The mammalian model for population studies of B chromosomes: the wood mouse (Apodemus)

B Chromosomes’ Sequences in Yellow-Necked Mice Apodemus flavicollis—Exploring the Transcription

B chromosomes (Bs) are highly polymorphic additional chromosomes in the genomes of many species. Due to the dispensability of Bs and the lack of noticeable phenotypic effects in their carriers, they were considered genetically inert for a long time. Recent studies on Bs in Apodemus flavicollis revealed their genetic composition, potential origin, and spatial organization in the interphase nucleus. Surprisingly, the genetic content of Bs in this species is preserved in all studied samples, even in geographically distinct populations, indicating its biological importance. Using RT-PCR we studied the transcription activity of three genes (Rraga, Haus6, and Cenpe) previously identified on Bs in A. flavicollis. We analysed mRNA isolated from spleen tissues of 34 animals harboring different numbers of Bs (0–3).The products of transcriptional activity of the analysed sequences differ in individuals with and without Bs. We recorded B-genes and/or genes from the standard genome in the presence of Bs, showing sex-dependent higher levels of transcriptional activity. Furthermore, the transcriptional activity of Cenpe varied with the age of the animals differently in the group with and without Bs. With aging, the amount of product was only found to significantly decrease in B carriers. The potential biological significance of all these differences is discussed in the paper.

New Data on Organization and Spatial Localization of B-Chromosomes in Cell Nuclei of the Yellow-Necked Mouse Apodemus flavicollis

The gene composition, function and evolution of B-chromosomes (Bs) have been actively discussed in recent years. However, the additional genomic elements are still enigmatic. One of Bs mysteries is their spatial organization in the interphase nucleus. It is known that heterochromatic compartments are not randomly localized in a nucleus. The purpose of this work was to study the organization and three-dimensional spatial arrangement of Bs in the interphase nucleus. Using microdissection of Bs and autosome centromeric heterochromatic regions of the yellow-necked mouse (Apodemus flavicollis) we obtained DNA probes for further two-dimensional (2D)- and three-dimensional (3D)- fluorescence in situ hybridization (FISH) studies. Simultaneous in situ hybridization of obtained here B-specific DNA probes and autosomal C-positive pericentromeric region-specific probes further corroborated the previously stated hypothesis about the pseudoautosomal origin of the additional chromosomes of this species. Analysis of the spatial organization of the Bs demonstrated the peripheral location of B-specific chromatin within the interphase nucleus and feasible contact with the nuclear envelope (similarly to pericentromeric regions of autosomes and sex chromosomes). It is assumed that such interaction is essential for the regulation of nuclear architecture. It also points out that Bs may follow the same mechanism as sex chromosomes to avoid a meiotic checkpoint.

B Chromosomes in Populations of Mammals Revisited

The study of B chromosomes (Bs) started more than a century ago, while their presence in mammals dates since 1965. As the past two decades have seen huge progress in application of molecular techniques, we decided to throw a glance on new data on Bs in mammals and to review them. We listed 85 mammals with Bs that make 1.94% of karyotypically studied species. Contrary to general view, a typical B chromosome in mammals appears both as sub- or metacentric that is the same size as small chromosomes of standard complement. Both karyotypically stable and unstable species possess Bs. The presence of Bs in certain species influences the cell division, the degree of recombination, the development, a number of quantitative characteristics, the host-parasite interactions and their behaviour. There is at least some data on molecular structure of Bs recorded in nearly a quarter of species. Nevertheless, a more detailed molecular composition of Bs presently known for six mammalian species, confirms the presence of protein coding genes, and the transcriptional activity for some of them. Therefore, the idea that Bs are inert is outdated, but the role of Bs is yet to be determined. The maintenance of Bs is obviously not the same for all species, so the current models must be adapted while bearing in mind that Bs are not inactive as it was once thought.

B Chromosome System in the Korean Field Mouse Apodemus peninsulae Thomas 1907 (Rodentia, Muridae)

In this paper, we analyzed B chromosome variation in Korean field mouse Apodemus peninsulae Thomas 1907 (Rodentia, Muridae) based on a 40-year study of karyotypes collected from geographically distant populations in East Siberia, North Mongolia, China, the Russian Far East, and Japan (Hokkaido). We developed the database of individual variants of B chromosome systems in A. peninsulae. In Siberian populations all animals had Bs. The karyotypes of the studied animals contain from 1 to 30 Вs differing in size and morphology. Analysis of B chromosome systems in 598 individuals from different localities of the range shows the presence of 286 variants of Вs combinations in these animals. Unique sets of B morphotypes make up most of these variants (64.7 ± 1.3%), probably suggesting that individual Bs systems normally result from stochastic processes in the populations. The proportion of animals with a large number of Bs gradually decreases, probably due to increased complexities in the inheritance of large numbers of Bs. A. peninsulae is thus proposed as a good model for studying the origin and evolution of extra elements in the karyotype.

Cryptosporidium apodemi sp. n. and Cryptosporidium ditrichi sp. n. (Apicomplexa: Cryptosporidiidae) in Apodemus spp

Faecal samples from striped field mice (n = 72) and yellow-necked mice (n = 246) were screened for Cryptosporidium by microscopy and PCR/sequencing. Phylogenetic analysis of small-subunit rRNA, Cryptosporidium oocyst wall protein and actin gene sequences revealed the presence of C. parvum, C. hominis, C. muris and two new species, C. apodemi and C. ditrichi. Oocysts of C. apodemi are smaller than C. ditrichi and both are experimentally infectious for yellow-necked mice but not for common voles. Additionally, infection by C. ditrichi was established in one of three BALB/c mice. The prepatent period was 7-9 and 5-6 days post infection for C. apodemi and C. ditrichi, respectively. The patent period was greater than 30 days for both species. Infection intensity of C. ditrichi ranged from 4000-50,000 oocyst per gram of faeces and developmental stages of C. ditrichi were detected in the jejunum and ileum. In contrast, neither oocysts nor endogenous developmental stages of C. apodemi were detected in faecal or tissue samples, although C. apodemi DNA was detected in contents from the small and large intestine. Morphological, genetic, and biological data support the establishment of C. apodemi and C. ditrichi as a separate species of the genus Cryptosporidium.

Spatial organization of fibroblast and spermatocyte nuclei with different B-chromosome content in Korean field mouse, Apodemus peninsulae (Rodentia, Muridae)

Korean field mouse (Apodemus peninsulae) shows a wide variation in the number of B chromosomes composed of constitutive heterochromatin. For this reason, it provides a good model to study the influence of the number of centromeres and amount of heterochromatin on spatial organization of interphase nuclei. We analyzed the three-dimensional organization of fibroblast and spermatocyte nuclei of the field mice carrying a different number of B chromosomes using laser scanning microscopy and 3D fluorescence in situ hybridization. We detected a co-localization of the B chromosomes with constitutive heterochromatin of the chromosomes of the basic set. We showed a non-random distribution of B chromosomes in the spermatocyte nuclei. Unpaired B chromosomes showed a tendency to occur in the compartment formed by the unpaired part of the XY bivalent.

The origin of B chromosomes in yellow-necked mice (Apodemus flavicollis)-Break rules but keep playing the game

B chromosomes (Bs) are known for more than hundred years but their origin, structure and pattern of evolution are not well understood. In the past few years new methodological approaches, involving isolation of Bs followed by whole DNA amplification, DNA probe generation, and fluorescent in situ hybridization (FISH) or the B chromosome DNA sequencing, has allowed detailed analysis of their origin and molecular structure in different species. In this study we explored the origin of Bs in the yellow-necked wood mouse, Apodemus flavicollis, using generation of microdissected DNA probes followed by FISH on metaphase chromosomes. Bs of A. flavicollis were successfully isolated and DNA was used as the template for B-specific probes for the first time. We revealed homology of DNA derived from the analyzed B chromosomes to the pericentromeric region (PR) of sex chromosomes and subtelomeric region of two pairs of small autosomes, but lower homology to the rest of the Y chromosome. Moreover, all analysed Bs had the same structure regardless of their number per individual or the great geographic distance between examined populations from the Balkan Peninsula (Serbia) and Eastern Europe (south region of Russia and central Belarus). Therefore, it was suggested that B chromosomes in A. flavicollis have a unique common origin from the PR of sex chromosomes, and/or similar evolutionary pattern.

Distribution of B chromosomes in age categories of the yellow-necked mouse Apodemus flavicollis (Mammalia, Rodentia)

The presence of B chromosomes (Bs) is frequently found in populations of yellow-necked mouse, Apodemus flavicollis. Differences in frequencies of Bs in age categories were analyzed in 717 animals in order to clarify the mechanisms of their maintenance in populations of A. flavicollis. The absence of statistically significant differences in the frequency of Bs between six age categories indicates that the maintenance of Bs in populations can be explained by in terms of their contribution to overall genetic diversity of the species rather than by parasitic behavior.

Do B chromosomes affect fecundity in yellow-necked mice Apodemus flavicollis (Rodentia, Mammalia)?

The effects of the presence of B chromosomes on fecundity of the yellow-necked mouse Apodemus flavicollis were studied in 46 females without and 28 with Bs from four localities in Serbia. Uterine inspection showed that there was no significant difference in the mean number of scars and embryos between females with and those without Bs. Thus, B chromosomes do not appear to affect the fecundity of females carrying them, indicating that the presence of Bs does not affect fitness characteristics.