Cytogenetical analyses in three fish species of the genus Pimelodus(Siluriformes: Pimelodidae) from rio São Francisco: considerations about the karyotypical evolution in the genus

Molecular Evidence Reveals Taxonomic Uncertainties and Cryptic Diversity in the Neotropical Catfish of the Genus Pimelodus (Siluriformes: Pimelodidae)

Pimelodus is the most speciose genus of the family Pimelodidae, and is amply distributed in the Neotropical region. The species-level taxonomy and phylogenetic relationships within this genus are still poorly resolved, however. These taxonomic problems and the general lack of data have generated major uncertainties with regard to the identification of specimens from different localities. In the present study, we applied a single-locus species delimitation approach to identify the MOTUs found within the genus Pimelodus and provide sound evidence for the evaluation of the species richness of this genus in the different river basins of the Neotropical region. The study was based on the analysis of sequences of the mitochondrial COI gene of 13 nominal species, which resulted in the identification of 24 consensus MOTUs. Only six nominal species were recovered as well-defined molecular entities by both the traditional barcoding analysis and the molecular delimitation methods, while the other seven presented cryptic diversity or persistent taxonomic uncertainties. The lineages identified from the Parnaíba ecoregions, Amazonas Estuary and Coastal Drainages may represent a much greater diversity of Pimelodus species than that recognized currently, although a more detailed study of this diversity will be necessary to provide a more definitive classification of the genus.

The Karyotypic Diversification of Calophysines and the Exallodontus-Propimelodus Clade (Pimelodidae, Siluriformes): A Cytotaxonomic and Evolutionary Approach in Pimelodidae Based on Ancestral State Reconstruction

Pimelodidae family is one of the most diverse and widely distributed fish groups in South America. Phylogenetic analysis in the family have recently indicated the existence of two main clades: "sorubiminae" and the OCP clade, including Pimelodus ornatus, "calophysines" and "pimelodines." The aim of this study was to investigate the karyotype of three Amazonian Pimelodidae species: Calophysus macropterus, Propimelodus eigenmanni, and Exallodontus aguanai associating them to the literature, seeking to reconstruct probable ancestral characters. C. macropterus has 2n = 50, 20m+20sm+10a (fundamental number [FN] = 90), simple interstitial nucleolar organizing regions (NORs), and four 5S rDNA sites terminals, two in synteny with the 18S rDNA. P. eigenmanni has 2n = 56, 28m+20sm+2st+6a (FN = 106), simple NORs, and two 5S rDNA sites terminals. E. aguanai has 2n = 56, 36m+12sm+2st+6a (FN = 106) and 18S and 5S rDNA sites interstitial syntenic in the chromosome 1. All species exhibited a higher amount of heterochromatin, differing from the pattern of the family, and strong marking associated with NORs. The integration between molecular phylogenetic data and karyotype data indicated a high probability that 2n = 56 and simple terminals NORs in the short arm are ancestral characters in Pimelodidae, evidenced in "sorubiminae." In the OCP clade derived traits were observed resulting from chromosomal changes that played a critical role in the karyotype evolution of the group.

Contributions to the systematic of Pimelodidae (Osteichthyes, Siluriformes): basic and molecular cytogenetics on seven species of Pimelodus from three Brazilian hydrographic systems

ABSTRACT Pimelodidae harbors several species and is widely distributed throughout the Neotropical region. Pimelodus is the genus with the largest number of species, however it is a polyphyletic group. Cytogenetic analyzes of the valid species still covers less than half of them. Herein, seven Pimelodus species from three Brazilian hydrographic systems were analyzed through basic (Giemsa, AgNORs and C banding) and molecular (5S and 18S rDNA-FISH) cytogenetic methods. All species had 2n=56 chromosomes with different karyotype formulas observed among the species. AgNORs were corresponding to 18S rDNA and localized on long arm of one chromosome pair in all species. Heterochromatin distribution follows the pattern commonly verified in the family and allows to identify each one of the studied species. 5S rDNA marker was interspecifically variable in number and position of cistrons. Pimelodus ortmanni had B chromosomes varying intra and inter-individually. We performed a discussion on our own and available cytogenetic data for Pimelodidae, and the associating of them with available phylogeny enable us identifying features that distinguish subgroups within Pimelodidae, such as NORs location (terminal/long arm for species belonging to “Iheringichthys-Parapimelodus” and “Pimelodus maculatus” subclades) and location of 5S rDNA sites (pericentromeric/interstitial/ long arm for species belonging to Pimelodus group).

Comparative Cytogenetics Analysis on Five Genera of Thorny Catfish (Siluriformes, Doradidae): Chromosome Review in the Family and Inferences About Chromosomal Evolution Integrated with Phylogenetic Proposals

Doradidae has been a target of phylogenetic studies over the last few years, but chromosomal information about the family is still scarce. Therefore, new cytogenetic data are provided herein and they are correlated with phylogenetic proposals to contribute to the knowledge of chromosomal evolution within doradids. Cytogenetic studies were performed on Trachydoras paraguayensis, Anadoras sp. "araguaia," Ossancora eigenmanni, Platydoras armatulus, and Rhinodoras dorbignyi. O. eigenmanni, P. armatulus, and R. dorbignyi had 2n = 58 chromosomes as found for most doradids, but T. paraguayensis and Anadoras sp. "araguaia" had 2n = 56 chromosomes, probably caused by a chromosomal reduction. There is a great maintenance of 2n = 58 verified in doradids, but karyotype formulas are diverse. Moreover, other markers (i.e., nucleolar organizer regions, heterochromatin distribution, and 5S and 18S rDNA) showed a great diversity among the analyzed species. Contrasting the variability in the chromosomal markers with the maintenance of diploid number, it is likely that inversions and translocations played an important role in chromosome differentiation in Doradidae. Herein, we created an integrative discussion linking cytogenetic data to phylogenetic proposals, based on morphological and genetic features, enabling us to identify possible cytogenetic traits, as well as probable chromosomal plesiomorphy and apomorphy of Doradidae species.

Chromosomal Mapping of Repeat DNA in Bergiaria westermanni (Pimelodidae, Siluriformes): Localization of 45S rDNA in B Chromosomes

The occurrence of repetitive DNA in autosomes and B chromosomes of Bergiaria westermanni was examined using conventional and molecular cytogenetic techniques. This species exhibited 2n = 56 chromosomes, with intra- and interindividual variation in the number of heterochromatic B chromosomes (from 0 to 4). The 5S rDNA was localized in pairs 1 and 5, and histone probes (H1, H3, and H4) and U2 small nuclear RNA were syntenic with 5S rDNA in pair 5. Histone sequences were also located in chromosome pair 14. The (GATA)n sequence was dispersed throughout the autosomes and B chromosomes, with clusters (microsatellite accumulation) in some chromosome regions. The telomeric probe revealed no signs of chromosomal rearrangements in the genome of B. westermanni. The 45S rDNA sites were detected in the terminal region of pair 27; these sites corresponded to a GC-rich heterochromatin block. In addition, 3 of the 4 B chromosomes also contained 45S rDNA copies. Silver nitrate staining in interphase nuclei provided indirect evidence of the expression of these rRNA genes in B chromosomes, indicating the probable origin of these elements. This report shows plasticity in the chromosomal localization of repeat DNA in B. westermanni and features a discussion of genomic diversification.

Cytogenetic studies in the redtail catfish, Phractocephalus hemioliopterus (Bloch & Schneider, 1801) (Siluriformes, Pimelodidae) a giant fish from Amazon basin

The objective of this study was to cytogenetically analyze Phractocephalus hemioliopterus comparing the findings with other data to infer relationships among Pimelodidae species. The results revealed a diploid number of 2n = 56 and the karyotype composed of 16 metacentric, 20 submetacentric, 6 subtelocentric and 14 acrocentric chromosomes (FN = 98). The Ag-NORs, 18S rDNA and CMA₃ signals were coincident in location occupying the short arm of an acrocentric chromosome pair (23^th), in a secondary constriction. The 5S rDNA genes were localized near the centromere on the short arms of one submetacentric chromosome pair. C-bands were localized predominantly in the terminal regions of chromosomes, including the AgNORs and a small metacentric pair with a conspicuous positive band on interstitial region. This chromosome pair could be considered a species-specific cytogenetic marker.

Pimelodus maculatus (Siluriformes, Pimelodidae): complete mtDNA sequence of an economically important fish from the São Francisco river basin

Pimelodus maculatus is an important commercial fish found in the São Francisco and Paraná river basins. NGS was used to sequence the mtDNA of P. maculatus. The mtDNA was annotated and aligned with that of 25 other fish species to enable phylogenetic analysis. The complete mtDNA molecule had 16,561 bp and its GC content was 43.7%; the structure was similar to that of other vertebrates: 2 rRNA, 22 tRNA, 13 protein-coding genes, and a D-loop region containing 914 bp. Phylogenetic analysis yielded a tree with a high bootstrap coefficient that was coherent with the actual phylogeny of the species.

Cytogenetic and Molecular Data Demonstrate that the Bryconinae (Ostariophysi, Bryconidae) Species from Southeastern Brazil Form a Phylogenetic and Phylogeographic Unit

Brycon spp. occur in Neotropical watersheds to the west and east of the Andes, and as they are sensitive to anthropogenic changes, many these species are endangered in southeastern Brazil. Coastal rivers in southeastern Brazil are characterized by the presence of relatively few freshwater fish species and high endemism of this fauna. The objective of this study was to examine whether Brycon spp. occurring in the coastal basins of southeastern Brazil are monophyletic, using cytogenetic data, mitochondrial, and nuclear molecular markers. All the species showed a diploid number of 50 chromosomes, a conserved number within the subfamily Bryconinae. However, the karyotypic formulas were unique to most species, including Brycon devillei (26m+22sm+2st), Brycon ferox (26m+12sm+12st), Brycon insignis (22m+20sm+8st), Brycon opalinus, and Brycon vermelha (24m+20sm+6st), indicating the prevalence of pericentric and paracentric inversions in the chromosomal evolution of these species. All of them had nucleolar organizer regions in the first pair of subtelocentric chromosomes and no equilocal distribution of heterochromatin in the first pair of chromosomes of the karyotype. These two features, not seen in any other Brycon spp. examined to date, indicate that Bryconinae species from the Brazilian southeastern coastal basins, including the monotypic genus Henochilus, are monophyletic. Also, this is the first study that reports NOR location and C-banding patterns as synapomorphies for a Neotropical fish species group. The monophyly was also supported by a phylogenetic analysis of 16S rDNA (16S), cytochrome oxidase subunit I (COI), alpha-myosin (MYH6) genes and S72 intron molecular data. Our results partially corroborate the “Brycon acuminatus” group proposed by Howes in 1982: our proposed clade keeps B. devillei, B. ferox, and B. insignis; but it also includes B. opalinus, B. vermelha, and H. weatlandii whereas it excludes B. nattereri. The phylogeographic unit formed by Bryconinae species in southeastern Brazil reflects the long and isolated paleohydrological history of these coastal basins relative to the continental watersheds.

Cytogenetics analyses among populations of the fish Iheringichtys labrosus (Kröyer, 1874) (Siluriformes, Pimelodidae). Karyotype analysis, C- banding and AgRONs distribution

Karyotypic data on Iheringichthys labrosus from several populations of Paraná River/Argentina are presented. The diploid number was 2n=56 and the karyotype consisted of 42m/sm + 14st/a (NF= 98). The AgNORs were observed in telomeres of the long arm of a st/a chromosome pair. These sites were also positive after C-banding. Heterochromatin was observed in the telomeric position in some chromosomal pairs and can be found in one or both metacentric chromosome arms. The obtained results were compared with those reported for different populations from Brazilian rivers and the similarity among them was evident. However, discordant chromosome formulae and karyotype features between samples should be carefully taken because, in some cases, they seem to be more technical artifacts than real differences.

Twenty years of physical mapping of major ribosomal RNA genes across the teleosts: A review of research

Molecular cytogenetic data on the number and position of 45S ribosomal RNA genes (rDNA; located in nucleolus organizing regions, NORs) detected by FISH in 330 species of 77 families and 22 orders of bony fishes (Teleostei) and, additionally, 11 species of basal ray-finned fishes are compiled and analyzed. The portion of species with single rDNA sites in the sample amounts to 72%. The percentage of species with multiple NORs decreases with increasing numbers of rDNA loci per genome, i.e. scarcely 3% of species carry 4 or more rDNA-bearing chromosome pairs. 43% of all rDNA sites analyzed occur terminally on the short arms of chromosomes or constitute them. In general, terminal rDNA sites account for 87% of all examined cases. Interspecific variation in the location of single rDNA sites among related taxa, polymorphisms of multiple NORs in some groups of teleosts and analytical outcomes on the subject are reviewed.

Chromosome mapping of repetitive sequences in four Serrasalmidae species (Characiformes)

The Serrasalmidae family is composed of a number of commercially interesting species, mainly in the Amazon region where most of these fishes occur. In the present study, we investigated the genomic organization of the 18S and 5S rDNA and telomeric sequences in mitotic chromosomes of four species from the basal clade of the Serrasalmidae family: Colossoma macropomum, Mylossoma aureum, M. duriventre, and Piaractus mesopotamicus, in order to understand the chromosomal evolution in the family. All the species studied had diploid numbers 2n = 54 and exclusively biarmed chromosomes, but variations of the karyotypic formulas were observed. C-banding resulted in similar patterns among the analyzed species, with heterochromatic blocks mainly present in centromeric regions. The 18S rDNA mapping of C. macropomum and P. mesopotamicus revealed multiple sites of this gene; 5S rDNA sites were detected in two chromosome pairs in all species, although not all of them were homeologs. Hybridization with a telomeric probe revealed signals in the terminal portions of chromosomes in all the species and an interstitial signal was observed in one pair of C. macropomum.

Cytogenetic and molecular analyses in troglobitic and epigean species of Pimelodella (Siluriformes: Heptapteridae) from Brazil

Samples from seven different locations of the genus Pimelodella were genetically examined, two caves (exclusively subterranean, upper Tocantins River and São Francisco River) and five epigean (from upper Paraná River basin). Cytogenetic analyses revealed the same diploid number (2n=46) for all species besides similarities in both number and location of nucleolar organizer regions and C bands. FISH with 5S rDNA probes and CMA3 staining indicated significant differences among the studied species. Application of PCR-RFLP in ATPase 6 and 8 mitochondrial genes allowed building a minimum evolution phenogram identifying the close evolutionary relationship among groups. Both chromosomal and molecular data were useful to infer the relationships among studied Pimelodella species.

Cytogenetic characterization of F1, F2 and backcross hybrids of the Neotropical catfish species Pseudoplatystoma corruscans and Pseudoplatystoma reticulatum (Pimelodidae, Siluriformes)

The cytogenetic characteristics of Pseudoplatystoma corruscans and Pseudoplatystoma reticulatum and their F1, F2 and backcross hybrids were assessed by using chromosome banding techniques. The diploid number of 56 chromosomes was constant in all species and lineages, with a karyotypic formula containing 20 metacentric, 12 submetacentric, 12 subtelocentric and 12 acrocentric chromosomes. Nucleolar organizer regions (NORs) were identified in two subtelocentric chromosomes in the parents and hybrids, with partial nucleolar dominance in F1 and F2 specimens. Heterochromatic blocks were detected in the terminal and centromeric regions of some chromosomes in all individuals. For parental and hybrid lineages, 18S ribosomal clusters corresponding to NORs and 5S ribosomal genes were identified in distinct pairs of chromosomes. The striking conservation in the chromosomal macrostructure of the parental species may account for the fertility of their F1 hybrids. Similarly, the lack of marked alterations in the chromosomal structure of the F1 hybrids could account for the maintenance of these features in post-F1 lineages.

Chromosomal analyses in Megalonema platanum (Siluriformes: Pimelodidae), an endangered species from South American rivers

This study presents chromosomal data of Megalonema platanum from rio Tibagi, Paraná, Brazil and from rio Paraná, Argentina. The diploid number was equal 54 with karyotype composition of 24m+16sm+2st+12a in both populations. The AgNOR sites were detected in the terminal position of a submetacentric pair of the two analyzed populations, coinciding with secondary constrictions on the short arm of pair 15. CMA3 and FISH with 18S rDNA probe displayed fluorescent signals that correspond to the AgNOR sites and secondary constriction. The presence of a small acrocentric supernumerary chromosome can be observed in M. platanum from rio Tibagi, with centromeric heterochromatin. Others heterochromatic blocks were evidenced in the terminal position of some chromosome and one metacentric large chromosome pair, probably the first pair, showed an interstitial heterochromatin. In the population of the rio Paraná were still observed heterochromatic blocks in both ends in some chromosomes. This work brings for the first time cytogenetic date of M. platanum, which is a very rare species in the rio Paraná basin and may be endangered.

Karyotype variability in neotropical catfishes of the family Pimelodidae (Teleostei: Siluriformes)

Karyotypic data are presented for four species of fish belonging to the Pimelodidae family. These species show a conserved diploid number, 2n = 56 chromosomes, with different karyotypic formulae. The analyzed species showed little amount of heterochromatin located preferentially in the centromeric and telomeric regions of some chromosomes. The nucleolus organizer regions activity (Ag-NORs) and the chromosomal location of ribosomal genes by fluorescent in situ hybridization (FISH), with 18S and 5S probes, showing only one chromosome pair marked bearer of ribosomal genes, the only exception was Pimelodus britskii that presented multiple NORs and syntenic location of the 18S and 5S probes. Non-Robertsonian events, as pericentric inversion and NORs duplication are requested to explain the karyotype diversification in Pseudoplatystoma from the rio Paraguay (MS), Pimelodus from the rio Iguaçu (PR), Sorubim from the rio Paraguay (MS) and Steindachneridion from the rio Paraíba do Sul (SP). The obtained data for the karyotype macrostructure of these species corroborates a conserved pattern observed in Pimelodidae. On the other hand, interspecific variations detected by molecular cytogenetics markers made possible cytotaxonomic inferences and differentiation of the species here analyzed.

Structural chromosome polymorphism in a Pimelodus maculatus La Cepède, 1803 population (Siluriformes, Pimelodidae) from the Paranapanema River basin, PR, Brazil

In the present cytogenetic study of Pimelodus maculatus, 13 specimens (8 males and 5 females) from the Congonhas Stream in Paraná State, Brazil, were examined using conventional staining. All of them showed a karyotype of 2n=56, with a chromosome distribution of 20m + 20sm + 10st + 6a. However, four individuals (2 males and 2 females) were found to have a variant karyotype (cytotype) with two heteromorphic chromosomes in the group of submetacentric chromosomes--one of them corresponds to the second largest chromosome of this group and the other is a chromosome of small size. This variation suggests the existence of a structural polymorphism in the studied population.

Cytogenetic study of two species of the family Pimelodidae (Siluriformes) collected in lago Guaíba, Rio Grande do Sul, Brazil

A cytogenetic study was conducted on specimens of Parapimelodus nigribarbis and Pimelodus maculatus collected in the lago Guaíba drainage, Porto Alegre, Rio Grande do Sul, Brazil. The two species had a diploid number of 56 chromosomes, with P. nigribarbis showing a karyotype of 20m + 20sm + 4st + 12a with FN of 100, and P. maculatus showing a karyotype of 24m + 20sm + 6st + 6a with FN of 106. NORs were demonstrated in both species in only one pair of subtelocentric chromosomes, in the terminal region of the long arm, which was coincident with C- banding and CMA3 staining, while DAPI staining was negative in these regions. Parapimelodus nigribarbis had a greater number of heterochromatic bands than did P. maculatus, which were distributed mainly in the terminal regions, where the latter species showed an interstitial band on the short arm of the first metacentric pair. C-banding plus CMA3 demonstrated heterochromatin that was associated with GC-rich NORs in both P. nigribarbis and P. maculatus, although other fluorescent regions were also observed in the former species. With C-banding plus DAPI, various chromosomal regions were stained in the two species, along with interstitial staining in P. maculatus, indicating that heterochromatin contained a greater quantity of AT-rich regions.