Mapping 18S ribosomal genes in fish of the genus Brycon (Characidae) by fluorescence in situ hybridization (FISH)

The present study provides data on the nucleolar organizer regions (NORs) o f seven Brycon species based on mapping of the 18S rRNA genes by fluorescen ce in situ hybridization (FISH). Fluorescent signals were observed on the t elomere of the long arm of two large submetacentric chromosomes, thus confi rming the number and location of NORs previously revealed by other classica l cytogenetic techniques. although there were no inter- or intra-individual variations in the number and location of the 18S loci, NOR size polymorphi sm was observed between homologous chromosomes. The clustering and conserva tion of NORs in a single chromosome pair indicates a high level of NOR stab ility among species of the genus Brycon.