Catfishes of the genus Loricariichthys are widely distributed in the Platina Basin. Considering that the cytogenetic knowledge of Loricariichythys is underestimated, this study assessed Loricariichthys anus and Loricariichthys platymetopon through different chromosome bandings, to define the mechanisms determining the variability in these species. Cytogenetic analyses evidenced a high degree of similarity in relation to the 2n (54 chromosomes), as well as to the distribution of heterochromatin. Despite the apparent conservatism, it was possible to differentiate between these species, especially in relation to the location of the 18S rDNA genes. An interpopulation variation in the karyotype formula was detected only in L. anus, showing the existence of different karyotypes, probably due to the geographical isolation between Laguna dos Patos and Tramandaí River. The maintenance of the 2n=54, along with the different karyotypes observed in L. anus, the differential nucleolus organizer regions position, as well as the sexual chromosome system ZZ/ZW in L. platymetopon, makes the participation of pericentric inversions in the karyotypic evolution of these species evident. These structural rearrangements were important for chromosome evolution of these two species, because they probably promoted the postzygotic barriers to reproduction, significantly contributing to the speciation process between them.

Karyotypes and other chromosomal markers as revealed by conventional and molecular cytogenetic protocols in four species of the catfish family Doradidae from the Araguaia-Tocantins river basin, namely Hassar wilderi, Leptodoras cataniae, Tenellus leporhinus and Tenellus trimaculatus were examined. All species had diploid chromosome number 2n = 58 and karyotypes dominated by biarmed chromosomes, simple NOR phenotype, that is, one chromosome pair bearing this site in terminal position, but some differences in karyotypes and distribution of constitutive heterochromatin, position of rDNA sites. Such characteristics appeared species-specific. A ZZ/ZW sex chromosome system was found in Tenellus trimaculatus, resulting likely from the amplification of the heterochromatin, followed by a paracentric inversion. Our results confirmed low karyotype differentiation observed until now among representatives of this endemic catfish family.