Additional or B-chromosomes are "mysterious" hereditary elements found in the genomes of many species of fungi, plants and animals. Previously thought to be genomic "junk", their genetic makeup is now known to include ribosomal genes, telomeric and centromeric repeats and even autosomal genes responsible for a range of functions from cell division to the role in evolution and speciation.
The complex of narrow-skulled voles of the subgenus Stenocranius is represented by several taxa of different evolutionary ages: the relict Lasiopodomys raddei and L. gregalis, which includes three genetic lines A, B and C, probably of independent taxonomic rank. In the Altai-Sayan region and in Transbaikalia, their ranges coexist without signs of sympatry.
Large-scale cytogenetic analysis of karyotypes of representatives of the subgenus revealed significant B-chromosomal polymorphism (1-5 additional chromosomes), mainly in samples from secondary contact zones of the ranges of these taxa in the Altai-Sayan region (Tuva).
For the first time, it was established that the frequency of B chromosomes is significantly higher in karyotypes of evolutionarily younger lines B and C of L. gregalis, and the karyotypes of all studied L. raddei were “clean of debris” (did not contain additional chromosomes).
Molecular cytogenetic analysis using fluorescently labeled probes (FISH) showed that all B chromosomes contain telomeric repeats, and only some contain ribosomal genes. Immunocytochemical analysis revealed similarities in the structure of B chromosomes and sex chromosomes in meiotic cells and their co-location in one chromatin domain, which may indirectly indicate homology between them. Possibly, such localization of B chromosomes allows them to overcome meiotic checkpoints and, thus, to persist in the genome.
Further research is aimed at finding answers to the questions "Why are they needed?" and "How did they arise?".
The work was carried out by a team of authors from the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences (IEE RAS) (PhD S.V. Pavlova and Yu.M. Kovalskaya, postgraduate I.A. Dvoyashov), the Institute of Molecular and Cellular Biology of the Siberian Branch of the Russian Academy of Sciences, the N.I. Vavilov Institute of General Genetics of the Russian Academy of Sciences, the Tuva Institute for Integrated Development of Natural Resources of the Siberian Branch of the Russian Academy of Sciences, and the Zoological Institute of the Russian Academy of Sciences as part of the RSF project (grant No. 22-24-00513).
The details of the study were published in the journal JEZ Part B: Molecular and Developmental Evolution (Q1) and presented at a conference on B chromosomes.