Abstract
Archolaemus, one of the five genera of Neotropical freshwater fish of the family Sternopygidae (Gymnotiformes), was long considered a monotypic genus represented by Archolaemus blax. Currently, it consists of six species, most of them occurring in the Amazon region. There are no cytogenetic data for species of this genus. In the present study, we used classical cytogenetics (conventional staining and C-banding) and molecular cytogenetics (probes of telomeric sequences and multigenic families 18S rDNA, 5S rDNA, and U2 snDNA) to study the karyotype of Archolaemus janeae from Xingu and Tapajós rivers in the state of Pará (Brazil). The results showed that the two populations have identical karyotypes with 46 chromosomes: four submetacentric and 42 acrocentric (2n = 46; 4m/sm + 42a). Constitutive heterochromatin occurs in the centromeric region of all chromosomes, in addition to small bands in the interstitial and distal regions of some pairs. The 18S rDNA occurs in the distal region of the short arm of pair 2; the 5S rDNA occurs in five chromosome pairs; and the U2 snDNA sequence occurs in chromosome pairs 3, 6, and 13. No interstitial telomeric sequence was observed. These results show karyotypic similarity between the studied populations suggesting the existence of a single species and are of great importance as a reference for future cytotaxonomic studies of the genus.