Abstract
A cytogenetic analysis by conventional Giemsa staining, silver staining, C-banding, and fluorescence in situ hybridization (FISH) was carried out on Brycon amazonicus from Caicara del Orinoco, Venezuela. The karyotype of this species is characterized by the presence of 2n = 50 chromosomes, a karyotypic formula 22m+14sm+14st, and a fundamental number of 100 chromosomal arms. Nucleolar organizer regions (NORs) and 18S rDNA genes are located in the terminal regions of the long arms of the second pair of subtelocentric chromosomes, corresponding to pair 13. C-banding revealed heterochromatin distribution in only seven chromosome pairs. The largest metacentric pair (number 1) possesses a paracentromeric block equilocally distributed in both chromosome arms, whereas in pairs 12 to 17 positive C band blocks were located in the paracentromeric region of the long arm, close to the centromere. Analysis performed with 5S rDNA gene revealed a terminal site located on the short arm of one small submetacentric chromosome (pair 15) corroborating previous studies with this repetitive gene showing an apparent conservation of 5S rDNA in the genome of these fish species. The data obtained in this study reinforce the chromosomal conservativeness in the species of the genus Brycon, related not only to the macro-chromosomal structure (diploid number, karyotypic formula, and fundamental number), but also to the repeated DNAs, such as heterochromatic regions and ribosomal DNAs. These data will contribute to a better understanding of chromosomal evolution in both Brycon and Characidae fishes.
Highlights
Characidae is the largest family of Characiformes and comprises about 55% of freshwater fishes of this group (Fink, Fink, 1981)
The largest metacentric pair possesses a paracentromeric block equilocally distributed on both chromosome arms, whereas in pairs 12 to 17 of the submetacentric series positive C-band blocks were located in the paracentromeric region of the long arm, close to the centromere (Fig. 1, b)
The presence of a large metacentric chromosome pair seems to be a common feature for species in the genus Brycon and has been perceived as a chromosome marker in the Bryconinae (Parada et al, 2003; López et al, 2008)
Summary
Characidae is the largest family of Characiformes and comprises about 55% of freshwater fishes of this group (Fink, Fink, 1981). Due to their great adaptive capacity, a large variety of species presenting distinct shape and life habits, which probably form a polyphyletic group (Nelson, 2006), are found within this family. Distributed from Southern Mexico to Argentina, Bryconinae is a Characidae subfamily composed of 43 species subdivided in three genera, among which Brycon Müller et Troschel, 1844 is the most diverse with 41 species until now. Human activities have threatened or led to the extinction of some populations (Braga, 1982), and conservation programs are badly needed
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