Triportheus fish species present 2n = 52 chromosomes. The karyotypes show similar macrostructure and a ZZ/ZW sex chromosome system, which probably represents synapomorphy for the genus. A probe of the Z chromosome was obtained from T. nematurus through microdissection, followed by unspecific amplification via DOP-PCR. This probe was used for WCP (whole chromosome painting) through fluorescent in situ hybridization (FISH) in several other Triportheus species, to analyze the differentiation of the ZZ/ZW system. The homology between this probe and chromosomes of species from other genera, putatively related to Triportheus, was also examined to search for evidence about evolution of their sex chromosomes. Complete homology was found among the Z chromosomes of all Triportheus species, while only small positive signals were found on the W chromosomes. Hybridization signals were absent in species from other genera. The present results reinforce both the conservative nature of Z chromosomes and the hypothesis that the ZZ/ZW sex chromosome system is a synapomorphic feature of Triportheus. On the other hand, besides reduction of size, W chromosomes have undergone accentuated composition changes in relation to Z chromosomes, since only a small region, usually located in the short arm, kept homology with the Z chromosomes.

The location of 18S and 5S rDNA sites was determined in eight species and populations of the fish genus Triportheus by using fluorescent in situ hybridization (FISH). The males and females of all species had 2n = 52 chromosomes and a ZZ/ZW sex chromosome system. A single 18S rDNA site that was roughly equivalent to an Ag-NOR was detected on the short arms of a submetacentric pair in nearly all species, and up to two additional sites were also observed in some species. In addition, another 18S rDNA cluster was identified in a distal region on the long arms of the W chromosome; this finding corroborated previous evidence that this cluster would be a shared feature amongst Triportheus species. In T. angulatus, a heterozygotic paracentric inversion involving the short arms of one homolog of a metacentric pair was associated with NORs. The 5S rDNA sites were located on the short arms of a single submetacentric chromosomal pair, close to the centromeres, except in T. auritus, which had up to ten 5S rDNA sites. The 18S and 5S rDNA sites were co-localized and adjacent on the short arms of a chromosomal pair in two populations of T. nematurus. Although all Triportheus species have a similar karyotypic macrostructure, the results of this work show that in some species ribosomal genes may serve as species-specific markers when used in conjunction with other putatively synapomorphic features.

Interspecific hybrids of fish can offer important advances in the zootechnical exploration of this animal group. However, hybrids can pose serious biological risks to the environment and natural populations, making genetic characterization and monitoring of hybrids produced in fish culture important concerns. In the present study, individuals of the hybrid ‘Piaupara’ were characterized by fluorescence in situ hybridization (FISH) using sex-specific probes to differentiate the respective parental contributions from female Leporinus macrocephalus (Piauçu) and male Leporinus elongatus (Piapara). The application of this technique demonstrated a difference in hybridization patterns between males and females of L. elongatus and L. macrocephalus, mainly due to the fact that the NOR-bearing chromosomes of L. elongatus contain some homology with the probe. This allowed us to detect a clear sex-specific distinction of the hybrid ‘Piaupara’, demonstrating that this technique is well-suited for the characterization of these hybrids. Moreover, this technique offers an important methodological advance for the study of diverse sex chromosome systems of Leporinus, supplying valuable information about the origin and establishment of a recently described sexual system in this genus.

In Prochilodus lineatus B-chromosomes are visualized as reduced size extra elements identified as microchromosomes and are variable in morphology and number. We describe the specific total probe (B-chromosome probe) in P. lineatus obtained by chromosome microdissection and a whole genomic probe (genomic probe) from an individual without B-chromosome. The specific B-chromosome was scraped and processed to obtain DNA with amplification by DOP-PCR, and so did the genomic probe DNA. Fluorescence in situ hybridization using the B-chromosome probe labeled with dUTP-Tetramethyl-rhodamine and the genomic probe labeled with digoxigenin-FITC permitted to establish that in this species supernumerary chromosomes with varying number and morphology had different structure of chromatin when compared to that of the regular chromosomes or A complement, since only these extra elements were labeled in the metaphases. The present findings suggest that modifications in the chromatin structure of B-chromosomes to differentiate them from the A chromosomes could occur along their dispersion in the individuals of the population.

Summary An X 1 X 2 Y sex chromosome system is reported for the first time in Gymnotus sp. The chromosome number observed was 2nϭ40 (14 M-SMϩ26 ST-A) in females and 2nϭ39 (15 M-SMϩ24 ST-A) in males, with the same fundamental number in both sexes (FNϭ54). The multiple sex chromosome system might have been originated by a Robertsonian translocation of an ancestral acrocentric Y-chromosome with an acrocentric autosome, resulting in a metacentric neo-Y chromosome observed in males. Single NORs were detected on the short arm of a middle-sized acrocentric chromosome pair. Constitutive heterochromatin was observed in the pericentromeric regions of several chromosome pairs, including the neo-Y chromosome and the NOR carrier chromosomes. The DAPI/CMA 3 stain revealed that all the pericentromeric heterochromatin are AϩT rich whereas the NORs were associated with GϩC rich base composition. The possible ancestral condition characterized by an undifferentiated Y-chromosome from all the Gymnotiformes fishes is discussed.