Abstract
Pairing of X and Y chromosomes at meiotic prophase in 14 species of the subfamily Microtinae (Clethrionomys rufocanus, C. rutilus, C. glareolus, Arvicola terrestris, Microtus guentheri, M. socialis, M. afghanus, M. bucharicus, M. oeconomus, M. arvalis, M. rossiaemeridionalis, M. kirgisorum, M. transcaspicus, M. (Pitymys) majori) was analysed in relation to their taxonomic position and variation in the morphology of their sex chromosomes. The sex chromosomes formed a synaptonemal complex (SC) at pachytene in all Clethrionomys species, Arvicola terrestris, and M. oeconomus, while they did not pair at all in M. (Pitymys) majori, Microtus socialis, M. guentheri, M. afghanus, M. bucharicus, M. arvalis, M. rossiaemeridionalis, M. kirgisorum, and M. transcaspicus. The X chromosome of these species varied in centromere position independently of pairing pattern. Insertion of heterochromatin of different size and location was found in some, but not in all species with asynaptic sex chromosomes. It is suggested that the sex chromosomes lost their ability to pair at male meiosis in the common ancestor of palearctic species of the genus Microtus. This event was not caused by a gross chromosomal rearrangement.
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