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.

A cytogenetic and morphometric study of four populations of characid fish assigned to the "complex" of Astyanax scabripinnis originating from three riverine basins, namely Ivaí (populations A and B), Paranapanema (C) and Paraná (D), was carried out. Karyological analysis showed the diploid chromosome number 2n = 48 for population B and 2n = 50 for the remaining populations. All populations under study differed in the composition of karyotypes and C-banding patterns. Canonical variable analysis used to assess the morphometric data revealed that 1) the four populations under study were entirely discriminated from each other and 2) the coefficients that most contributed to this pattern were standard length, rostrodorsal distance, eye diameter and snout length. The results indicate that the differences in karyotypes were paralleled by the morphological differentiation of the populations under study and contributed to the problem of systematics of A. scabripinnis "complex".
Foram realizados estudos citogenéticos e morfométricos de quatro populações de peixes characídeos, designados de" complexo" de Astyanax scabripinnis, pertencentes a três bacias hidrográficas diferentes: bacia do Ivaí (populações A e B), do Paranapanema (população C) e do Paraná (população D). A análise cariotípica mostrou um número diplóide de 2n = 48 cromossomos para a população B e de 2n = 50 cromossomos para as demais populações. Todas as populações analisadas diferiram na composição cariotípica e no padrão de banda C. A análise de variável canônica, aplicada aos dados morfométricos, revelou que: 1) as quatro populações estudadas foram totalmente discriminadas entre si, e 2) os coeficientes que mais contribuíram para isto foram o comprimento padrão, distância rostro-dorsal, diâmetro do olho e comprimento do focinho. Portanto, estes resultados indicam que as diferenças cariotípicas foram acompanhadas pelas diferenças morfológicas nas populações e contribuíram para o problema da sistemática do "complexo" A. scabripinnis

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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.

Chromosomes of Astyanax scabripinnis from the Tatupeba stream, Ivaí Basin (state of Paraná, Brazil), were analyzed. Astyanax scabripinnis population presents 3 different diploid numbers (2n = 46, 48 and 50) and B chromosomes in each cytotype. Eighty per cent of the females among individuals of cytotype I (2n = 50) has a metacentric B macrochromosome, whereas three different types of B chromosomes were identified in individuals of cytotype II (2n = 48). Cytotype III (2n = 46) showed two B chromosomes of different morphologic types (metacentric macrochromosomes and acrocentric) in all specimens and cells analyzed. Constitutive heterochromatin pattern for the three cytotypes showed weak markings in centromeric regions and conspicuous blocks in the telomere regions of ST and A chromosomes. Whereas C-banding showed that B chromosomes were totally or partially heterochromatic, a discussion on their behavior and origin was also undertaken.

Cytogenetic studies of three species of the Pimelodidae family were carried out. The results showed a diploid chromosome number of 2n =46 for Pimelodella sp1, 2n = 52 for Pimelodella sp2 and 2n = 50 for Pinirampus pirinumpu. Chromosome polymorphism was detected by of G‐banding analysis in Pimeladella spl and by C‐banding in P. pirinampu. In Pimelodella spl the 12th chromosome pair presented three G‐bands on the long arm while its homologous showed only two G‐bands. The 21st chromosome pair of P. pirinampu showed centromeric and interstitial heterocromatin block on one of the homologues and absence of an interstitial on other. However, the latter presented a telomeric band. In both cases the polymorphism was detected in all specimens analyzed. A deletion and an inversion rearrangement are suggested, respectively. A single NOR system was observed for three species. Considerations about the karyotype variability and evolution of the Pimelodidae family are presented.

Cytogenetic analysis of two populations of Astyunax scabripinnis from the Yukatan stream (Ivaí river basin) and from the Água do Rancho river (Paraná basin) revealed the presence of extra chromosomes of 2 different types. The population from the Yukatan stream presented a diploid number of 2n = 50 (3M, 15SM, 2ST and 5A) and a large metacentric B macrochromosome similar in size to the first in the complement in 100%1 of the cells analyzed (2n = 51) in 31% of the females. The C‐banding pattern of this chromosome was partially heterochromatic. The population from the Água do Rancho river presented a diploid number of 2n = 50 (3M, 14SM, 8A) and a B microchromosome in 32% of the females analyzed, with a range of 0‐2 chromosomes per metaphase. These chromosomes were found to be fully heterochromatic when submitted to C banding. Both populations presented extra chromosomes exclusively found in females. Aspects related to the morphology of these B chromosomes and a hypothesis for their origin are discussed.

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