Cytogenetic data on Astyanax jacuhiensis ( Characidae ) in the lago Guaíba and tributaries , Brazil

Cytogenetic analyses were performed in Astyanax jacuhiensis from lago Guaíba, Brazil. The diploid number was 50, with a karyotype composed of 8m+30sm+4st+8a chromosomes, FN = 92. The AgNORs were observed in 2 to 5 chromosomes, with intraand interindividual variation. The sm pair 8 observed always carried NORs on the short arms, presenting size heteromorphism between homologous. Fluorescence in situ hybridization (FISH) with an 18S rDNA probe only confirmed the location of ribosomal cistrons in the sm pair 8, and heteromorphism of these regions between the homologous chromosomes. C-banding revealed the occurrence of weak C-positive heterochromatin in the pericentromeric regions of several chromosomes, in addition to more evident bands interstitially located on some chromosome pairs and in the terminal region of the short arms in pair 8. C-banding plus CMA 3 revealed light fluorescent signals in different chromosomes of the karyotype, with a strong terminal site in pair 8, indicating the occurrence of several GC-rich heterochromatic regions in this species. Our results provide the first description of the Astyanax jacuhiensis karyotype, showing karyotype similarities when compared to various populations of A. altiparanae and A. bimaculatus, indicating that chromosomal features are very similar for these three species.

Astyanax jacuhiensis was first described as Tetragonopterus jacuhiensis (Cope, 1984).It was later transferred to the genus Astyanax (Fowler, 1906) and considered synonymous with A. bimaculatus (Eigenmann, 1921).This species was recently referred to as A. jacuhiensis (Lima et al., 2003), for the endemic population of the Jacuí River, RS, Brazil, corresponding to its typelocality.
The present study presents the first karyotype description of A. jacuhiensis, using AgNOR conventional and fluorochrome stainings, in addition to C-banding and fluorescence in situ hybridization (FISH) with 18S rDNA probe.
Silver nitrate staining showed 2 to 5 chromosomes with probable Ag-NORs sites: on the short arms of sm pair 8 displaying size heteromorphism, on the short arms of st pair 20, and on the long arm of one chromosome of m pair 4, which could correspond to system of multiple NORs.Pair 8 was the most frequent AgNOR-bearing chromosome, being visualized in all metaphases analyzed (Fig. 1b).However, the fluorescence in situ hybridization showed that only sm pair 8 displayed 18S rDNA sites, also confirming the size heteromorphism between homologous (Fig. 1c), thereby characterizing a simple NOR system in A. jacuhiensis.AgNOR heteromorphism is frequently seen in fish, and can be ascribed to variations in the number of copies of ribosomal cistrons between homologous through unequal crossing-overs, transposition or rearrangements, such as deletions and duplications (Galetti-Jr et al., 1995).The other chromosomal sites that were positive for silver nitrate staining, but have proved to be negative after FISH with 18S rDNA probe, may represent only a kind of heterochromatin with some affinity to silver nitrate, irrespective of whether they do or do not carry any site of rDNA.
Heterochromatin has been an important marker for the characterization and differentiation of several species and populations.Its distribution pattern in A. jacuhiensis resembled that found in some populations of A. altiparanae (Daniel-Silva & Almeida-Toledo, 2001;Fernandes & Martins-Santos, 2004), and A. bimaculatus (Paganelli, unpubl. data), where interstitial C-positive bands were also observed.In fact, in addition to discrete pericentromeric bands, A. jacuhiensis showed conspicuous interstitial C-positive blocks in several pairs of the karyotype (Fig. 2a), as well as in the terminal region of pair 8, in correspondence with the NOR location (Fig. 2b-c).
Up to four CMA 3 positive signals were found in A. jacuhiensis (Fig. 1d), where pair 8 was always seen with a strong terminal staining, and demonstrating the same heteromorphism observed with AgNORs and 18S rDNA sites.C-banding plus CMA 3 staining (CB+CMA 3 ) evidenced some pericentromeric and interstitial positive sites.However, pair 8 showed strong heteromorphic sites corresponding to the NOR regions (Fig. 2d).Therefore, A. jacuhiensis contains a class of GC-rich heterochromatin in some chromosome pairs and the ribosomal cistrons appear to be interspersed with this kind of heterochromatin.
Astyanax bimaculatus, from the Paraná River, has a karyotypic structure similar to that found in various populations of A. altiparanae (Jorge & Moreira Filho, 2001).In addition, A. jacuhiensis also showed similarities in the diploid number, karyotype formula, and chromosome banding when compared to various populations of A. altiparanae and A. bimaculatus, indicating that these three species are very similar in relation to their chromosomal features.It is important to note that these
cytotaxonomic data somehow supportEigenmann's proposal  (1921).However, further cytogenetic and taxonomic studies should be conducted to better clarify this question.