NUCLEOLUS ORGANIZER REGIONS AND B-CHROMOS OMES OF WOOD MICE (MAMMALIA, RODENTIA, APODEMUS)

Distribution of nucleolus organizer regions (NORs) in karyotypes was s tudied in 10 species of wood mice, including Apodemus flavicollis, A. sylvaticus, A. uralensis (= A. microps), A. fulvipectus (= A. falzfein i), A. ponticus, A. hyrcanicus, A. mystacinus, A. agrarius, A. peninsu lae, and A. speciosus. Peculiarities of NOR location in karyotypes can be used in interspecific diagnostics of wood mice. Intraspecific poly morphism of A. sylvaticus, A. agrarius, and A. peninsulae in terms of the number of NORs and their localization in chromosomes can serve as evidence for karyological differentiation in certain populations of th ese species. The minimum number of active NORs in mice of the genus Ap odemus is two to four. Two A. Flavicollis wood mice with karyotypes co ntaining one small acrocentric B-chromosome (2n = 49) were identified among animals captured in Estonia. In A. peninsulae, B-chromosomes wer e found among animals captured in the following regions: the vicinity of Kyzyl (one mouse with 17 microchromosomes, 2n = 65); the vicinity o f Birakan (two mice with one metacentric chromosome each, 2n = 49); an d in the Ussuri Nature Reserve (one mouse with five B-chromosomes, inc luding three metacentric and two dotlike chromosomes; 2n = 65). In the latter animal, the presence of NORs on two metacentric B-chromosomes was revealed; this is the first case of identification of active NORs on extra chromosomes of mammals.