Presence of the 54-chromosome common vole (Mammalia)... 433 Presence of the 54-chromosome common vole

We report a new finding of the 54-chromosome sibling species of the common vole in East Siberia- the first description from Olkhon Island (Lake Baikal). The karyotype of a male specimen revealed by routine staining and C-banding demonstrates the unambiguous presence of Microtus rossiaemeridionalis Ognev, 1924 (recently often regarded as as junior synonym of M. levis Miller, 1908). Comparison with conspe-cific specimens from the European part of the species range (from the left bank of the river Volga) shows that the vole of the island population has a smaller X-chromosome due to a reduced quantity of C-positive heterochromatin. This is just the third example of this type of X-chromosome variant with previous cases on an Arctic island (Svalbard) and the West Siberian lowland (Novosibirsk) and the only one on a lake island. Although M. rossiaemeridionalis is largely monomorphic in its karyotype, our data show that one specific type of X-chromosome variant is remarkably widespread, though rare