Mapping of the 18 S and 5 S ribosomal RNA genes in Astyanax altiparanae Garutti & Britski , 2000 ( Teleostei , Characidae ) from the upper Paraná river basin , Brazil

Fluorescence in situ hybridization (FISH) was undertaken in order to determinate the chromosomal distribution pattern of 18S and 5S ribosomal DNAs (rDNA) in four populations of the characid fish Astyanax altiparanae from the upper Paraná river basin, Brazil. The 18S rDNA probe FISH revealed numerical and positional variations among specimens from the Keçaba stream compared to specimens of the other populations studied. In contrast to the variable 18S rDNA distribution pattern, highly stable chromosomal positioning of the 5S rDNA sites was observed in the four A. altiparanae populations. Divergence in the distribution pattern of 18S and 5S rDNA sites is also discussed.


Introduction
Piscine nucleolar organizer regions (NORs) have been extensively analyzed using silver nitrate staining (Ag-NOR) due to the simplicity of this technique.According to Miller et al. (1976) this methodology detects only the nucleolar regions that were active in the preceding interphase, and is most suitable for the study of NOR expression.Fluorescence in situ hybridization (FISH) is the best method for characterizing NORs for determining the location of both active and inactive ribosomal DNA (rDNA) and almost always allows detection of a larger number of NORs than can be detected using Ag-NOR banding and is also more precise in identifying NORs.In higher eukaryotes the rDNA is organized into two distinct gene classes, the major class (45S rDNA) transcribing 18S, 5.8S, and 28S rRNA genes and the minor class (5S rDNA) that transcribes 5S rRNA genes.The 45S rDNA active sites have shown to have positional coincidence with chromosome NORs but the 5S rDNA sites are unrelated to NORs.
In Astyanax altiparanae, previously known as Astyanax bimaculatus for the upper Paraná river in Brazil (Garutti and Britski, 2000), cytogenetic studies in different populations have shown a constant diploid number of 2n = 50 chromosomes, although with differences in their karyotype formulae and with regard to number and position of NORs (Daniel-Silva and Almeida-Toledo, 2001;Pacheco et al., 2001;Fernandes and Martins-Santos, 2004).
Multiple Ag-NORs have been a common characteristic in A. altiparanae, with the number reaching 10 NOR-bearing chromosomes for an A. altiparanae specimen from the Índios river in the Brazilian state of Paraná (Fernandes and Martins-Santos, 2004).
In the study described in this paper, FISH was used to determine the chromosomal location of 18S and 5S rDNA sites in four A. altiparanae populations with the aim of contributing to the better understanding of the genomic organization of this species.

Results and Discussion
The four A. altiparanae populations revealed a monomorphic macrokaryotype constitution, with 2n = 50 chromosomes (6 M, 26 SM, 6 ST and 12A).Thus, specimens of A. altiparanae from Tatupeba, Keçaba and Maringá streams presented karyotype formulae identical to the A. altiparanae specimens from the Paraná river previously studied by Fernandes and Martins-Santos (2004).
The 18S-FISH technique revealed a bright fluorescence signal spread at the telomeric region of seven chromosomes (the 2A short arm and five other chromosomes) for Keçaba stream specimens and the telomeric region of four chromosomes (the 2A short arm and two other chromosomes) for Paraná river, Tatupeba and Maringá stream specimens (Figure 1).Almeida-Toledo et al. (2002) also reported four chromosomes (2 A and 2 M) were marked with a 28S rDNA probe in A. altiparanae specimens, although in two metacentric chromosomes the probes were pericentromeric.The same chromosomal location of 45S rDNA (18S or 28S rDNA) on the short arm of 2 acrocentric A. altiparanae chromosomes was seen in our present study and was also observed for specimens from the Mogi-Guaçu river in the Brazilian state of Paraná (Almeida-Toledo et al., 2002), indicating that these are marker chromosomes for this species.On the other hand, the other sites seem not to be conserved among A. altiparanae populations, which differ in the position (telomeric or pericentromeric) and type of chromosomes.Numerical and positional variations of the 18S rDNA sites reported in specimens of A. altiparanae from the Keçaba stream in comparison to the other populations analyzed have also been recorded in other Astyanax species, including A. scabripinnis (Ferro et al., 2001;Souza et al., 2001;Mantovani et al., 2005;Fernandes and Martins-Santos, in press) and Prochilodus lineatus (Jesus and Moreira-Filho, 2003).According to Schweizer and Loidl (1987), the proximity of telomeric regions within interphase nuclei would facilitate genetic material transference as predicted by Rabl's model.In distinct A. scabripinnis populations this model has been suggested to explain heterochromatin dispersion in the telomeric regions (Souza et al., 1996;Mantovani et al., 2000;Fernandes and Martins-Santos, 2003).Therefore, the telomeric location of the 18S rDNA sites in the four A. altiparanae populations would facilitate transference events, which seems to have occurred in the case of A. altiparanae from the Keçaba stream.
Sequential Ag-staining of an 18S-FISH slide of a specimen from the Tatupeba stream revealed that of the four marked chromosomes three were Ag-NOR positive (Figure 3a events or unequal crossing-over and not the differential expression of NORs. In contrast to the variability detected regarding the 18S rDNA distribution pattern, we observed a highly conserved chromosomal position of 5S rDNA sites in the four A. altiparanae populations.The 5S-FISH method revealed bright fluorescence signal spread over the pericentromeric region of a single, probably submetacentric, chromosomal pair (Figure 2).Considering that 5S rDNA sequences were not localized in the terminal regions of chromosomes the events that dispersed the 18S rDNA may not have been acting upon the 5S rDNA sites.Moreover, the 5S rDNA interstitial position has been found in most species of several orders.For these reasons, the highly conserved chromosomal position of 5S rDNA sites observed in the four A. altiparanae populations may have derived from the interstitial localization of these sites in the chromosomes.The 5S rDNA genes situated in a single chromosomal locus have also been identified in A. altiparanae and A. lacustris (Almeida-Toledo et al., 2002) and other piscine species, including the Atlantic salmon (Pendás et al. 1994), Anguilla anguilla (Martinez et al. 1996), Prochilodus lineatus (Jesus and Moreira-Filho, 2003), Neoplecostomus microps and Harttia loricariformis (Kavalco et al. 2004), possibly corresponding to a more ancestral condition in fishes.
Sequential Ag-staining of 5S-FISH slides of A. altiparanae specimens from Maringá (Figure 3c, d) and Keçaba (Figure 3e, f) streams revealed that 5S rDNA was not located on the same Ag-NOR chromosomes.Therefore, investigations utilizing double FISH with the two rDNA probes should be carried out in order to prove the different chromosomal location of 18S and 5S rDNA in these specimens.Different chromosomal sites for NOR and 5S rDNA have also been reported for Anguilla anguilla (Martinez et al. 1996), Salmo trutta (Moran et al. 1996), Leporinus elongatus, Leporinus obtusidens and Leporinus friderici (Martins and Galetti 1999), Oreochromis niloticus (Martins et al. 2000) and A. scabripinnis (Fernandes and Martins-Santos, in press).According to Lucchini et al. (1993) and Suzuki et al. (1996), this arrangement is frequently observed in vertebrates.However, Almeida-Toledo et al. (2002) detected in situ signals for the major rDNA (28S rDNA) co-localized with the 5S rDNA clusters in the pericentromeric region of one marker chromosome in five Astyanax species and Mantovani et al. (2005) used double FISH to show that the 45S and 5S rDNA loci were syntenic in an A. scabripinnis chromosome.
There are still only a few studies which have used FISH to investigate the genus Astyanax, and the majority of these studies have been limited to A. scabripinnis (Souza et  al., 2001;Ferro et al., 2001;Mantovani et al., 2005;Fernandes and Martins-Santos, in press).In A. altiparanae, only one Mogi-Guaçu river population (Almeida-Toledo et al., 2002) and the populations analyzed in the present study have been reported as utilizing the FISH technique with rDNA probes.Our results are important for the better characterization of the chromosomal location of Astyanax altiparanae 5S, 18S or 28S rDNA and may also aid cytotaxonomic studies of related species.

FernandesFigure 1 -
Figure 1 -Fluorescence in situ hybridization (FISH) Astyanax altiparanae metaphases showing the chromosomal location of 18S rDNA sites in populations from the Keçaba stream (a), Paraná river (b), Tatupeba stream (c) and Maringá stream (d).The arrows indicate the ribosomal cistrons carrier chromosomes and the arrowheads the ribosomal cistrons in the short arm of 2 acrocentric chromosomes of four populations.
Figure 2 -Fluorescence in situ hybridization (FISH) Astyanax altiparanae metaphases showing the chromosomal location of 5S rDNA sites in populations from the Maringá stream (a), Paraná river (b), Keçaba stream (c) and Tatupeba stream (d).The arrows indicate the ribosomal cistrons carrier chromosomes.