Analyses of the Structure of NORs in Two Species of South American Sorubiminae Fishes (Siluriformes) by Means of Several Cytogenetic Techniques

Several cytogenetic techniques (AgNOR, C- G- and RE bandings, DAPI, CMA₃ and FISH) were applied in order to analyze the structure and variability of NORs in the fish species Steindachneridion melanodermatum and S. scripta. Ag-NORs were observed on the short arm of the first acrocentric chromosome pair, coincidentally with a strong C-positive band on a large secondary constriction. In addition, NORs showed bright fluorescent signals when stained with CMA₃ and treated for FISH with rDNA 18S. However, they showed negative coloration after G- and restriction enzyme banding and DAPI staining. The results evidence a substantial size polymorphism in these regions. The NOR bearing chromosomes in both species may be considered homologues because they maintain conserved characteristics, such as being interspersed with a GC-rich heterochromatin and possessing target sequences for Alu I, BamHI and EcoRI.