Summary The ribosomal RNA genes of Pinirampus pirinampu have been localized by fluorescence in situ hybridization (FISH) with 18S rDNA probe. This method allowed the location of FITC signals on the short arm of the biggest subtelocentric chromossomic pair and evidenced a size heteromorphism of hybridization signals between homologous chromosomes. This difference has been attributed to variation in the amount of rRNA genes per cluster, probably due structural modifications (i.e. duplications by unequal crossing over or transpositions) of NOR involving homologous segments. Key words Pinirampus pirinampu, Chromosomes, FISH, rRNA genes.Genes of ribosomal RNA are arranged in long tandem repeated segments, which each gene unit presents conserved coding regions for the 18S, 5.8S and 28S RNAs, separated by intergenic spacers or IGS (Miller et al. 1983). The chromosomal location and activity of the NORs are easily demonstrated by silver staining, once the AgNO3 react with acid proteins associated with rDNA (Jordan 1987). These regions are important chromosome markers, used in animal and plant studies (Almeida-Toledo et al. 1996). Variation on the number, position and size of Ag-NORs patterns have been found in different fish groups, which can be at intra-and/or interspecific levels (Almeida- Toledo et al. 1985, Pendas et al. 1993a, b, Rossi et al. 1997.Treatment by specific fluorochromes, such as chromomycin A3 (CMA3) and/or Mithramycin, has been commonly employed to localize some GC-rich heterochromatin (Schmid 1980, Schweizer 1980, has been also used to evidence NORs since these regions are associated to GC-rich DNA families (Schmid and Guttenbach 1989). In Pinirampus pirinampu of Tibagi's river, the NOR pair is the first subtelocentric one and silver impregnation has evidenced a conspicuous size heteromorphism among the homologues (Swarca et al. 1999). When stained with CMA3 several blocks have been found distributed on the centromeres or telomeric regions. In the last case the fluorochrome binds besides the Ag-NOR bearing chromosome regions, evidencing differences in homologue size (Swarca et al. 1999). Furthermore, the most precise procedure that reveals all the chromosomes involved in nucleolar organization as well the exact location of the ribosomal genes is represented by the fluorescence in situ hybridization (FISH) with rDNA probes. The aim of this communication is to verify the nature of the NOR polymorphism previously reported in P pirinampu as well as to the determined the precise location and relative amount of rDNA loci by FISH.
Material and methodsSpecimens of Pinirampus pirinampu were collected at Tibagi river, Parana, Brazil. The specimens were deposited at the Zoological Museum of the Universidade Estadual de Londrina, Parana, * Corresponding author , e-mail: anadias@uel.br Brazil, and received the numbers 1150 and 1151. Mitotic chromosomes were prepared from pooled cephalic kidney. The 18S rDNA segment containing 1700 by of the fish Oreochromis niloticus was used for in situ hybridization and labeled ...