Chromosomal novelties and the level of meiotic and mitotic abnormalities were studied in a hybrid zone between two chromosomally differentiated Spalax cytotypes of 2n = 58 and 2n = 52. These cytotypes differ by five Rb fusions, four centromeric shifts accompanied by heterochromatin deletion, one paracentric inversion, and the Y-chromosome reorganization. Among 149 specimens studied, 82 were hybrids with 64 different karyotypes ranging in diploid numbers from 2n = 50 to 2n = 60. Nine hybrid specimens were mosaics for the chromosome numbers due to occurrence of cell lines with different Robertsonian chromosome arrangements, and six specimens possessed variable number of B-chromosomes. Mosaicism of B-chromosomes was found also in meiotic cells however chromatid breaks and abnormal chromosome pairing during meiosis occurred very rarely. All these results imply some local genomic instability resulting in the spontaneous process of reversible Rb fusions.
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