Electric fish of the order Gymnotiformes are endemic to the Neotropical region, and their highest diversity is observed in the Amazon region. The family Gymnotidae, which consists of the genera Electrophorus and Gymnotus, is a natural group and is located at the base of the phylogeny of the order. Gymnotus is a widely distributed and specious genus with high karyotypic diversity, especially concerning to the diploid number and the locations of repetitive sequences. Our karyotyping results in five species of the family Gymnotidae (Gymnotus ucamara, Gymnotus cf. stenoleucus, Gymnotus cf. pedanopterus, Gymnotus mamiraua, and Gymnotus carapo “Maranhão”) corroborate the proposal of plasticity of the diploid number in this group. Moreover, in this study, we propose that the 5S ribosomal DNA (rDNA) sequences were species-specific markers that act as a potential biogeographical marker for the genus. Besides, the sequence's location, particularly in G. mamiraua from Central Amazon, shows a close relationship with 5S of the Gymnotus species, with 54 chromosomes, from the Paraná-Paraguay basin in the Center-South of Brazil. Considering that the ancestral diploid number for Gymnotidae is 52 chromosomes, we also suggest that the trend in the family is toward a decrease in the chromosome number. However, the carapo clade stands out in this regard, with an increase and a decrease in chromosome number; this pattern may be reinforced with the ecologic behaviors and the geodispersal patterns of this clade.
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