Variability in Size of the Nuclear Genome in Pygmy Wood Mouse Sylvaemus uralensis (Rodentia, Muridae)

Abstract

Earlier, in an integral genetic study, the Asian and European races were distinguished within the species Sylvaemus uralensis (pygmy wood mouse) and the European race was divided into the East European and South European forms. Each of these groups differed from the others, in particular, in the quantity of the centromeric heterochromatin in karyotypes of the animals. To establish the pattern of its changes in S. uralensis, in the present study the DNA content in splenocyte nuclei in all races and forms of pygmy wood mice was assessed using DNA flow cytometry. The heterochromatin amount in karyotypes and genome size were shown to be correlated. The East European chromosomal race of S. uralensis (Central Chernozem and Non-Chernozem regions of Russia, Crimea Peninsula, Middle Volga region, and Southern Ural) and the Asian race of this species (East Kazakhstan, Uzbekistan, and East Turkmenistan), which have respectively the highest and the lowest amounts of centromeric heterochromatin in the karyotype, exhibit the greatest difference in the DNA content in the genome. On average, the difference is approximately 8% in males and 6.7% in females; in both cases, the ranges of variability were distinctly different. Against the general background of the trait variation, the Asian race, whose members have the smallest DNA amount in their cells, looks homogeneous. The genome of the South European chromosomal form of S. uralensis (Caucasus, Transcaucasia, Carpathians, and Balkan Peninsula), which exhibits an intermediate content of the centromeric heterochromatin in the karyotype, is smaller that the genome of the East European race (by 3.2% in the group of males and by 1.9%, in the group of females), but larger than that of the Asian race (by 5% in either sex). Thus, the variability of size of centromeric C-blocks in pygmy wood mouse is likely to be associated with elimination (or, conversely, an increase in the amount) of the genetically inert chromatin. It is suggested that a significant contribution to the variability of genome size in S. uralensis is made by heterochromosomes, or, more precisely, their variable regions, which seem to be largely heterochromatic.