Most mammalian species have heteromorphic sex chromosomes in males, except for a few enigmatic groups such as the mole voles Ellobius, which do not have the Y chromosome and Sry gene. The Ellobius (XX ♀♂) system of sex chromosomes has no analogues among other animals. The structure and meiotic behaviour of the two X chromosomes were investigated for males of the sibling species Ellobius talpinus and Ellobius tancrei. Their sex chromosomes, despite their identical G-structure, demonstrate short synaptic fragments and crossover-associated MLH1 foci in both telomeric regions only. The chromatin undergoes modifications in the meiotic sex chromosomes. SUMO-1 marks a small nucleolus-like body of the meiotic XX. ATR and ubiH2A are localized in the asynaptic area and the histone γH2AFX covers the entire XX bivalent. The distribution of some markers of chromatin inactivation differentiates sex chromosomes of mole voles from those of other mammals. Sex chromosomes of both studied species have identical recombination and meiotic inactivation patterns. In Ellobius, similar chromosome morphology masks the functional heteromorphism of the male sex chromosomes, which can be seen at meiosis.

Evolutionary history and taxonomic position for cryptic species may be clarified by using molecular and cytogenetic methods. The subterranean rodent, the Alay mole vole Ellobiusalaicus Vorontsov et al., 1969 is one of three sibling species constituting the subgenus Ellobius Fischer, 1814, all of which lost the Y chromosome and obtained isomorphic XX sex chromosomes in both males and females. E.alaicus is evaluated by IUCN as a data deficient species because their distribution, biology, and genetics are almost unknown. We revealed specific karyotypic variability (2n = 52–48) in E.alaicus due to different Robertsonian translocations (Rbs). Two variants of hybrids (2n = 53, different Rbs) with E.tancrei Blasius, 1884 were found at the Northern slopes of the Alay Ridge and in the Naryn district, Kyrgyzstan. We described the sudden change in chromosome numbers from 2n = 50 to 48 and specific karyotype structure for mole voles, which inhabit the entrance to the Alay Valley (Tajikistan), and revealed their affiliation as E.alaicus by cytochrome b and fragments of nuclear XIST and Rspo1 genes sequencing. To date, it is possible to expand the range of E.alaicus from the Alay Valley (South Kyrgyzstan) up to the Ferghana Ridge and the Naryn Basin, Tien Shan at the north-east and to the Pamir-Alay Mountains (Tajikistan) at the west. The closeness of E.tancrei and E.alaicus is supported, whereas specific chromosome and molecular changes, as well as geographic distribution, verified the species status for E.alaicus. The case of Ellobius species accented an unevenness in rates of chromosome and nucleotide changes along with morphological similarity, which is emblematic for cryptic species.

A comparative genomic analysis was carried out in the mole vole sibling species Ellobius tancrei and E. talpinus. Performing fluorescent in situ hybridisation (Zoo-FISH) using chromosome paints from the field vole Microtus agrestis showed no differences in the allocation of syntenic groups in the karyotypes of these sibling species. The only difference between their karyotypes was the position of the centromere in one pair of chromosomes, which is assumed to be the result of an inversion. To verify this hypothesis, we analysed chromosome synapsis in prophase I of meiosis. We utilised a synaptonemal complex (SC) surface-spreading technique to visualise the process of chromosome synapsis in the spermatocytes and oocytes of first-generation hybrids and back-crosses of these sibling species. In prophase I of meiosis, immunocytochemical and electron microscopy analyses revealed that all bivalents had been fully adjusted. Even in the case of a submetacentric-acrocentric bivalent with different centromere locations, synapsis of SC lateral elements was fulfilled along the entire length of the chromosomes and the formation of an inversion loop was not observed. We hypothesise that a possible mechanism leading to the change in centromere position is the repositioning and/or generation of a neocentromere. Despite the great similarity in the karyotypes of these sibling species, they exhibited significant genomic diversification, which manifested as hybrid sterility and parous female death.

Knowing whether triploid hybrids resulting from natural hybridization of parthenogenetic and bisexual species are fertile is crucial for understanding the mechanisms of reticulate evolution in rock lizards. Here, using males of the bisexual diploid rock lizard species Darevskia raddei nairensis and Darevskia valentini and a triploid hybrid male Darevskia unisexualis × Darevskia valentini, we performed karyotyping and comparative immunocytochemistry of chromosome synapsis and investigated the distribution of RAD51 and MLH1 foci in spread spermatocyte nuclei in meiotic prophase I. Three chromosome sets were found to occur in cell nuclei in the D. unisexualis × D. valentini hybrid, two originating from a parthenogenetic D. unisexualis female and one from the D. valentini male. Despite this distorted chromosome synapsis and incomplete double-strand breaks repair in meiotic prophase I, the number of mismatch repair foci in the triploid hybrid was enough to pass through both meiotic divisions. The defects in synapsis and repair did not arrest meiosis or spermatogenesis. Numerous abnormal mature spermatids were observed in the testes of the studied hybrid.

Abstract. The synaptonemal complex (SC) surface-spreading technique was used to visualize the process of chromosome synapsis in spermatocytes and oocytes of E. talpinus Pallas, 1770, a species with the XX sex chromosome system in both males and females. We used electron microscopy and immunofl uorescent localization of synaptonemal complex protein (SCP3) and centromeric proteins to analyze the structure and behaviour of synaptonemal complexes in prophase I of meiosis, aiming to reveal signs of meiotic sexual dimorphism in this species. We present evidence of considerable differences in the structure and behaviour of the axial structures of sex bivalents in male and female meiosis, despite the isomorphic G-and C-banding patterns of mitotic sex chromosomes. During meiotic prophase I, the sex bivalent in females behaved as autosomal bivalents, but it was not involved in the formation of the bouquet confi guration or it was the fi rst to leave it. The XX chromosomes of males formed closed sex bivalents. Only short tracts of SC were formed at both ends of the sex bivalent, while large middle segments of the lateral elements remained unpaired. The male sex chromosomes also formed characteristic "sex bodies". In fact, electron microscopy revealed dense nucleolus-like bodies associated with unpaired parts of the axial elements. These regions of the sex chromosomes were poorly immunostained, because the distribution of SCP3 had a peculiar powder-like pattern, but SCP3 was not associated with the nucleolus-like bodies. We also revealed signs of sexual dimorphism in the dynamics of formation and destruction of autosomal SCs. In males, the total SC length was shorter than in females. The chromosome bouquet confi guration was preserved up to the stage of early pachytene in females. The bouquet confi guration in males was not expressed. At late pachytene, gaps were revealed in the structure of autosomal SCs in spermatocytes immunostained with antibodies to SCP3. The pattern of distribution of these gaps was comparable with the G-banding patterns of mitotic chromosomes.