Recent and ancient introgression of mitochondrial DNA between the red (Clethrionomys rutilus) and bank (Clethrionomys glareolus) voles (Rodentia, Cricetidae)

Abstract

The purposes of this study were (1) to determine the proportion and geographic distribution of voles with the red-vole mitochondrial genome in the populations of bank voles and (2) to reveal interspecific hybrids. The main results of analysis of the mitochondrial (cytochrome b ) and nuclear molecular markers are as follows. (1) The distribution area of the bank voles with the foreign mitochondrial genome is a narrow crescentshaped band extending from the Southern Urals to the northwest of the European part of Russia. Relative proportion of such voles in populations is small (3‐5%), except for the White Sea coast and islands, which are inhabited by the voles carrying solely the foreign mitochondrial haplotype. (2) Bank voles with the red-vole mitochondrial genome have originated as a result of ancient hybridization and subsequent introgression; however, judging by the hybrid found in the Urals population, hybridization may also occur at present. Interspecific hybridization with subsequent gene introgression is widely spread in natural populations of many species. Gene transfer from species to species via introgression is sometimes regarded as a possible way to acquire new genetic material that is then subject to selection [1, 2]. The list of close mammalian species for which examples of hybridization and introgression are known steadily grows. Although hybrid zones have been studied for over 70 years, the interest of evolutionary biologists to this phenomenon does not diminish with the years. Furthermore, since the end of 1980s, when it became possible to apply molecular‐genetic approaches to analysis of such zones, the interest to this problem significantly increased. This is quite natural because the study of such zones helps to better understand the nature of genetic differences between species, the role of natural selection in maintenance of boundaries between species, and the evolution of mechanisms underlying reproductive isolation [3, 4]. One of the most interesting examples of a possible hybrid zone is the wide sympatric area of two vole species—the bank vole ( Cl. glareolus Shreiber, 1780) and the red vole ( Cl. rutilus Pallas, 1779). Both species are the most common and widely spread rodents of the forest zone. The wide sympatric area of these species formed gradually upon their radiation in the postpleistocene period from periglacial refugia together with the distribution of the forest zone. Judging by paleontological data, these species diverged from one another 2.5 million