Multiple Chromosomal Polymorphism in Birch Mice of the subtilis Group (Rodentia, Dipodoidea, Sicista) from the Saratov Right Bank of the Volga River

Cytogenetic analysis (routine, G-, C-, AgNOR-chromosome banding) allowed us to obtain new data on chromosomal polymorphism in the population (n = 13) of the Sicista sp. 1 representative of the Sicista subtilis group from the Saratov region, right bank of Volga River. It was shown that in the studied polymorphic population of a species from the northern part of the species' range (Voskresenskii district of Saratov region) the diploid number of chromosomes varies from 2n = 22 to 2n = 24, and the number of autosome arms ranges from NFa = 39 to NFa = 44, and the sample includes the following six karyotype variants: with 2n = 24, NFa = 44; 2n = 24, NFa = 43; 2n = 23, NFa = 40; 2n = 23, NFa = 41; 2n = 22, NFa = 39; 2n = 22, NFa = 40. Based on the G-banding method, the determined intrapopulation chromosomal polymorphism in Sicista sp. 1 was defined by two chromosomal rearrangements: tandem translocation involving two biarmed autosomes nos. 4 and 10 and a pericentric inversion of pair no. 6. In the studied sample (n = 13), a stable advantage of heterozygotes was revealed for both structural chromosomal mutations and the chromosomal polymorphism is characterized as balanced or heterozygous. The potential role of the identified polymorphism in chromosomal evolution and the adaptive strategy of the species is discussed.