This study provides new insight into the chromosomal diversification in Loricariinae. We analyzed nine species from different Brazilian hydrographic basins, using conventional and molecular cytogenetic methods, aiming to understand the karyotypic diversification, and contribute with cytotaxonomic markers in this group considered one of the most diverse of Loricariidae. Our results evidenced a high karyotypic variability in diploid number (2n) ranging from 2n = 54 (Loricariichthys platymetopon and Loricariichthys anus), 2n = 60 (Rineloricaria reisi and Rineloricaria parva), 2n = 62 (Proloricaria prolixa), 2n = 64 (Loricaria cataphracta complex species), 2n = 66 (Sturisoma barbatum), and 2n = 68 (Pyxiloricaria menezesi). Different patterns of 18S and 5S ribosomal DNA (rDNA) were also identified, while slight divergences in heterochromatin distribution were observed. This high variability is probably related with independent events of Robertsonian translocations, pericentric inversions, and different mechanisms of rDNA sites dispersion (nonreciprocal translocation and transposable element [TEs] co-localization). In addition, our study provides a set of efficient chromosomal markers for the characterization of all analyzed species, and certainly, in future analyzes, will contribute as a useful cytotaxonomic tool in groups where the traditional taxonomy based on morphological data are not sufficient to clarify their relationship.

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Basic and molecular cytogenetic techniques were carried out in 3 Neotropical region populations of catfishes, two of Trachelyopterus galeatus (one from the marshlands of Paraguay River basin and another from Lago Catalão, Amazon River basin) and one of Trachelyopterus porosus, a sympatric population to T. galeatus from the Amazon River basin. This study aimed to describe and understand the structure and evolution of Trachelyopterus B chromosomes, mainly through physical mapping of repetitive elements. A diploid number of 58 chromosomes was found for all individuals, as well as the presence of B chromosomes. For T. porosus this is the first report of a supernumerary. The sympatric species of T. galeatus and T. porosus from Amazon River had 1–3 B chromosomes and T. galeatus from Paraguay River had 1–2 B chromosomes, all of them showed intra- and interindividual numerical variation. Two females of T. porosus exhibited a new variant B chromosome (B2), previously not seen in Auchenipteridae, which might have originated from B1 chromosomes. All B chromosomes were entirely heterochromatic. In contrast to all complement A and B2 chromosomes, in which the telomeric sequences were found in the telomeric regions, B1 chromosomes of all populations were totally marked by (TTAGGG)n probes. (GATA)n sequence sites were found through all complement A chromosomes, but B1 and B2 chromosomes exhibited only a clustered block in one of the chromosome arms. The most frequent B chromosomes (B1) in all populations/species, including those previously studied in Auchenipteridae catfishes, share the following characteristics: totally heterochromatic, small, metacentric, with accumulation of repetitive (TTAGGG)n sequences, and a low number of (GATA)n copies, which might suggest a common ancient origin in Trachelyopterus species/populations.

Despite conservation of the diploid number, a huge diversity in karyotype formulae is found in the Ancistrini tribe (Loricariidae, Hypostominae). However, the lack of cytogenetic data for many groups impairs a comprehensive understanding of the chromosomal relationships and the impact of chromosomal changes on their evolutionary history. Here, we present for the first time the karyotype of Panaqolus tankei Cramer & Sousa, 2016. We focused on the chromosomal characterization, using conventional and molecular cytogenetic techniques to unravel the evolutionary trends of this tribe. P. tankei, as most species of its sister group Pterygoplichthini, also possessess a conserved diploid number of 52 chromosomes. We observed heterochromatin regions in the centromeres of many chromosomes; pairs 5 and 6 presented interstitial heterochromatin regions, whereas pairs 23 and 24 showed extensive heterochromatin regions in their q arms. In situ localization of 18S rDNA showed hybridization signals correlating with the nucleolus organizer regions, which are located in the q arms of pair 5. However, the 5S rDNA was detected in the centromeric and terminal regions of the q arms of pair 8. (TTAGGG)n hybridized only in the terminal regions of all chromosomes. Microsatellite in situ localization showed divergent patterns, (GA)15 repeated sequences were restricted to the terminal regions of some chromosomes, whereas (AC)15 and (GT)15 showed a scattered hybridization pattern throughout the genome. Intraspecific comparative genomic hybridization was performed on the chromosomes of P. tankei to verify the existence of sex-specific regions. The results revealed only a limited number of overlapping hybridization signals, coinciding with the heterochromatin in centromeric regions without any sex-specific signals in both males and females. Our study provides a karyotype description of P. tankei, highlighting extensive differences in the karyotype formula, the heterochromatin regions, and sites of 5S and 18S rDNA, as compared with data available for the genus.