Antagonist sexual selection is the driving force behind the origin and diversification of sex chromosomes such as XX/XY and ZZ/ZW. However, chromosome mobility, mainly in fishes, may result in the formation of chromosomes of recent origin, a process known as turnover. The family Gymnotidae, which is composed of the genera Electrophorus+Gymnotus, presents a multiple system of the type X1X1X2X2/X1X2Y, which has been described for Gymnotus pantanal. This article describes the karyotype of three Amazon Gymnotus species, revealing the presence of both simple and multiple systems: Gymnotus carapo "Catalaõ" 2n=40 XX/XY, Gymnotus coropinae 2n=49/50 X1X1X2X2/X1X2Y, and Gymnotus sp. "Negro" 2n=50 XX/XY. Our hypothesis is that the simple system present in G. carapo "Catalaõ" is ancestral in relation to G. pantanal's multiple system and that the diversification of the subsequent multiple system occurred after the final separation of the Amazon and Paraná basins. Moreover, G. coropinae's multiple system may have originated from the simple system present in Gymnotus sp. "Negro." The distant position between the species in the Gymnotidae family's phylogeny in addition to differences in sex chromosome formula and number between Clade G1 G. coropinae and G. sp. "Negro" species and "Carapo" Clade. G. carapo and G. pantanal species suggest that both sequences of sexual systems occurred independently, supporting other proposed models and highlighting the fact that species of the genus Gymnotus may serve as a model for studying sex chromosome turnover. © 2014 Mary Ann Liebert, Inc.
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