POST-ZYGOTIC MODIFICATIONS AND INTRA-INDIVIDUAL AND INTER-INDIVIDUAL NUCLEOLAR ORGANIZING REGION VARIATIONS IN FISH - REPORT OF A CASE INVOLVING LEPORINUS-FRIDERICI

Silver nitrate staining, a rapid and efficient method, has proven to b e excellent for nucleolar organizing region (NOR) studies in fish. Som e fish appear to have only two NOR-bearing chromosomes in their karyot ype, whereas others probably have several. In the present study we ana lyzed the NORs of Leporinus friderici, a species that, on the basis of previous studies, has been considered as representative of species wi th NORs carried by a single chromosome pair. The analyses were perform ed by a combination of three methods, i.e. silver nitrate staining, st aining with the GC-specific fluorochrome chromomycin A(3), and in site hybridization with digoxigenin-labeled probes. The results showed tha t, although more frequent and conspicuous in a single chromosome pair, the NORs of this species are present in multiple chromosomes. Intra- and inter-individual variations observed by the three methods strongly suggest the occurrence of post-zygotic modifications involving NORs. NOR identification in fish, almost exclusively performed by the silver nitrate method, is currently being re-evaluated by methods such as ch romomycin A(3) staining and in situ hybridization, which may provide i mportant information leading to a better understanding of chromosome e volution in these animals.