The family Parodontidae comprises three genera: Parodon, Apareiodon and Saccodon, including 32 species considered valid. These animals are distributed in South America and part of Panama, except in the South Atlantic coastal basin in the state of Bahia, Patagonia and the Amazon River channel. In general, the karyotype macrostructure of the family appears to be conserved, with a diploid number of 54 chromosomes, karyotypes consisting of metacentric chromosomes (m), submetacentric (sm) and subtelocentric (st) and centromeric heterochromatic blocks in almost all chromosome complement, as well as terminal blocks. However, there is diversity interspecific karyotype in micro-structure of this group. Chromosome analysis in Parodontidae species are found only for the Iguacu, Paraguay, Parana and Sao Francisco river basins. The Tocantins-Araguaia river basin is one of the largest basins of Brazil and presents a high level of endemism of species of fish. However, cytogenetic studies involving species in this area are still restricted. Considering the small number of species belonging to the family Parodontidae and the importance of endemism in the ichthyofauna of the Tocantins-Araguaia, chromosome studies in Apareiodon species of this basin, absent until now, will assist in resolving taxonomic problems in the family, and contribute to establish a model of the chromosome in this group, emphasizing the role of repetitive DNA in this process.
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