A cytogenetic analysis based on the integration of a number of different chromosomal methodologies, including chromosome microdissection was carried out to characterize the chromosomally polymorphic Hypostomusregani population from the Paraguay River basin, state of Mato Grosso do Sul in Brazil. All specimens had 2n=72 (FN=116) but two distinct karyotype formulas: karyomorph A (12m+14sm+18s+28a) and karyomorph B (13m+14sm+17st+28a). Karyomorph A and B differed only for pair 19 that consisted of two subtelocentrics in karyomorph A and a large metacentric and a subtelocentric in karyomorph B. This heteromorphism was due to extensive heterochromatinization of the short arm of the large metacentric, as highlighted by C-banding. The microdissection of the large metacentric of pair 19 allowed the production of a probe, named HrV (Hypostomusregani Variant), that hybridized to the whole p arm of the large metacentric and the pericentromeric region of the short arm of its (subtelocentric) homologue (karyomorph B) and of both homologs of pair 19 in karyomorph A. Additional cytogenetic techniques (FISH with 18S and 5S rDNA probes, CMA3 and DAPI staining) allowed a finer distinction of the two karyomorphs. These results reinforced the hypothesis that the novel large metacentric of H.regani (karyomorph B) was the result of the amplification of heterochromatin segments, which contributed to karyotypic diversification in this species.
Apteronotus include a large number of recognized species, but few have been cytogenetically studied. This study establishes the first cytogenetic description of A. ellisi collected from the upper Paraguay River basin, which presented 2n=52 chromosomes, karyotype composed of 20 metacentric, 20 submetacentric, eight subtelocentric and four acrocentric chromosomes, and fundamental number as 100 for both sexes. Heteromorphic sex chromosomes were absent. A pair of nucleolar organizing regions (NORs) was detected in the submetacentric chromosome pair 13 by silver-staining. Heterochromatic regions were observed in the long arms of the NORbearing chromosome pair. Besides the present data are valuable to help in understanding karyotypic evolution in Apteronotidae. Data from NORs confirmed the tendency of this family in presenting simple NORs sites, similar to the other Gymnotiformes clades. Yet, the presence of little heterochromatin can be used as cytogenetic markers for A. Ellis, and centric fusions/fissions appear to be an important mechanism in the karyotype evolution and differentiation among Apteronotus species.
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