Geographical distribution and chromosomal study of the underground vole Microtus thomasi in Albania and Montenegro

In order to better define the geographical distribution of the underground vole Microtus thomasi and the chromosomal variability, within its natural populations, we studied 77 individuals from 14 localities of Albania and Montenegro. Chromosomal preparations were obtained from bone marrow and testicular material, on which a karyological analysis, based on C-banding patterns was performed. The examined individuals belonged to the chromosomal races “thomasi” (2n=44, FN=44), “subalpine” (2n=42, FN=42) and “Rb-subalpine” (2n=40, FN=42), which are also distributed in Greece. However, the C-banding pattern revealed an extensive sex chromosome polymorphism, demonstrated by three different X and three different Y chromosomal variants. Taking under consideration all available chromosomal data for M. thomasi, it seems that the species could possess the highest chromosomal variability, within its genus. It is proposed that due to the limited mobility of the underground vole and the rough, mountainous terrain of the Balkan Peninsula, it is possible that several small populations were isolated, in which inbreeding and random genetic drift led to the fixation of different chromosomal mutations, giving rise to the extensive chromosomal variability, observed today.