| Summary: | <p>Abstract</p> <p>Background</p> <p>Cytogenetic studies were carried out on samples of <it>Eigenmannia virescens </it>(Sternopygidae, Gymnotiformes) obtained from four river systems of the Eastern Amazon region (Para, Brazil).</p> <p>Results</p> <p>All four populations had 2n = 38, with ZZ/ZW sex chromosomes (Z, acrocentric; W, submetacentric). Constitutive heterochromatin (CH) was found at the centromeric regions of all chromosomes. The W chromosome had a heterochromatic block in the proximal region of the short arm; this CH was positive for DAPI staining, indicating that it is rich in A-T base pairs. The nucleolar organizer region (NOR) was localized to the short arm of chromosome pair 15; this result was confirmed by fluorescent <it>in situ </it>hybridization (FISH) with human 45S rDNA, and CMA<sub>3 </sub>staining indicated that the region is G-C rich. FISH with telomeric probes did not show any evidence of interstitial telomeric sequences (ITS).</p> <p>Conclusion</p> <p>Previous studies have shown that the species <it>Eigenmannia </it>sp. 2 and <it>E. virescens </it>have differentiated sex chromosomes, and diverse sex chromosome systems have been described for <it>E. virescens </it>specimens obtained from different Brazilian rivers. A comparative analysis of the present data and prior reports suggests that the sex chromosomes of <it>Eigenmannia </it>may have arisen independently in the different populations.</p>
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