Cytotoxicity of metals is important because some metals are potential mutagens able to induce tumors in humans and experimental animals. Chromium can damage DNA in several ways, including DNA double strand breaks (DSBs) which generate chromosomal aberrations, micronucleus formation, sister chromatid exchange, formation of DNA adducts and alterations in DNA replication and transcription. In our study, water samples from three sites in the Córrego dos Bagres stream in the Franca municipality of the Brazilian state of São Paulo were subjected to the comet assay and micronucleus test using erythrocytes from the fish Oreochromis niloticus. Nuclear abnormalities of the erythrocytes included blebbed, notched and lobed nuclei, probably due to genotoxic chromium compounds. The greatest comet assay damage occurred with water from a chromium-containing tannery effluent discharge site, supporting the hypothesis that chromium residues can be genotoxic. The mutagenicity of the water samples was assessed using the onion root-tip cell assay, the most frequent chromosomal abnormalities observed being: c-metaphases, stick chromosome, chromosome breaks and losses, bridged anaphases, multipolar anaphases, and micronucleated and binucleated cells. Onion root-tip cell mutagenicity was highest for water samples containing the highest levels of chromium.

In freshwater Neotropical fishes, supernumerary chromosomes or B chromosomes have been found in 61 species so far, distributed in 16 families of seven different orders and in distinct hydrographic basins. The order Characiformes is the one that possesses the most species with these chromosomes, including 31 species from six different families. The order Siluriformes has 21 species from five families, with Rhamdia, of the family Heptapteridae, being the most common genus where these chromosomes are found. The order Perciformes has five species with B chromosomes in only one family, Cichlidae, and the orders Beloniformes, Cyprinodontiformes, Gymnotiformes and Synbranchiformes contain only one species each with extra chromosomes. They are found as microchromosomes in Schizodon, Astyanax, Moenkhausia, Cyphocharax, Steindachnerina, Prochilodus, Rhamdia, Iheringichthys and Loricaria, among others, up to supernumerary macrochromosomes as in Astyanax, varying in morphology as well as number, showing evidence of the wide diversity of B chromosomes in Neotropical fishes.

SUMMARY -In fish cytogenetics the methods of chromosome preparations still need to be improved in order to obtain good metaphase figures for banding technics and more accurate analysis. In this paper we describe a very fast and simples culture method from solid fish tissues. Results of its application to different species as well as about 5-BrdU incorporation, both for chromosome banding and sister chromatid differentiation are reported. The data emphasized the advantage of this short term culture method wich provide excellent chromosome preparations suitables for basic and applied cytogenetic studies.

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Abstract-Fifty three species (48 species valid) belonging to the Pimelodidae, Heptapteridae and Pseudopimelodidae families have been studied cytogenetically, and in the present paper the chromosome number, the presence of B chromosomes and other relevant data were examined. The diploid number varies from 2n=42 to 2n=58 and were detected three cases of triploids with 3n=87 in Rhamdia species. The karyotypes show a high fundamental number because there are constituted predominantly by meta/submetacentric chromosomes.

The occurrence of natural triploidy in two specimens of Astyanax scabripinnis collected in the Araqua river and in the C6rrego das Pedras stream, respectively, is noticed. The triploid specimen from the Araqua river presented one macro B-chromosome and the triploid specimen from C6rrego das Pedras stream presented two macro B-chromosomes. The C-banding performed in the specimen from the Araqua river confirmed the triploidy and showed that the macro B-chromosome was entirely heterochromatic. Ag-NOR characterization showed two chromosomes involved in the nucleolar activity in both specimens. Some aspects related to the origin of triploid fishes are described.

A cytogenetic analysis of thirteen specimens of Bryconamericus aff. Iheringii revealed a diploid number of 52 chromosomes and a karyotype of 8M+22SM+10ST+12A with a fundamental number (FN) of 92. The nucleolus organizer regions (NORs) were studied by means of AgNO 3 and CMA 3 staining as well as by FISH using an 18S rDNA probe. NORs were found to be located at the terminal position on the short arm of the submetacentric chromosome pair. C-banding showed strong telomeric and centromeric staining in the majority of the chromosomes, and a similar pattern was observed after treatment with AluI restriction enzyme

Only 33 species among about 300 belonging to the families Pimelodidae and Rhamdiidae have been studied cytogenetically. The diploid number varies from 2n = 46 to 2n = 63 chromosomes, with the karyotypes often being of the meta/submetacentric type. As a result, there is generally a very elevated fundamental number.