“…Cytogenetics has been an important tool in cytotaxonomy and has proved to be very useful in understanding the evolutionary processes behind the diversification of Gymnotus . The Gymnotiformes order has considerable variation, not only in diploid number (from 2 n = 24 in Apteronotus albifrons , Howell, 1972 ; Almeida-Toledo et al, 1981 ; Mendes et al, 2012 ; to 2 n = 74 in Rhabdolichops cf eastward , Suárez et al, 2017 ) but also in the karyotype formula and location of repetitive sequences (Fernandes et al, 2005 ; Almeida-Toledo et al, 2007 ; Silva et al, 2009 ; da Silva et al, 2013 ; Jesus et al, 2016 ; Araya-Jaime et al, 2017 ; Batista et al, 2017 ; Sousa et al, 2017 ; Takagui et al, 2017 ). Recently, fluorescence in situ hybridization (FISH), has played an important role in understanding the genome structure of fish species (Yi et al, 2003 ; Cabral-de-Mello and Martins, 2010 ; Martins et al, 2011 ; Vicari et al, 2011 ; Gornung, 2013 ; Knytl et al, 2013 ; Yano et al, 2017 ) and molecular cytogenetic studies in Gymnotiformes have shown dynamic reorganization, including pericentric inversions observed through repetitive DNA position (Fernandes et al, 2017 ), sequence dispersion via transposable elements and the association between different repetitive sequences (Utsunomia et al, 2014 ; da Silva et al, 2016 ; Machado et al, 2017 ) and the presence of different sex chromosome systems (Margarido et al, 2007 ; Henning et al, 2008 , 2011 ; da Silva et al, 2011 , 2014 ; Almeida et al, 2015 ).…”