ISSN 1062-3590, Biology Bulletin, 2016, Vol. 43, No. 9, pp. 1120–1128. © Pleiades Publishing, Inc., 2016. Original Russian Text © M.I. Baskevich, S.G. Potapov, L.A. Khlyap, N.M. Okulova, U.M. Ashibokov, M.P. Grigoriev, T.K. Dzagurova, 2015, published in Zoologicheskii Zhur- nal, 2015, Vol. 94, No. 8, pp. 963–971. 1120 Chromosomal and Molecular Studies of Cryptic Species of the Subgenus Terricola (Rodentia, Arvicolinae, Microtus) in the Caucasian Region: Analysis of New Records M. I. Baskevich a, *, S. G. Potapov a , L. A. Khlyap a , N. M. Okulova a , U. M. Ashibokov b , M. P. Grigoriev b , and T. K. Dzagurova c a Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow, 119071 Russia b Stavropol Antiplague Scientific Research Institute, Stavropol, 355035 Russia c Chumakov Institute of Poliomyelitis and Viral Encephalitis, Moscow oblast, 142782 Russia *e-mail: mbaskevich@mail.ru Received May 21, 2014 AbstractUsing chromosomal and molecular (cytb) markers, the specific identity of 78 individuals of cryptic species of voles of the subgenus Terricola was ascertained. The animal samples were obtained at 13 localities of the Caucasian region, from Krasnodar krai to North Ossetia in the Greater Caucasus and in the Stavropol Upland (forest island in the steppe) in Ciscaucasia, which had not been covered earlier in genetic studies. In the total sample, two chromosomal forms (cryptic species), namely, Microtus (Terricola) majori (2n = 54, NF = 60) and M. (T.) daghestanicus (2n = 54, NF = 58), were identified. The specific identity of most animals tested was confirmed by karyological means, and for individuals from two localities on the south slopes of the Greater Caucasus (Adlerskii district of Krasnodar krai), it was confirmed exclusively by means of molecular (cytb) markers. The last two records were used for evaluation of the differential role of sibling-species of the subgenus Terricola in circulation of the pathogen in mountain loci of hemorrhagic fever with renal syndrome (HFRS) in the Western Caucasus. For the first time, using the example of M. (T.) majori from the Ciscauca- sia, which was compared with those of the mountain part of the species’ natural habitat, the role of isolation factors in morphogenesis of the subgenus Terricola was studied. As a whole, the results obtained specify the character of the geographical distribution and biotope attachment of cryptic species of the subgenus Terricola in the Caucasian region. Keywords: voles of the subgenus Terricola, sibling-species, the Caucasian region, chromosomes, molecular markers, diagnostics, distribution, hemorrhagic fever with renal syndrome (HFRS), role of factors of isola- tion in speciation DOI: 10.1134/S1062359016090016 INTRODUCTION Pine voles were initially considered as a Holarctic taxon of the Microtinae subfamily (Miller, 1912; Ognev, 1950; Ellerman and Morrison-Scott, 1951; Gromov and Polyakov, 1977; et al.). It was shown later, based on study of biochemical and paleontolog- ical data, that pine voles of the Old and New World have an independent history and the validity of the name Terricola Fatio 1867 for Palearctic forms (Сhaline, 1987; Chaline et al., 1988), living on a sig- nificant part of the European continent and in the Caucasus, was proved (Wilson and Reeder, 2005). According to the latest molecular-genetic data, the subgenus status of Terricola in the structure of the genus Microtus has been confirmed (Potapov et al., 1999; Jaarola et al., 2004). We adopt this point of view in this article. Intensive studies of karyotypes of Caucasian repre- sentatives of pine voles (subgenus Terricola) have detected a wide range of chromosomal variability in this group of rodents (Ivanov and Tembotov, 1972; Khatukov et al., 1978; Lyapunova et al., 1988; Akh- verdyan et al., 1988, 1992). It was shown that pine voles of the Caucasus and Transcaucasia are a quite complex group in a taxonomical sense, within which rates of chromosomal evolution were ahead of those of morphological one . This resulted in similarities of the morphological traits and significant differentiation of chromosome sets. At the present time, there are 12 known karyomorphs in the region, and two subse- quent cryptic species of pine voles are recognized (Akhverdyan, 1989; Akhverdyan et al., 1992): (1) the Major’s pine vole Microtus (Terricola) majori Thomas 1906, represented by a forest form that is widespread in the Greater and Lesser Caucasus and characterized