FLUORESCENCE IN-SITU HYBRIDIZATION WITH RDNA PROBES ON CHROMOSOMES OF2 NUCLEOLUS ORGANIZER REGION PHENOTYPES OF A SPECIES OF EIGENMANNIA (PISCES, GYMNOTOIDEI, STERNOPYGIDAE)

Nucleolus organizer regions (NORs) were analysed in two related and ge ographically close populations of Eigenmannia sp.1 (Pisces, Gymnotoide i, Sternopygidae) using silver staining and fluorescence in site hybri dization (FISH). The two populations differed in their AS-NOR phenotyp es, displaying fixed differences in the NOR-bearing chromosome pairs. FISH with rDNA probes showed that these differences were due to the lo cation of rDNA cistrons. This finding, showing fixed NOR differences b etween two populations belonging to the same species in a connected ri ver system, is highly significant in terms of evolutionary change, pos sibly indicating an initial step of genetic differentiation. This resu lt also has important implications from the cytosystematic point of vi ew, as NORs usually have a very constant karyotypic location in fish s pecies and have been used as species-specific chromosome markers.