The phylogeography of the Holarctic Microtus oeconomus in Russian Far East, near the Beringia, was studied using mitochondrial cytochrome b (Cytb) gene sequences. In comparison with the Cytb data of the Holarctic Myodes rutilus and the Palaearctic My. rufocanus previously studied by us, those of Mi. oeconomus showed similar local differentiation patterns to those of My. rutilus and obviously differed from those of My. rufocanus. Further, the current and previous Cytb data indicated that the

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... tic distances of specimens between the Magadan and Kamchatka/Kuril Archipelago regions were clearly higher in Mi. oeconomus and My. rutilus and lower in My. rufocanus. On the basis of the genetic distances and the geological splitting time between Kamchatka and Kuril Archipelago as 10,000 years ago previously reported, we could estimate the divergence time between the two region populations of Holarctic arvicolids as a few ten thousand years at most. Namely, in the Russian Far East, a past geographic and/or environmental barrier seemed to have appeared and to have inhibited genetic communication between the Magadan and Kamchatka/Sakhalin/Kuril Archipelago regions in Mi. oeconomus and My. rutilus, before My. rufocanus had expanded its distribution there.