Cytogenetic and molecular characteristics of Potamotrygon motoro and Potamotrygon sp. (Chondrichthyes, Myliobatiformes, Potamotrygonidae) from the Amazon basin: Implications for the taxonomy of the genus.

The chromosomes of two freshwater stingrays, Potamotrygon motoro and Potamotrygon sp., from the Amazon River basin in Brazil were investigated using integrated molecular (cytochrome c oxidase subunit 1) and cytogenetic analyses. Potamotrygon motoro presented intraspecific variation in the diploid number, with 2n=66 in the females and 2n=65 in the males, while Potamotrygon sp. had a karyotype with 66 chromosomes, in both sexes. The C-banding revealed the presence of heterochromatic blocks accumulated in the centromeric region of all the chromosomes in both species. The FISH assays with 18S DNA probes highlighted the terminal region of three or four chromosome pairs in P. motoro and seven chromosomes in Potamotrygon sp. The rDNA 5S sequences were found in only one chromosomal pair in both species. The interspecific genetic distance based on the COI sequences, between P. motoro and Potamotrygon sp. from Amazon River was 10.8%, while that between the Amazonian P. motoro and Potamotrygon amandae from the Paraná River was 2.2%, and the genetic distance between Potamotrygon sp. and P. amandae was 11.8%. In addition to the new insights on the cytogenetics of the study species, the results of the present study confirmed the existence of heteromorphic sex-linked chromosomes in P. motoro.

Uncovering the Ancestry of B Chromosomes in Moenkhausia sanctaefilomenae (Teleostei, Characidae).

B chromosomes constitute a heterogeneous mixture of genomic parasites that are sometimes derived intraspecifically from the standard genome of the host species, but result from interspecific hybridization in other cases. The mode of origin determines the DNA content, with the B chromosomes showing high similarity with the A genome in the first case, but presenting higher similarity with a different species in the second. The characid fish Moenkhausia sanctaefilomenae harbours highly invasive B chromosomes, which are present in all populations analyzed to date in the Parana and Tietê rivers. To investigate the origin of these B chromosomes, we analyzed two natural populations: one carrying B chromosomes and the other lacking them, using a combination of molecular cytogenetic techniques, nucleotide sequence analysis and high-throughput sequencing (Illumina HiSeq2000). Our results showed that i) B chromosomes have not yet reached the Paranapanema River basin; ii) B chromosomes are mitotically unstable; iii) there are two types of B chromosomes, the most frequent of which is lightly C-banded (similar to euchromatin in A chromosomes) (B1), while the other is darkly C-banded (heterochromatin-like) (B2); iv) the two B types contain the same tandem repeat DNA sequences (18S ribosomal DNA, H3 histone genes, MS3 and MS7 satellite DNA), with a higher content of 18S rDNA in the heterochromatic variant; v) all of these repetitive DNAs are present together only in the paracentromeric region of autosome pair no. 6, suggesting that the B chromosomes are derived from this A chromosome; vi) the two B chromosome variants show MS3 sequences that are highly divergent from each other and from the 0B genome, although the B2-derived sequences exhibit higher similarity with the 0B genome (this suggests an independent origin of the two B variants, with the less frequent, B2 type presumably being younger); and vii) the dN/dS ratio for the H3.2 histone gene is almost 4-6 times higher for B chromosomes than for A chromosome sequences, suggesting that purifying selection is relaxed for the DNA sequences located on the B chromosomes, presumably because they are mostly inactive.

Origin of B chromosomes in the genus Astyanax (Characiformes, Characidae) and the limits of chromosome painting.

Eukaryote genomes are frequently burdened with the presence of supernumerary (B) chromosomes. Their origin is frequently investigated by chromosome painting, under the hypothesis that sharing the repetitive DNA sequences contained in the painting probes is a sign of common descent. However, the intragenomic mobility of many anonymous DNA sequences contained in these probes (e.g., transposable elements) adds high uncertainty to this conclusion. Here we test the validity of chromosome painting to investigate B chromosome origin by comparing its results for seven B chromosome types in two fish species genus Astyanax, with those obtained (1) by means of the physical mapping of 18S ribosomal DNA (rDNA), H1 histone genes, the As51 satellite DNA and the (AC)15 microsatellite, and (2) by comparing the nucleotide sequence of one of these families (ITS regions from ribosomal DNA) between genomic DNA from B-lacking individuals in both species and the microdissected DNA from two metacentric B chromosomes found in these same species. Intra- and inter-specific painting suggested that all B chromosomes that were assayed shared homologous DNA sequences among them, as well as with a variable number of A chromosomes in each species. This finding would be consistent with a common origin for all seven B chromosomes analyzed. By contrast, the physical mapping of repetitive DNA sequences failed to give support to this hypothesis, as no more than two B-types shared a given repetitive DNA. Finally, sequence analysis of the ITS regions suggested that at least some of the B chromosomes could have had a common origin.

Microstructural chromosome reorganization in the genus Trichomycterus (Siluriformes: Trichomycteridae)

Trichomycterus is a specious fish genus within Trichomycterinae and displays remarkable karyotype diversity. However, knowledge about their genomic structure and location of repetitive sequence is still limited. In order to better understand the karyotype diversification, we analyzed nine species of Trichomycterus using classical and molecular cytogenetic techniques. Results revealed a conserved diploid chromosome number of 2n=54 chromosomes in all analyzed species, although remarkable differences on the constitutive heterochromatin distribution were observed. In addition, while the 18S rDNA showed a conserved distribution pattern, the 5S rDNA sites showed a quite diverse location considering the analyzed species. Remarkably, both ribosomal genes were co-located in all species, except in T . iheringi , suggesting that co-localization is probably an ancestral condition in Trichomycterus . Finally, three analyzed species showed heterochromatic B chromosomes, reinforcing the intense genomic reorganization occurring in Trichomycterus . Our results showed that chromosomal variations are not restricted to differences in karyotype formula as previously proposed, but also to modifications on the microstructural level of resolution.

Trichomycterus é um especioso gênero dentro de Trichomycterinae e exibe marcante diversidade cariotípica. No entanto, o conhecimento sobre sua estrutura genômica e localização de seqüências repetitivas ainda é restrita. Para um melhor conhecimento sobre a sua diversificação cariotípica, nós analisamos nove especies de Trichomycterus usando técnicas de citogenética clássica e molecular. Os resultados revelaram um conservado número diploide de 2n = 54 cromossomos em todas as espécies analisadas, embora diferentes marcações na distribuição da heterocromatina constitutiva tenham sido observadas. Além disso, enquanto o DNAr 18S mostrou um padrão de distribuição conservado, os sítios de DNAr 5S mostraram uma localização bastante diversa, considerando as espécies analisadas. Ambos os genes ribossomais foram co-localizados em todas as espécies, exceto em T. iheringi , sugerindo que a co-localização é provavelmente uma condição ancestral em Trichomycterus . Finalmente, três espécies analisadas mostraram cromossomos B heterocromáticos, reforçando uma intensa reoganização genômica ocorrendo em Trichomycterus . Nossos resultados mostraram que variações cromossômicas não estão restritas à diferenças na fórmula cariotípica, como proposto anteriormente, mas também às alterações a níveis de resolução estrutural.

Repetitive DNA Sequences and Evolution of ZZ/ZW Sex Chromosomes in Characidium (Teleostei: Characiformes).

Characidium constitutes an interesting model for cytogenetic studies, since a large degree of karyotype variation has been detected in this group, like the presence/absence of sex and supernumerary chromosomes and variable distribution of repetitive sequences in different species/populations. In this study, we performed a comparative cytogenetic analysis in 13 Characidium species collected at different South American river basins in order to investigate the karyotype diversification in this group. Chromosome analyses involved the karyotype characterization, cytogenetic mapping of repetitive DNA sequences and cross-species chromosome painting using a W-specific probe obtained in a previous study from Characidium gomesi. Our results evidenced a conserved diploid chromosome number of 2n = 50, and almost all the species exhibited homeologous ZZ/ZW sex chromosomes in different stages of differentiation, except C. cf. zebra, C. tenue, C. xavante and C. stigmosum. Notably, some ZZ/ZW sex chromosomes showed 5S and/or 18S rDNA clusters, while no U2 snDNA sites could be detected in the sex chromosomes, being restricted to a single chromosome pair in almost all the analyzed species. In addition, the species Characidium sp. aff. C. vidali showed B chromosomes with an inter-individual variation of 1 to 4 supernumerary chromosomes per cell. Notably, these B chromosomes share sequences with the W-specific probe, providing insights about their origin. Results presented here further confirm the extensive karyotype diversity within Characidium in contrast with a conserved diploid chromosome number. Such chromosome differences seem to constitute a significant reproductive barrier, since several sympatric Characidium species had been described during the last few years and no interespecific hybrids were found.

Extensive spreading of interstitial telomeric sites on the chromosomes of Characidium (Teleostei, Characiformes).

Characidium comprises several species of small freshwater fish that display conserved diploid chromosome numbers and karyotypic formulae. In this study, a comparative cytogenetic analysis using telomeric DNA probes was carried out in nine species of Characidium; a molecular phylogenetic analysis with mitochondrial DNA was also performed in order to investigate the direction of the evolutionary chromosome changes observed here. Our results showed the existence of species with several and variable interstitial telomeric sites (ITSs), with other species showing only terminal signals in their chromosomes. Molecular phylogenetic data suggested that these ITSs emerged once in the evolutionary history of Characidium and were later differentially spread in distinct species/populations of this clade. Additionally, the origin of an exclusive acrocentric pair found in C. pterostictum, C. serrano and C. timbuiense was also investigated, revealing that this pair possibly had a common origin to these species. These results evidence the occurrence of intense and continuous genomic changes among species of Characidium.

Single origin of sex chromosomes and multiple origins of B chromosomes in fish genus Characidium.

Chromosome painting with DNA probes obtained from supernumerary (B) and sex chromosomes in three species of fish genus Characidium (C. gomesi, C. pterostictum and C. oiticicai) showed a close resemblance in repetitive DNA content between B and sex chromosomes in C. gomesi and C. pterostictum. This suggests an intraspecific origin for B chromosomes in these two species, probably deriving from sex chromosomes. In C. oiticicai, however, a DNA probe obtained from its B chromosome hybridized with the B but not with the A chromosomes, suggesting that the B chromosome in this species could have arisen interspecifically, although this hypothesis needs further investigation. A molecular phylogenetic analysis performed on nine Characidium species, with two mtDNA genes, showed that the presence of heteromorphic sex chromosomes in these species is a derived condition, and that their origin could have been unique, a conclusion also supported by interspecific chromosome painting with a CgW probe derived from the W chromosome in C. gomesi. Summing up, our results indicate that whereas heteromorphic sex chromosomes in the genus Characidium appear to have had a common and unique origin, B chromosomes may have had independent origins in different species. Our results also show that molecular phylogenetic analysis is an excellent complement for cytogenetic studies by unveiling the direction of evolutionary chromosome changes.

Scattered organization of the histone multigene family and transposable elements in Synbranchus.

The fish species Synbranchus marmoratus is widely distributed throughout the Neotropical region and exhibits a significant karyotype differentiation. However, data concerning the organization and location of the repetitive DNA sequences in the genomes of these karyomorphs are still lacking. In this study we made a physical mapping of the H3 and H4 histone multigene family and the transposable elements Rex1 and Rex3 in the genome of three known S. marmoratus karyomorphs. The results indicated that both histone sequences seem to be linked with one another and are scattered all over the chromosomes of the complement, with a little compartmentalization in one acrocentric pair, which is different from observations in other fish groups. Likewise, the transposable elements Rex1 and Rex3 were also dispersed throughout the genome as small clusters. The data also showed that the histone sites are organized in a differentiated manner in the genomes of S. marmoratus, while the transposable elements Rex1 and Rex3 do not seem to be compartmentalized in this group.

Delimiting the origin of a B chromosome by FISH mapping, chromosome painting and DNA sequence analysis in Astyanax paranae (Teleostei, Characiformes).

Supernumerary (B) chromosomes have been shown to contain a wide variety of repetitive sequences. For this reason, fluorescent in situ hybridisation (FISH) is a useful tool for ascertaining the origin of these genomic elements, especially when combined with painting from microdissected B chromosomes. In order to investigate the origin of B chromosomes in the fish species Astyanax paranae, these two approaches were used along with PCR amplification of specific DNA sequences obtained from the B chromosomes and its comparison with those residing in the A chromosomes. Remarkably, chromosome painting with the one-arm metacentric B chromosome probe showed hybridization signals on entire B chromosome, while FISH mapping revealed the presence of H1 histone and 18S rDNA genes symmetrically placed in both arms of the B chromosome. These results support the hypothesis that the B chromosome of A. paranae is an isochromosome. Additionally, the chromosome pairs Nos. 2 or 23 are considered the possible B chromosome ancestors since both contain syntenic H1 and 18S rRNA sequences. The analysis of DNA sequence fragments of the histone and rRNA genes obtained from the microdissected B chromosomes showed high similarity with those obtained from 0B individuals, which supports the intraspecific origin of B chromosomes in A. paranae. Finally, the population hereby analysed showed a female-biased B chromosome presence suggesting that B chromosomes in this species could influence sex determinism.

Karyotype diversity and patterns of chromosomal evolution in Eigenmannia (Teleostei, Gymnotiformes, Sternopygidae).

Conventional (Giemsa, C-banding, Ag - NORs) and molecular [5S rDNA, 18S rDNA, (TTAGGG)n] cytogenetic techniques were employed to study six species of the genus Eigenmannia Jordan & Evermann, 1896. They exhibited diploid chromosome numbers ranging from 2n=28 (Eigenmannia sp.1) to 2n=38 (Eigenmanniavirescens (Valenciennes, 1836)). The C-banding results revealed that species with the lowest 2n have less heterochromatin content and that morphologically differentiated sex chromosomes observed in two species showed distinct patterns of heterochromatin. While the X1, X2 and Y-chromosomes of Eigenmannia sp.2 showed only centromeric heterochromatin, the XY sex chromosomes of Eigenmanniavirescens possessed large heterochromatic blocks in the terminal position, particularly on the X chromosome. The nucleolus organizer regions (NORs) were located in different positions when compared to the 5S rDNA sites. Additionally, the presence of minor ribosomal gene sites on the sex chromosome pair of Eigenmanniavirescens represented a new type of the sex chromosomes in this group. The telomeric probe (TTAGGG)n hybridized to the terminal portion of all chromosomes in all species examined however, interstitial telomeric sites were found in the metacentric pair No. 2 in Eigenmannia sp.1. The analyzes confirmed some hypotheses about karyotype evolution in the genus Eigenmannia, and brought new information about the distribution of the genetic material in the chromosomes of the samples analyzed providing new insights for understanding the process differentiation in the genomes of species under study.

Chromosomal organization of repetitive DNA sequences in Astyanax bockmanni (Teleostei, Characiformes): dispersive location, association and co-localization in the genome.

Repetitive DNA sequences constitute a great portion of the genome of eukaryotes and are considered key components to comprehend evolutionary mechanisms and karyotypic differentiation. Aiming to contribute to the knowledge of chromosome structure and organization of some repetitive DNA classes in the fish genome, chromosomes of two allopatric populations of Astyanax bockmanni were analyzed using classic cytogenetics techniques and fluorescent in situ hybridization, with probes for ribosomal DNA sequences, histone DNA and transposable elements. These Astyanax populations showed the same diploid number (2n = 50), however with differences in chromosome morphology, distribution of constitutive heterochromatin, and location of 18S rDNA and retroelement Rex3 sites. In contrast, sites for 5S rDNA and H1, H3 and H4 histones showed to be co-located and highly conserved. Our results indicate that dispersion and variability of 18S rDNA and heterochromatin sites are not associated with macro rearrangements in the chromosome structure of these populations. Similarly, distinct evolutionary mechanisms would act upon histone genes and 5S rDNA, contributing to chromosomal association and co-location of these sequences. Data obtained indicate that distinct mechanisms drive the spreading of repetitive DNAs in the genome of A. bockmanni. Also, mobile elements may account for the polymorphism of the major rDNA sites and heterochromatin in this genus.

Chromosome painting of Z and W sex chromosomes in Characidium (Characiformes, Crenuchidae).

Some species of the genus Characidium have heteromorphic ZZ/ZW sex chromosomes with a totally heterochromatic W chromosome. Methods for chromosome microdissection associated with chromosome painting have become important tools for cytogenetic studies in Neotropical fish. In Characidium cf. fasciatum, the Z chromosome contains a pericentromeric heterochromatin block, whereas the W chromosome is completely heterochromatic. Therefore, a probe was produced from the W chromosome through microdissection and degenerate oligonucleotide-primed polymerase chain reaction amplification. FISH was performed using the W probe on the chromosomes of specimens of this species. This revealed expressive marks in the pericentromeric region of the Z chromosome as well as a completely painted W chromosome. When applying the same probe on chromosome preparations of C. cf. gomesi and Characidium sp., a pattern similar to C. cf. fasciatum was found, while C. cf. zebra, C. cf. lagosantense and Crenuchus spilurus species showed no hybridization signals. Structural changes in the chromosomes of an ancestral sexual system in the group that includes the species C. cf. gomesi, C. cf. fasciatum and Characidium sp., could have contributed to the process of speciation and could represent a causal mechanism of chromosomal diversification in this group. The heterochromatinization process possibly began in homomorphic and homologous chromosomes of an ancestral form, and this process could have given rise to the current patterns found in the species with sex chromosome heteromorphism.

Karyotypic conservatism in samples of Characidium cf. zebra (Teleostei, Characiformes, Crenuchidae): Physical mapping of ribosomal genes and natural triploidy.

Basic and molecular cytogenetic analyses were performed in specimens of Characidium cf. zebra from five collection sites located throughout the Tietê, Paranapanema and Paraguay river basins. The diploid number in specimens from all samples was 2n = 50 with a karyotype composed of 32 metacentric and 18 submetacentric chromosomes in both males and females. Constitutive heterochromatin was present at the centromeric regions of all chromosomes and pair 23, had additional interstitial heterochromatic blocks on its long arms. The nucleolar organizer regions (NORs) were located on the long arms of pair 23, while the 5S rDNA sites were detected in different chromosomes among the studied samples. One specimen from the Alambari river was a natural triploid and had two extra chromosomes, resulting in 2n = 77. The remarkable karyotypic similarity among the specimens of C. cf. zebra suggests a close evolutionary relationship. On the other hand, the distinct patterns of 5S rDNA distribution may be the result of gene flow constraints during their evolutionary history.

Karyotypic diversity in four species of the genus Gymnotus Linnaeus, 1758 (Teleostei, Gymnotiformes, Gymnotidae): physical mapping of ribosomal genes and telomeric sequences.

Conventional (Giemsa, C-Banding, Ag-NORs, CMA3) and molecular (5S rDNA, 18S rDNA, telomeric sequences) cytogenetic studies were carried out in specimens of ten distinct fish populations of the genus Gymnotus (Gymnotus sylvius Albert and Fernandes-Matioli, 1999, Gymnotus inaequilabiatus Valenciennes, 1839, Gymnotus pantherinus Steindachner, 1908, and G. cf. carapo Linnaeus, 1758) from different Brazilian hydrographic basins. Gymnotus sylvius presented a diploid number of 40 chromosomes (22m+12sm+6st), Gymnotus pantherinus presented 52 chromosomes (32m+18sm+2st), while Gymnotus inaequilabiatus (42m+10sm+2a)and Gymnotus cf. carapo (38m+12sm+4st) presented 54 chromosomes. The C-banding technique revealed centromeric marks in all chromosomes of all species. Besides that, conspicuous blocks of heterochromatin were found interstitially on the chromosomes of Gymnotus inaequilabiatus, Gymnotus cf. carapo,and Gymnotus pantherinus. All four species showed single nucleolus organizing regions confirmed by results obtained through Ag-NORs and FISH experiments using 18S rDNA probes, which showed the NORs localized on the first chromosome pair in Gymnotus inaequilabiatus, Gymnotus cf. carapo,and Gymnotus pantherinus, and on pair 2 in Gymnotus sylvius. CMA3 staining revealed additional unrelated NORs marks in Gymnotus sylvius and Gymnotus pantherinus. The 5S rDNA probes revealed signals on one pair in Gymnotus sylvius and two pairs in Gymnotus pantherinus; Gymnotus inaequilabiatus had about seventeen pairs marked, and Gymnotus cf. carapo had about fifteen pairs marked. It is considered that the high amount of heterochromatin identified in the chromosomes of Gymnotus inaequilabiatus and Gymnotus cf. carapo could have facilitated the dispersion of 5S rDNA in these species. Interstitial signals were detected on the first metacentric pair of Gymnotus sylvius by telomeric probes (TTAGGG)n indicating the possible occurrence of chromosomal fusions in this species. The present study reveals valuable cytotaxonomic markers for this group and allows a more precise evaluation of the processes involved in the karyotype differentiation and the interrelationships among different species of the genus Gymnotus.

Karyotypic conservatism in samples of Characidium cf. zebra (Teleostei, Characiformes, Crenuchidae): physical mapping of ribosomal genes and natural triploidy

Basic and molecular cytogenetic analyses were performed in specimens of Characidium cf. zebra from five collection sites located throughout the Tietê, Paranapanema and Paraguay river basins. The diploid number in specimens from all samples was 2n = 50 with a karyotype composed of 32 metacentric and 18 submetacentric chromosomes in both males and females. Constitutive heterochromatin was present at the centromeric regions of all chromosomes and pair 23, had additional interstitial heterochromatic blocks on its long arms. The nucleolar organizer regions (NORs) were located on the long arms of pair 23, while the 5S rDNA sites were detected in different chromosomes among the studied samples. One specimen from the Alambari river was a natural triploid and had two extra chromosomes, resulting in 2n = 77. The remarkable karyotypic similarity among the specimens of C. cf. zebra suggests a close evolutionary relationship. On the other hand, the distinct patterns of 5S rDNA distribution may be the result of gene flow constraints during their evolutionary history.