Polymorphism of the nucleolus organizer regions (NORs) in Physalaemus petersi (Amphibia, Anura, Leptodactylidae) detected by silver staining and fluorescence in situ hybridization.

The nucleolus organizer regions (NORs) of both karyotypes I and II of Physalaemus petersi (Jiménez de la Espada, 1872) from the Brazilian Amazon were studied by Giemsa staining, and by the Ag-NOR method. Karyological group I specimens were also studied by the fluorescence in situ hybridization (FISH) technique. Multiple NOR-bearing chromosomes were detected in both karyotypes. The coincident results of the Ag-NOR and FISH methods rule out the occurrence of silent NORs in this anuran. There was no intraindividual NOR variability in either group, but interindividual variability of NORs was high in group I. Seven different patterns of active NOR distribution were definitely recognized among fifteen specimens. This was considered to be a NOR site polymorphism. These results, combined with the C-band polymorphism previously reported for P. petersi, demonstrate a high rate of chromosome evolution in this group.