Supernumerary chromosomes contribute to karyotypic diversity within cryptic species of the subgenus Stenocranius (Cricetidae, Rodentia): comparative analyses of chromosomes and synaptonemal complexes

The subgenus Stenocranius includes two cryptic species, Lasiopodomys raddei and L. gregalis; the latter has three allopatric and genetically well-isolated lineages having unclear taxonomic rank. To determine possible karyotypic differences, 121 individuals from 37 localities (among them, 15 are previously unpublished sampling sites) were analysed. Besides differences in some chromosome traits, higher intra-specific karyotypic variation within L. gregalis is mostly due to various numbers of small heterochromatic acrocentric B chromosomes (Bs) whose number does not vary intra-individually . The Early Pleistocene relic L. raddei carries stable initial karyotype 2 n = 36 without Bs, while among the L. gregalis lineages, the number of Bs varies from one to five: increasing from older lineage A to evolutionarily younger lineages B and C. In both cryptic species, the distribution pattern of ribosomal DNA varies in A chromosome sets and on Bs, whereas telomeric sequences are located only at the ends of all chromosomes, including Bs. Immunodetection of several meiotic proteins indicates that meio-Bs are transcriptionally inactive and have a pattern of meiotic behaviour similar to that of sex chromosomes. Further research is needed to clarify the nature, mechanisms of inheritance and stability of supernumerary chromosomes in narrow-headed voles.