PATTERN OF X-Y CHROMOSOME-PAIRING IN MICROTINE RODENTS

Pairing of X and Y chromosomes at meiotic prophase in 14 species of th e subfamily Microtinae (Clethrionomys rufocanus, C. rutilus, C. glareo lus, 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 rel ation to their taxonomic position and variation in the morphology of t heir sex chromosomes. The sex chromosomes formed a synaptonemal comple x (SC) at pachytene in all Clethrionomys species, Arvicola terrestris, and M. oeconomus, while they did not pair at all in M. (Pitymys) majo ri, Microtus socialis, M. guentheri, M. afghanus, M. bucharicus, M. ar valis, M. rossiaemeridionalis, M. kirgisorum, and M. transcaspicus. Th e X chromosome of these species varied in centromere position independ ently of pairing pattern. Insertion of heterochromatin of different si ze and location was found in some, but not in all species with asynapt ic sex chromosomes. It is suggested that the sex chromosomes lost thei r 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 c hromosomal rearrangement.