Korean field mouse (Apodemus peninsulae) is widely distributed throughout northeastern Asia, including the Russian Far East, northern China, the Korean Peninsula, Sakhalin, and Hokkaido. This mouse species is characterized by a high frequency of animals with B chromosomes differing in their number, morphology, and DNA composition in different geographical regions. For the first time a comparative analysis of DNA probes from B chromosomes with metaphase chromosomes of mice from Transbaikalia, the Far East (including the Russian Far East), Japan, and South Korea was conducted by in situ hybridization. B chromosomes in mice from the Russian Far East were shown to exhibit low variability in DNA content; however, the DNA composition of B chromosomes in species from Transbaikalia and Japan were highly variable. B chromosomes in A. peninsulae from the South Korean population demonstrate minor differences from those from the Russian Far East. We discuss the origin of B chromosomes in the studied region in comparison with previously obtained data for mice from Siberia and the Baikal region, as well as the dispersal routes of the Korean field mouse.

Materials on the new findings of the striped field mouse (Apodemus agrarius) in Transbaikalia, within an earlier disjunction area in the habitat of this species (in the basins of the Onon and Ingoda rivers), are presented. Possible causes of the expansion of the species range are discussed. It is most probable that the species was introduced from the basin of the Argun River with a cargo of agricultural products to the lower reaches of the Onon River or the upper reaches of the Shilka River. Currently, the striped field mouse is a common species inhabiting a floodplain shrub biotope in the lower reaches of the Ingoda and Onon rivers.

Closely related mammalian species often have differences in chromosome number and morphology, but there is still a debate about how these differences relate to reproductive isolation. To study the role of chromosome rearrangements in speciation, we used the gray voles in the Alexandromys genus as a model. These voles have a high level of chromosome polymorphism and substantial karyotypic divergence. We investigated testis histology and meiotic chromosome behavior in the captive-bred colonies of Alexandromys maximowiczii, Alexandromys mujanensis, two chromosome races of Alexandromys evoronensis, and their interracial and interspecies hybrids, to explore the relationship between karyotypic differences and male hybrid sterility. We found that the seminiferous tubules of the males of the parental species and the interracial hybrids, which were simple heterozygotes for one or more chromosome rearrangements, contained germ cells at all stages of spermatogenesis, indicating their potential fertility. Their meiotic cells displayed orderly chromosome synapsis and recombination. In contrast, all interspecies male hybrids, which were complex heterozygotes for a series of chromosome rearrangements, showed signs of complete sterility. Their spermatogenesis was mainly arrested at the zygotene- or pachytene-like stages due to the formation of complex multivalent chains, which caused extended chromosome asynapsis. The asynapsis led to the silencing of unsynapsed chromatin. We suggest that chromosome asynapsis is the main cause of meiotic arrest and male sterility in the interspecies hybrids of East Asian voles.

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.