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.

Hoplias malabaricus is a species widely distributed throughout Brazil. Cytogenetic studies indicate the occurrence of extensive chromosomal rearrangements in population differentiation and speciation of the group that demonstrated an independent origin of sex chromosome systems. Seven karyomorphs were characterized for the species and are located in specific river basins, while others are distributed throughout several different basins. However, there are few studies linking the geographical distribution of H. malabaricus karyomorphs to the Brazilian hydrographic basins. This article provides new chromosomal information on five populations of H. malabaricus collected in a South Atlantic basin. The samples were analyzed by conventional and molecular cytogenetic techniques. Two karyomorphs, A (2n=42 m/sm) and C (2n=40 m/sm), were detected, and remarkable differences in the distribution of heterochromatin and GC-rich blocks were observed in the karyomorphs. A review of existing data is presented here, where we observe that dispersion is associated with the genesis of the South and Central America river basins. Coastal drainages represent an ancestral biogeographical component for many groups of fish, representing older basins, such as the basins of the Eastern Atlantic and San Francisco river, suggesting that existing karyomorphs found in these basins may represent a basal karyotype (karyomorph F) within H. malabaricus. The current cytogenetic data, including this article, for different karyomorphs of H. malabaricus in conjunction with the geological history of the continent allow us to determine that the ancestral group is most likely karyomorph F.

The nucleolus is an important nuclear structure where transcription of ribosomal DNA (rDNA) takes place. During mitotic division, the nucleolus passes through different processes that inactivate rDNA transcription; in meiosis, its reassembly takes place during telophase II. The objective of this study was to identify the activity patterns and localization of nucleolar organizer regions (NORs) during meiotic division in fish species of the family Curimatidae. For this analysis, the meiotic division in five curimatid species was studied using silver nitrate impregnation, fluorescent in situ hybridization (FISH), and base-specific fluorochrome staining. Silver nitrate staining indicated the presence of a nucleolus in interphase nuclei, one chromosome pair in the spermatogonial metaphases, and one bivalent at the pachytene stage. No Ag-NORs were identified for cells at the diplotene, diakinesis, metaphase I, or metaphase II stages; however, FISH confirmed the presence of Ag-NORs in the nuclei, in spermatogonia, and at the pachytene phase. FISH identified this region during the other stages of meiosis, as did fluorochrome CMA3 staining, which revealed fluorescent marks corresponding to NORs during all stages of meiosis analyzed. The gene activity and localization of this ribosomal sequence during the different stages involved will also be discussed.