ABSTRACT
Since the first description of sympatric sibling species and allopatric chromosomal forms of the common vole (Microtus arvalis group) in 1969, the search of their range boundaries has been Continually going on up to the present time. Based on the cytogenetic material presented in the present study, the geographical distribution of two karyoforms (2n = 46: NF = 84 and NF = 72) was examined in 20 geographic localities of European Russia (within the interval between the longitude 34 degrees-56 degrees East and 51 degrees-61 degrees). The places of findings of two karyoforms, "arvalis" and "obscurus", in Russia have been supplemented and were shown to be significantly closer; whereas the well-known distribution boundaries for the "obscurus" form were specified in the northern and southern parts of Russia.A direct evidence of parapatric distribution and hybrid zone formation between the "arvalis" and "obscurus" karyoforms was obtained, which suggests the possibility of studying the evolutionary relations ships between two genomes in the Russian part of the group range under the natural experimental conditions-the hybrid zone at the boundary between the "arvalis" and "obscurus".
