Abstract
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.
Highlights
In recent years, there has been a significant deepening and expansion of ideas about what constitutes the genome of a species
We characterized the details of repetitive DNA distribution using highthroughput sequencing [20] and fluorescence in situ hybridization (FISH) [41]
Cells 2021, 10, x section of B chromosomes (Bs), autosomes, and sex chromosomes from karyotypes of A chromosomes (As). flavicollis individuals [39,41,57,58], we continued the analysis of FISH patterns, which were produced by in situ hybridization of the probes with metaphase chromosomes of yellow-necked mice from Serbia, Belarus, and Russia, including specimens from several additional localities (Table 1)
Summary
There has been a significant deepening and expansion of ideas about what constitutes the genome of a species. The application of whole genome sequencing has made a huge contribution, but it became evident that the same species genomes can vary on different levels, including the chromosomal one. Bs can vary in number and size in some species Another example of chromosome variability is the directed elimination of specific genomic regions in somatic cells: the phenomenon is known as “chromatin diminution” and was first described in the 1880s for Ascaris megalocephala [1,2]; later, several other species with chromatin diminution were described, including zebra finch, where large, mainly heterochromatic chromosomes that presented only in germ cells were found [3]. It was shown that such germline-restricted chromosomes (GRC) are probably presented in germ cells of all songbirds [4]. The question of the location, timing and significance of the expression of these genes remains open
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