Fucoidan-active α-L-fucosidases of the GH29 and GH95 families from a fucoidan degrading cluster of the marine bacterium Wenyingzhuangia fucanilytica.

In present work we provide the bioinformatic and biochemical characterization of six α-L-fucosidases that belong to the 29 and 95 families of glycoside hydrolases (GH) from the fucoidan-degrading locus of the marine bacterium Wenyingzhuangia fucanilytica CZ1127T. The fucosidases FucWf1GH29, FucWf2GH29, FucWf3GH29 and FucWf6GH29 are relegated to the subfamily A of the GH29 family. The fucosidase FucWf4GH29 bears a distant resemblance to the GH29 and does not belong to either the GH29A or the GH29B subfamilies. Apparently, FucWf4GH29 is the first representative of a new subfamily within the GH29 family of α-L-fucosidases. For the first time the specificity of fucosidases has been studied using a series of fucoidan-related sulfated oligosaccharides. Studied α-L-fucosidases are able to cleave l-fucose from sulfated fucooligosacchrides after their treatment with exo-sulfatases. All studied α-L-fucosidases are cleaving the α-1→3- and α-1→4-linked terminal l-fucose in sulfated fucooligosaccharides. However, only FucWf3GH29 is able to cleave off an α-1→2-linked l-fucose. The fucosidase FucWf5GH95 of the GH95 family is shown to have higher activity on fucoidans than fucosidases of the GH29 family. Supposedly, the α-l-fucosidase FucWf5GH95 participates in fucoidan debranching. The obtained data indicate different roles of fucosidases of the GH29 and GH95 families in the process of fucoidan degradation by the marine bacteria W. fucanilytica CZ1127T.

[Choice of optimal treatment policy in patients with descending aortic dissection]. / Vybor optimal'noi taktiki lecheniia patsientov s rassloeniem niskhodiashchei aorty.

Contemporary approaches to treatment of patients with dissection of the descending aorta (DA), as well as the data of international registries and guidelines are contradictory. The endovascular method of treatment of patients with descending aortic dissection has been recognized to be revolutionary. However, there seems to be no commonly accepted concept concerning appropriate use of currently available techniques of treating patients with DA dissection. This study was aimed at analysing the experience gained in using various methods of treatment of patients with descending aortic dissection (DAD). We retrospectively examined the results of surgical treatment of 78 patients with DAD. Of these, 45 patients were found to have Stanford type B dissection and 33 patients had type A dissection, who underwent treatment during ten years from January 2008. The decision concerning the type of care for patients with DAD (conservative, surgical, endovascular or combined methods of treatment) directly depends on the patient's state stability, technical feasibility of the intended procedure, and the presence of concomitant pathology. The findings obtained in our study demonstrated better immediate outcomes of the endovascular approach compared with the hybrid and surgical ones. In order to achieve complete haemodynamic correction of the pathology, to minimize the risk of aortic remodelling and the development of aorta-related complications, the treatment should not be limited to endoprosthetic repair of the thoracic portion of the DA with a stent graft, but should rather be supplemented with stenting of more distal segments, including with uncoated stents at the level of the visceral branches. Only such staged approach may provide better conditions for obliteration of the aortic false lumen and formation of a 'neo-frame' of the aortic wall.

[Telomere Recombination in Normal Mammalian Cells].

Two mechanisms of telomere length maintenance are known to date. The first includes the use of a special enzymatic telomerase complex to solve the problems that arise during the replication of linear DNA in a normal diploid and part of tumor cells. Alternative lengthening of telomeres (ALT), which is based on the homologous recombination of telomere DNA, represents the second mechanism. Until recently, ALT was assumed to be expressed only in 15-20% of tumors lacking active telomerase and, together with telomerase reactivation represented one of two possibilities to overcome the replicative senescence observed in somatic mammalian cells due to aging or during cell culturing in vitro. Previously described sporadic cases of combinations of the two mechanisms of telomere length maintenance in several cell lines in vitro were attributed to the experimental design rather than to a real biological phenomenon, since active cellular division without active telomerase was considered to be the "gold standard" of ALT. The present review describes the morphological and functional reorganizations of mammalian telomeres observed with ALT activation, as well as recently observed,and well-documented cases of combinations between ALT-like and telomerase-dependent mechanisms in mammalian cells. The possible role of telomere recombination in telomerase-dependent cells is discussed.

HUMAN KARYOTYPE ANALYSIS IN DIAGNOSIS VERIFICATION.

When analyzing a patient's karyotype using classic cytogenetic tools, clinical cytogeneticists frequently face a problem of whether the observed morphological variant of a chromosome is the norm or pathology. Here we present three cases, when the use of additional approaches allowed us to accurately and reliably describe the chromosomal abnormalities and to provide a substantiated medical and genetic prognosis. Translocations were preliminary diagnosed in the first two patients. This opinion was subsequently challenged, as these patients were the carriers of rare variants of normal chromosome polymorphisms (21pstkstkpss and 20cenh+). Thus, these diagnostic measures helped the wife of the first patient to maintain the pregnancy, whereas the second patient was referred for IVF. In the third case, the preliminary diagnosis trisomy of chromosome 22 has not been confirmed. This patient turned out to be a carrier of a supernumerary marker chromosome invdup(15)(q13), which offers a much more favorable medical prognosis.

Telomere Length of Individual Chromosomes in Patients with Rheumatoid Arthritis.

We analyzed telomere length of individual chromosomes in peripheral blood lymphocytes of healthy individuals and patients with rheumatoid arthritis. Quantitative fluorescent in situ hybridization and subsequent computer analysis of metaphase chromosomes showed that distribution of telomere length on individual chromosomes is different under normal and pathological conditions. Patients with rheumatoid arthritis had significantly shorter chromosome 4p telomeres, which can be essential for pathogenesis of this multifactorial disease. Additionally, disease activity inversely correlated with telomere length on chromosome 10p carrying genes involved in T cell differentiation and proliferation.

[Comparative Analysis of DNA Homology in Pericentric Regions of Chromosomes of Wood Mice from Genera Apodemus and Sylvaemus].

In the present study, an analysis of the DNA homology of the pericentric chromosomal regions and pericentric heterochromatin in distantly related species of wood mice (species from the Apodemus genus, as well as from the Apodemus and Sylvaemus genera) was conducted by fluorescent in situ hybridization (FISH) with microdissected DNA probes obtained from the corresponding chromosomal regions of these species. Cross-hybridization of microdissected DNA probes obtained from pericentric C-positive blocks of chromosomes of Sylvaemus species with chromosomes of Apodemus species, as well as DNA probes from pericentric C-positive blocks of chromosomes of Apodemus species with chromosomes of Apodemus and Sylvaemus species, showed that DNA repeats homologous to the pericentric regions in other species represented. dispersed repeats in C-negative chromosomal regions, as well as in several regions bordering pericentric C-positive and C-negative regions in heterochromosomes and autosomes and in distal regions in the long arms of several autosomes. The results indicate that the level of DNA homology in pericentric chromosomal regions decreases with an increase in the differentiation level and a decrease in the kinship between the compared forms and species of wood mice. Most likely, degeneration of the DNA repeats is accompanied by a gradual destruction of repeat clusters and their replacement by new, nonhomologous repeats in almost all pericentric regions (some old repetitive sequences might be "extruded" into interstitial or telomeric regions of chromosomes). These processes, which are observed in some species from Sylvaemus genus in distantly related species of Sylvaemus and Apodemus genera, have almost achieved the final stages.

[Features of the B chromosome in Korean wood mice Apodemus peninsulae (Thomas, 1906) from Transbaikalia and the Far East identified by the FISH method].

Korean field mice (Apodemus peninsulae) are 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.

[Fluorescence in situ hybridization with DNA probes derived from individual chromosomes and chromosome regions].

A significant part of the eukaryotic genomes consists of repetitive DNA, which can form large clusters or distributed along euchromatic chromosome regions. Repeats located in chromosomal regions make a problem in analysis and identification of the chromosomal material with fluorescence in situ hybridization (FISH). In most cases, the identification of chromosome regions using FISH requires detection of the signal produced with unique sequences. The feasibility, advantages and disadvantages of traditional methods of suppression of repetitive DNA hybridization, methods of repeats-free probe construction and methods of chromosome-specific DNA sequences visualization using image processing of multicolor FISH results are considered in the paper. The efficiency of different techniques for DNA probe generation, different FISH protocols, and image processing of obtained microscopic images depends on the genomic size and structure of analyzing species. This problem was discussed and different approaches were considered for the analysis of the species with very large genome, rare species and species which specimens are too small in size to obtain the amount of genomic and Cot-1 DNA required for suppression of repetitive DNA hybridization.

A case report of Pallister-Killian syndrome with an unusual mosaic supernumerary marker chromosome 12 with interstitial 12p13.1-p12.1 duplication.

Pallister-Killian syndrome (PKS) is a rare inherited disease with multiple congenital anomalies, profound intellectual disability, and the presence in the karyotype of sSMC - i(12)(p10). The frequency of PKS may be underestimated due to problems with cytogenetic diagnosis caused by tissue-specific mosaicism and usually a low percentage of peripheral blood cells containing sSMC. Such tissue-specific mosaicism also complicates a detailed analysis of the sSMC, which, along with the assessment of mosaicism in different tissues, is an important part of cytogenetic diagnosis in PKS. Unfortunately, a full-fledged diagnosis in PKS is either practically impossible or complicated. On the one hand, this is due to problems with the biopsy of various tissues (skin biopsy with fibroblast culture is most often used in practice); on the other - a low percentage of dividing peripheral blood cells containing sSMC, which often significantly complicates the analysis of its composition and organization. In the present study, a detailed analysis of sSMC was carried out in a patient with a characteristic clinical picture of PKS. A relatively high percentage of peripheral blood cells with sSMC (50%) made it possible to perform a detailed molecular cytogenetic analysis of de novo sSMC using chromosomal in situ suppression hybridization (CISS-hybridization), multicolor FISH (mFISH), multicolor chromosome banding (MCB), array CGH (aCGH), and quantitative real-time PCR (qPCR), and short tandem repeat (STR) - analysis. As a result, it was found that the sSMC is not a typical PKS derivative of chromosome 12. In contrast to the classical i(12)(p10) for PKS, the patient's cells contained an acrocentric chromosome consisting of 12p material. Clusters of telomeric repeats were found at the both ends of the sSMC. Furthemore, the results of aCGH and qPCR indicate the presence of interstitial 8.9 Mb duplication at 12p13.1-p12.1 within the sSMC, which leads to different representations of DNA from different segments of 12p within cells containing sSMC. The obtained data raise the question of the instability of the sSMC and, as a consequence, the possible presence of additional rearrangements, which, in traditional cytogenetic analysis of patients with PKS, are usually described as i(12)(p10).

[Organization of eukaryotic chromosomes: from Kol'tsov's studies up to present day].

N. K. Kol'tsov ideas and views on the organization of eukaryotic chromosomes, including the notion of a giant hereditary molecule (genoneme) and its structural functional organization, are considered. Different approaches to chromosome studies are discussed, ranging from the examination of a chromosome as a stained cell organelle and the visualization of individual chromosomes in a living cell to the identification of topological domains of human and murine chromosomes using 3C and 5C technologies. The prospects of studies of chromosome organization using up-to-date methods of cytology, molecular biology, and bioinformatics are discussed.

Chromosomal localization of ribosomal and telomeric DNA provides new insights on the evolution of gomphocerinae grasshoppers.

Chromosome location of ribosomal DNA (rDNA) and telomeric repeats was analysed in mitotic chromosomes of 15 species of Gomphocerinae grasshoppers belonging to the tribes Arcypterini, Gomphocerini, Stenobothrini, and Chrysochraontini. Two types of rDNA distribution were found in the Gomphocerini tribe. Type 1, found in 9 species, was characterized by the presence of rDNA in the short arm of the long biarmed chromosomes 2 and 3 and, in some species, also in the X chromosome. Type 2 was found only in Aeropus sibiricus and Stauroderus scalaris and consisted in the presence of pericentromeric rDNA blocks in all chromosomes. A comparison of rDNA distribution in Gomphocerini species with 2n ♂ = 23, 2n ♂ = 21, and 2n ♂ = 17 suggested the possible involvement of chromosome 6 in the ancestral karyotype (2n ♂ = 23) in 1 of the 3 centric fusions that decreased the chromosome number in these species. In the tribe Stenobothrini, Stenobothrus eurasius carried a single rDNA cluster in the X chromosome, likewise 2 Spanish species previously analysed, but Omocestus viridulus unusually showed a single rDNA cluster in the longest autosome. Telomeric repeats were located primarily on the ends of chromosome arms. In 2 species, however, we observed the presence of interstitial clusters outside telomeric regions. The first one, Aeropus sibiricus, exhibited a polymorphic interstitial site of telomeric repeats in chromosome 6 as a consequence of a paracentric inversion. Most remarkably, Chorthippus jacobsoni showed the presence of telomeric repeats in the pericentric regions of the 3 biarmed chromosome pairs originated by centric fusion, thus suggesting that these rearrangements were not of the Robertsonian type but true centric fusion with a probable generation of dicentric chromosomes.

[Mammalian telomere biology].

Review is devoted to detailed consideration of the functioning in normal and immortal cells of one of the main chromosomal regions, telomeres, being dynamic nucleoprotein structures that cap the ends of eukaryotic chromosomes, protecting them from degradation and end-to-end fusion. The role of telomeres in maintenance of genome stability and cell division was also analyzed. Telomere function depends on many interrelated parameters such as telomerase activity, status of the telomere safety complex shelterin and telomere associated proteins (factors of replication, recombination, and reparation of DNA breaks, and so on). We have focused on mechanisms of telomere length control in normal and immortal cells as well as in cells containing active telomerase and cells wherein it is absent. We have analyzed the features attributed to alternative telomere lengthening, namely in view of recently discovered additional mechanism of telomere shortening by trimming of t-cycles. We have viewed a possibility of expression in normal mammalian cells of both telomerase dependent and recombinational ways of telomere length control and the role of shelterin proteins in choice of the one of them as the dominant way. The role oftelomeres in spatial organization of nucleus, in mitosis and meiosis has been also considered. Diversity of telomere organization in mammalians including unusual telomeres in Iberian shrews has been discussed.

Generation of iPS cell line (ICGi040-A) from skin fibroblasts of a patient with ring small supernumerary marker chromosome 4.

Human induced pluripotent stem cell (iPSC) line, ICGi040-A, was obtained from skin fibroblasts derived from a male patient with mosaic ring small supernumerary marker chromosome 4 (sSMS(4)) and infertility. ICGi040-A cells have karyotype 47,XY,+r(4) in 97% of cells and express a set of pluripotent markers, as well as are able to differentiate in vitro into derivatives of all three embryonic germ layers.

A method for generating selective DNA probes for the analysis of C-negative regions in human chromosomes.

Linker-adapter polymerase chain reaction (LA-PCR) is among the most efficient techniques for whole genome DNA amplification. The key stage in LA-PCR is the hydrolysis of a DNA sample with restriction endonucleases, and the choice of a restriction endonuclease (or several endonucleases) determines the composition of DNA probes generated in LA-PCR. Computer analysis of the localization of the restriction sites in human genome has allowed us to propose an efficient technique for generating DNA probes by LA-PCR using the restriction endonucleases HaeIII and RsaI. In silico hydrolysis of human genomic DNA with endonucleases HaeIII and RsaI demonstrate that 100- to 1,000-bp DNA fragments are more abundant in the gene-rich regions. Applying in situ hybridization to metaphase chromosomes, we demonstrated that the produced DNA probes predominantly hybridized to the C-negative chromosomal regions, whereas the FISH signal was almost absent in the C-positive regions. The described protocol for generating DNA probes may be successfully used in subsequent cytogenetic analysis of the C-negative chromosomal regions.

[Comparative FISH analysis of C-positive regions of chromosomes of wood mice (Rodentia, Muridae, Sylvaemus)].

The homology of DNA of C-positive centromeric regions of chromosomes in wood mice of the genus Sylvaemus (S. uralensis, S. fulvipectus, S. sylvaticus, S. flavicollis, and S. ponticus) was estimated for the first time. DNA probes were generated by microdissection from the centromeric regions of individual autosomes of each species, and their fluorescence in situ hybridization (FISH) with metaphase chromosomes of representatives of all studied wood mouse species was carried out. Unlike in the chromosomal forms and races of S. uralensis, changes in the DNA composition of the chromosomal centromeric regions in the wood mouse species of the genus Sylvaemus (including closely related S. flavicollis and S. ponticus) are both quantitative and qualitative. The patterns of FISH signals after in situ hybridization of the microdissection DNA probes with chromosomes of the species involved in the study demonstrate significant differences between C-positive regions of wood mouse chromosomes in the copy number and the level of homology of repetitive sequences as well as in the localization of homologous repetitive sequences. It was shown that C-positive regions of wood mouse chromosomes can contain both homologous and distinct sets of repetitive sequences. Regions enriched with homologous repeats were detected either directly in C-positive regions of individual chromosomes or only on the short arms of acrocentrics, or at the boundary of C-positive and C-negative regions.

Discovery of a fucoidan endo-4O-sulfatase: Regioselective 4O-desulfation of fucoidans and its effect on anticancer activity in vitro.

Fucoidans are a class of sulfated fucose-containing bioactive polysaccharides produced by brown algae. The biological effects exhibited by fucoidans are thought to be related to their sulfation. However, the lack of methods for sulfation control does not allow for a reliable conclusion about the influence of the position of certain sulfate groups on the observed biological effects. We identified the gene encoding the endo-acting fucoidan sulfatase swf5 in the marine bacterium Wenyingzhuangia fucanilytica CZ1127T. This is the first report on the sequence of fucoidan endo-sulfatase. Sulfatase SWF5 belongs to the subfamily S1_22 of the family S1. SWF5 was shown to remove 4O-sulfation in fucoidans composed from the alternating α-(1→3)- and α-(1→4)-linked residues of sulfated L-fucose but not from fucoidans with the α-(1→3)-linked backbone. The endo-sulfatase was used to selectively prepare 4O-desulfated fucoidan derivatives. It was shown that the 4O-desulfated fucoidans inhibit colony formation of DLD-1 and MCF-7 cells less effectively than unmodified fucoidans. Presumably, 4O-sulfation makes a significant contribution to the anticancer activity of fucoidans.

[The structure of long telomeres in chromosomes of the Iberian shrew].

It is shown that the size, localization, and structure of telomeres in the Iberian shrew (Sorex granarius) are not characteristic of mammals. In this species, long telomeres of an average size of 213 kb are localized on the short arms of all 32 acrocentrics; ribosomal blocks and active nucleolus-organizing regions (NORs) were also discovered there. At the remaining chromosome ends the average size of telomeres is 3.8 kb. However, in a closely related species, Sorex araneus, all telomeres have size similar to that of human telomeres, i.e., 6.8-15.2 kb. Despite the fact that some long telomeres contain ribosomal repeats in addition to telomeric ones, the long telomeres have preserved asymmetry of G- and C-rich strands as in functional telomeres. It is probable that long telomeres were formed in meiosis at the stage of chromosome bouquet as a result of global reorganization of the chromosome ends. The provoking factors for such reorganization might be the fission of several metacentrics and the necessity of telomerization of the resulting acrocentrics.

[Comparative FISH analysis of C-positive blocks of centromeric chromosomal regions of pygmy wood mice Sylvaemus uralensis (Rodentia, Muridae)].

The composition and homology of centromeric heterochromatin DNA has been compared in representatives of the Asian race and two chromosomal forms (Eastern European and Southern European) of the European race of the pygmy wood mouse Sylvaemus uralensis by means of in situ hybridization with metaphase chromosomes of microdissection DNA probes obtained from centromeric C-blocks of mice of the Southern European chromosomal form and the Asian race. Joint hybridization of both DNA probes yielded all possible variants of centromeric regions in terms of the presence of repetitive sequences homologous to those of some or another dissection region, which indicates a diversity of centromeric regions differing in DNA composition. However, most variations of the fluorescent in situ hybridization (FISH) patterns are apparently related to quantitative differences of repetitive elements of the genome. Experiments with the DNA probe obtained from the genome of the Southern European form of the pygmy wood mouse have shown that the number of intense FISH signals roughly corresponds to the number of large C-segments in representatives of the European race, which is characterized by a large amount of the centromeric C-heterochromatin in the karyotype. However, intense signals have been also detected in experiments on hybridization of this probe with chromosomes of representatives of the Asian race, which has no large C-blocks in the karyotype; thus, DNA sequences homologous to heterochromatic ones are also present in nonheterochromatic regions adjacent to C-segments. Despite the variations of the numbers of both intense and weak FISH signals, all chromosomal forms/races of S. uralensis significantly differ from one another in these characters. The number of intense FISH signals in DNA from the samples of pygmy wood mice from eastern Turkmenistan (the Kugitang ridge) and southern Omsk oblast (the vicinity of the Talapker railway station) was intermediate between those in the European and Asian races, which is apparently related to a hybrid origin of these populations (the hybridization having occurred long ago in the former case and recently in the latter case).

Generation of microdissected DNA probes from metaphase chromosomes when chromosome identification by routine staining is impossible.

Application of microdissected DNA libraries and DNA probes in numerous and various modern molecular cytogenetic studies showed them as an efficient and reliable tool in the analysis of chromosome reorganization during karyotypic evolution and in the diagnosis of human chromosome pathology. An important advantage of DNA probe generation by metaphase chromosome microdissection followed by sequence-independent polymerase chain reaction in comparison with the method of DNA probe generation using chromosome sorting is the possibility of DNA probe preparation from chromosomes of an individual sample without cell line establishment for the production of a large number of metaphase chromosomes. One of the main requirements for successful application of this technique is a possibility for identification of the chromosome of interest during its dissection and collection of its material from metaphase plates spread on the coverslip. In the present study, we developed and applied a technique for generation of microdissected DNA probes in the case when chromosome identification during microdissection appeared to be impossible. The technique was used for generation of two sets of Whole Chromosome Paints (WCPs) from all chromosomes of two species of free-living flatworms in the genus Macrostomum, M. mirumnovem and M. cliftonensis. The single-copy chromosome technique including separate collection of all chromosomes from one metaphase plate allowed us to generate WCPs that painted specifically the original chromosome by Chromosome In Situ Suppression Hybridization (CISS-Hybridization). CISS-Hybridization allowed identifying the original chromosome(s) used for DNA probe generation. Pooled WCPs derived from homologous chromosomes increased the intensity and specificity of chromosome painting provided by CISS-Hybridization. In the result, the obtained DNA probes appeared to be good enough for application in our studies devoted to analysis of karyotypic evolution in the genus Macrostomum and for analysis of chromosome rearrangements among the worms of laboratory cultures of M. mirumnovem.

Regions enriched for DNA repeats in chromosomes of Macrostomum mirumnovem, a species with a recent Whole Genome Duplication.

The free-living flatworm Macrostomum mirumnovem is a neopolyploid species whose genome underwent a recent Whole Genome Duplication (WGD). In the result of chromosome fusions of the ancient haploid chromosome set, large metacentric chromosomes were formed. In addition to three pairs of small metacentrics, the current karyotype of M. mirumnovem contains two pairs of large metacentric chromosomes, MMI1 and MMI2. The generation of microdissected DNA libraries enriched for DNA repeats followed by DNA probe preparation and fluorescent in situ hybridization (FISH) were performed. The DNA probes obtained marked chromosome regions enriched for different DNA repeats in the M. mirumnovem chromosomes. The size and localization of these regions varied in different copies of large chromosomes. They varied even in homologous chromosomes, suggesting their divergence due to genome re-diploidization after a WGD. Besides the newly formed chromosome regions enriched for DNA repeats, B chromosomes were found in the karyotypes of the studied specimens of M. mirumnovem. These B chromosomes varied in size and morphology. FISH with microdissected DNA probes revealed that some Bs had a distinct DNA content. FISH could paint differently B chromosomes in different worms and even in the same sample. B chromosomes could carry a bright specific fluorescent signal or could show no fluorescent signal at all. In latter cases, the specific FISH signal could be absent even in the pericentromeric region of the B chromosome. Possible mechanisms of B chromosome formation and their further evolution are discussed. The results obtained indicate an important role that repetitive DNAs play in genome re-diploidization initiating a rapid differentiation of large chromosome copies. Taking together, karyotype peculiarities (a high level of intraspecific karyotypic diversity associated with chromosome number variation, structural chromosomal rearrangements, and the formation of new regions enriched for DNA repeats) and some phenotypic features of M. mirumnovem (small body size, short lifecycle, easy maintenance in the laboratory) make this species a perspective model in the studies of genomic and karyotypic evolution in species passed through a recent WGD event.