Karyotype description and comparative chromosomal mapping of rDNA and U2 snDNA sequences in and (Teleostei, Gymnotiformes, Sternopygidae).

The genus Jordan et Evermann,1896 includes electric fishes endemic to the Neotropical region with extensive karyotype variability and occurrence of different sex chromosome systems, however, cytogenetic studies within this group are restricted to few species. Here, we describe the karyotypes of (Schreiner et Miranda Ribeiro, 1903) and (Reinhardt, 1852) and the chromosomal locations of 5S and 18S rDNAs (ribosomal RNA genes) and U2 snDNA (small nuclear RNA gene). Among them, 18S rDNA sites were situated in only one chromosomal pair in both species, and co-localized with 5S rDNA in . On the other hand, 5S rDNA and U2 snRNA sites were observed on several chromosomes, with variation in the number of sites between species under study. These two repetitive DNAs were observed co-localized in one chromosomal pair in and in four pairs in . Our study shows a new case of association of these two types of repetitive DNA in the genome of Gymnotiformes.