Silent ribosomal cistrons are located at the pairing segment of the postreductional sex chromosomes of Apodemus sylvaticus (Rodentia, Muridae)

We analysed the karyotype of the rodent species Apodemus sylvaticus by G- a nd C-banding, Ag-NOR-staining and in situ hybridization, with special atten tion to the sex chromosomes. NOR-bearing chromosome pairs were identified w ith simultaneous staining of G bands and NORs. In situ hybridization with a n rDNA probe revealed the presence of silent ribosomal cistrons in both sex chromosomes. Studies of meiosis demonstrated that these inactive ribosomal cistrons are located in the pairing segment, which occupies the proximal p ortion of the telocentric sex chromosomes, and may thus be involved in post reduction caused by an obligatory chiasma in this position.