences in M. m. castaneus and M.m. domesticus subspecies.

Abstract—The variability of three pine vole species, as well differences between them, and the phylogenetic relationships were determined by analysis of nucleotide sequence fragments of nuclear genes BRCA1, exon 11 (1473 bp) and XIST (415 bp). The data obtained by us demonstrate significant genetic disunity of the species and M. majori separation related to M. daghestanicus–M. subterraneus. Studying the XIST gene revealed M. majori differentiation into close North Caucasian and Trans-Caucasian population groups. Fragments of both nuclear genes exhibited unexpectedly high intraspecific variability in M. daghestanicus out of clear relations with the geographic origin of the specimens. We propose that differences in the character of genetic polymorphism between M. daghestanicus and M. majori are due to their ecological peculiarities.

Robertsonian translocations are common chromosomal alterations. Chromosome variability affects human health and natural evolution. Despite the significance of such mutations, no mechanisms explaining the emergence of such translocations have yet been demonstrated. Several models have explored possible changes in interphase nuclei. Evidence for non-homologous chromosomes end joining in meiosis is scarce, and is often limited to uncovering mechanisms in damaged cells only. This study presents a primarily qualitative analysis of contacts of non-homologous chromosomes by short arms, during meiotic prophase I in the mole vole, Ellobius alaicus, a species with a variable karyotype, due to Robertsonian translocations. Immunocytochemical staining of spermatocytes demonstrated the presence of four contact types for non-homologous chromosomes in meiotic prophase I: (1) proximity, (2) touching, (3) anchoring/tethering, and (4) fusion. Our results suggest distinct mechanisms for chromosomal interactions in meiosis. Thus, we propose to change the translocation mechanism model from 'contact first' to 'contact first in meiosis'.

Results of the analysis of IRBP (interphotoreceptor retinoid-binding protein) nuclear DNA gene variability in birch mice of the genus Sicista reflect monophyly of the betulina group and confirm the substantial genetic isolation of the sibling species S. betulina and S. strandi within this group. Using Tamura’s 3‑parameter model T92, it is shown that the mean genetic distances between S. betulina and S. strandi are 0.7%, comparable to the interspecific genetic distances for sibling species from other groups of birch mice. Their intraspecific differentiation, which is particularly well-expressed in S. strandi, is also identified. Differentiation between the northern and southern populations of S. strandi, comparable to the interspecific differences (D = 0.8%), and isolation of the Carpathian specimen of S. betulina from other European and Siberian sample sets (D = 0.2%) are shown.

Microsatellite analysis was used to examine intraspecific polymorphism in two extensive continental isolates of the striped field mouse (Apodemus agrarius Pallas, 1771) separated by the Baikal disjunction. Striped field mice from the western isolate (from the European and Kazakh-Siberian parts of the range) and from the eastern isolate (from the territory of the Middle Amur Region and Coastal Territory of Far East) were tested. The analysis used 180 specimens collected from 33 localities and five microsatellite loci developed earlier for the genus Apodemus. The work was carried out based on the summation of local samples in each of the aforementioned geographical regions. It was shown that allelic diversity and the number of specific alleles were higher in the eastern isolate that may be the result of the longer habitation of the striped field mouse in the eastern part of the range. The limited number of specific alleles in the western isolate as compared to the eastern one can be determined by the founder effect and may reflect the direction of the historical migration of the species from east to west. Our results demonstrate no more than a population level of differentiation within the continental isolates of the striped field mouse and indicate no more than a subspecies level of differences between these isolated forms, i.e., the relatively recent penetration of A. agrarius to western Eurasia.

To better understand the evolutionary history of oriental wildlife newcomers in Europe, we studied the phylogeography and demographic history of the striped field mouse, Apodemus agrarius , throughout its Palaearctic distribution area. Genetic datasets including cytochrome b gene sequences and microsatellite markers were analysed using a large range of population genetics methodologies, including coalescent models and approximate Bayesian computations. Our results showed high mitochondrial genetic homogeneity among A. agrarius populations throughout the Palaearctic region, but microsatellite markers detected a finer population structure with the genetic differentiation of populations from the Eastern and Western distribution ranges. The Western colonisation likely originated from Far East Russian populations during one of the last interglacials. After their colonisation of the Central Asia and Western regions, the Central Palaearctic populations became isolated from their Eastern relatives. Our coalescent-based approaches suggested a separation between these two distribution ranges around 38 kya or more recently (around 11 kya). Limited gene flow still happened between populations in the two main distribution ranges, mainly from the Eastern to Western populations. Our study, for the first time, provides an overview of the evolutionary and demographic history of the striped field mouse throughout the Palaearctic region. A. agrarius appears to be an Asiatic immigrant and a relatively new member of the European fauna community. This study further confirms the important role of Far East Asian regions as a source of European biodiversity.

Intraspecific variability of craniometric characters (absolute measurements and indices) has been studied in the pygmy wood mouse Sylvaemus uralensis and the yellow-necked mouse S. flavicollis from a number of localities of Eastern Europe. A total of 478 skulls of Sylvaemus specimens were measured and analyzed. Cluster analysis of skull measurements showed that both S. uralensis and S. flavicollis could be split into two groups, the northern and the southern ones, which completely correspond to the genetically described intraspecific structure of S. flavicollis, but only partly in regards to S. uralensis. The northern and southern groups of both species are distinguished for the first time using discriminant analysis. Indices (relative skull measurements) related to the character of feeding are discussed. The intraspecific craniometric variability of S. flavicollis is suggested to be related to distinctions in the diet and the ways of getting food, whereas for S. uralensis no relation between the values of the indices of the dental apparatus and feeding features in different parts of its distribution range has been revealed.

Genetic sex determination is realized by a complex cascade of genes. In mammals, sex is determined by genes whose precise temporal and spatial regulation leads to the development of testes or ovaries from bipotential embryonic gonads. Genetic development programs along the female or male pathway are not completely different, a significant part of the genes involved in both pathways. In placental mammals, activation of the Y-linked gene Sry (sex-determining region Y) initiates testicular development. When Sry is not present, as in XX individuals, the bipotential gonads develop into ovaries. Y chromosome and Sry gene were lost in two rodents genera, mole voles genus Ellobius and spiny rats Tokudaia. In four mole vole species without Y chromosomes, a few Y-linked genes escaped to the X chromosome and exposed in male and female genomes. Two X chromosomes in males and females of E. talpinus and E. tancrei are morphologically identical, but functional differences are evident in meiosis. So far, no explanation has been received for existence of males and females in mole voles and spiny rats along with the Sry loss and possible upregulation of the Sox9 gene. Several genes, usually localized in a Y-chromosome, exist in female and male genomes of mole voles, such as a factor spermatogonial proliferation (Eif2s3y, Eukaryotic translation initiation factor 2 subunit 3, Y-linked). In species with two X-chromosomes in individuals of both sexes, the morphological identity of sex chromosomes masks functional heteromorphism, which can be detected in the meiosis only. The existence of males and females for the mole voles Ellobius and the spiny rats Tokudaia in the absence of the Sry gene and a possible violation of expression of the Sox9 gene has not yet been explained. To date, it remains unclear how variable the cascade of genes involved in the sex determination can be. Study of different deviations in the structure of the genome, such as XX or X0 females and males, full or partial loss of the Y chromosome, as well as in the structure of the gonads, may be crucial for understanding the sex determination genetics.