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We present the karyotype characteristics of four populations of mole rats ( Nannospalax xanthodon ) with 50 chromosomes investigated by C-banding and AgNOR staining. The populations studied originated from the Asiatic part of Turkey and the collecting sites were separated by geographic distance of several hundreds of kilometres. The comparative analysis showed that these populations differ, in spite of the identical diploid number of chromosomes, in various discrete cytogenetic markers. The overall amount of C-heterochromatin varied between populations, and also variation in the number and localization of C-heterochromatic autosomal short arms was observed. The nucleolar organizer regions occurred on two, three or four autosomal pairs. They were found usually in the telomeric position and were associated with the C-heterochromatic arms. These results indicate that a cytotype with the particular number of chromosomes may include several populations with different karyotypes that have evolved independently. We therefore assume that convergent evolution of karyotypes with the same diploid chromosome number is not exceptional in mole rats. [ABSTRACT FROM AUTHOR]