Chromosomal location of 5S and 18S rRNA genes in Prochilodus lineatus (Characiformes, Prochilodontidae)

Nucleolar organizer regions (NORs) have been extensively studied in fish through the classic cytogenetic technique of staining by silver nitrate. However, the inconvenience of this methodology is that it detects only NORs that were active in the preceding interphase. Several recent studies have reevaluated nucleolar regions using the isotopic or non-isotopic in situ hybridization methodology to locate these ribosomal genes in the genome of different species. In the study reported on herein, the physical location of the 18S rDNA was identified in the chromosomes of Prochilodus lineatus by fluorescent in situ hybridization (FISH), in a search for possible additional nucleolar organizer regions not detected by conventional techniques. FISH analysis with 18S ribosomal DNA revealed the presence of additional ribosomal cistrons in relation to the earlier described for the species through the silver nitrate, suggesting the occurrence of numerical and structural polymorphism in this class of ribosomal DNA. The 5S rDNA sites were also investigated through FISH, evidencing the occurrence of a single locus for this genic family, together with an organization sintenic to the commonly active 18S rDNA sites.