Extensive polymorphism and chromosomal characteristics of ribosomal DNA in the characid fish Triportheus venezuelensis (Characiformes, Characidae)

Mauro Nirchio,Angel Granado,Irani Alves Ferreira,Ernesto Ron,Claudio Oliveira

doi:10.1590/s1415-47572007000100007

Abstract

The karyotype and chromosomal characteristics of the characid fish Triportheus venezuelensis were investigated using differential staining techniques (C-banding, Ag-NOR staining) and fluorescent in situ hybridization (FISH) with an 18S rDNA probe. The diploid chromosome number (2n = 52), karyotype composition and sex chromosome determination system of the ZZ/ZW type were the same as previously described in other species of the genus Triportheus. However, extensive variation regarding nucleolus organizer regions (NOR) different from other species was observed. 18S rDNA sequences were distributed on nine chromosome pairs, but the number of chromosomes with Ag-NORs was usually lower, reaching a maximum of four chromosomes. When sequential staining experiments were performed, it was demonstrated that: 1.) active NORs usually corresponded to segments with 18S rDNA genes identified in FISH experiments; 2.) several 18S rDNA sequences were not silver-stained, suggesting that they do not correspond to active NORs; and 3.) some chromosomes with silver-stained regions did not display any 18S rDNA signals. These findings characterize an extensive polymorphism associated with the NOR-bearing chromosomes of T. venezuelensis and emphasize the importance of combining traditional and molecular techniques in chromosome studies.

Highlights

Two distinct sets of multiple rRNA genes, usually located on distinct chromosomes, operate in eukaryote genomes: the 18S, 5.8S and 28S major genes and the 5S minor genes (Hadjiolov, 1985)
We confirmed this in T. venezuelensis, reinforcing the hypothesis that this group represents a monophyletic unit in the Characidae (Malabarba, 2004)
The heterochromatinization of the W chromosome in the Triportheus is thought to be associated with a reduction in the size of this chromosome during the evolution of the ZW sex chromosome system (Bertollo and Cavallaro, 1992; Artoni et al, 2001), but a robust phylogeny to test this hypothesis is still lacking for the group

Summary

Introduction

Two distinct sets of multiple rRNA genes, usually located on distinct chromosomes, operate in eukaryote genomes: the 18S, 5.8S and 28S major genes and the 5S minor genes (Hadjiolov, 1985). The eukaryotic major rRNA genes are grouped, in order to form RNA polymerase I transcription units, and multiple copies of these units are typically found clustered in long direct tandem arrays cytologically identified as the nucleolus organizer regions (NORs) (Hadjiolov, 1985; Drouin and Moniz-de-Sá, 1995). The technique most commonly used to detected NORs is the silver nitrate (Ag) impregnation method in which silver binds to NOR proteins such as the RNA polymerase I subunit which is part of the active site of ribosomal genes (Roussel and Hernandez-Verdun, 1994; Whitehead et al, 1997). Silver nitrate may bind to other proteins present in the nuclei, some chromosome structures visualized by silver nitrate may not correspond to ribosomal genes (Dobigny et al, 2002)

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Conclusion