Sex chromosome polymorphism in Bulgarian populations of Microtus guentheri (Danford & Alston, 1880)

Sex chromosome polymorphism in Bulgarian populations of Guenther's vole (Microtus guentheri Danford & Alston, 1880) has been described. All studied individuals of Guenther's vole have the diploid chromosome number 2n = 54 (FNa = 52). The autosomes are acrocentric. All autosomes in the karyotype have constitutive heterochromatin, located in the centromeric region. In three autosomal pairs distal heterochromatin has been described. The sex chromosomes exhibit interpopulation variation in morphology and heterochromatin patterns. In most studied individuals, the X chromosomes are submetacentric, but there are individuals with acrocentric X chromosomes. The Y chromosome is completely heterochromatic. The acrocentric X chromosomes have a large centromeric heterochromatic block, which probably originated as a result of random amplification of sequences as have been proposed in other species of the genus Microtus.