Localisation of NORs and counterstain-enhanced fluorescence studies inPerca fluviatilis (Pisces, Percidae)

Fast satellite DNA evolution in Nothobranchius annual killifishes

Satellite DNA (satDNA) is a rapidly evolving class of tandem repeats, with some monomers being involved in centromere organization and function. To identify repeats associated with (peri)centromeric regions, we investigated satDNA across Southern and Coastal clades of African annual killifishes of the genus Nothobranchius. Molecular cytogenetic and bioinformatic analyses revealed that two previously identified satellites, designated here as NkadSat01-77 and NfurSat01-348, are associated with (peri)centromeres only in one lineage of the Southern clade. NfurSat01-348 was, however, additionally detected outside centromeres in three members of the Coastal clade. We also identified a novel satDNA, NrubSat01-48, associated with (peri)centromeres in N. foerschi, N. guentheri, and N. rubripinnis. Our findings revealed fast turnover of satDNA associated with (peri)centromeres and different trends in their evolution in two clades of the genus Nothobranchius.

Insights into the Karyotype Evolution of Charinidae, the Early-Diverging Clade of Whip Spiders (Arachnida: Amblypygi)

Whip spiders (Amblypygi) represent an ancient order of tetrapulmonate arachnids with a low diversity. Their cytogenetic data are confined to only a few reports. Here, we analyzed the family Charinidae, a lineage almost at the base of the amblypygids, providing an insight into the ancestral traits and basic trajectories of amblypygid karyotype evolution. We performed Giemsa staining, selected banding techniques, and detected 18S ribosomal DNA and telomeric repeats by fluorescence in situ hybridization in four Charinus and five Sarax species. Both genera exhibit a wide range of diploid chromosome numbers (2n = 42-76 and 22-74 for Charinus and Sarax, respectively). The 2n reduction was accompanied by an increase of proportion of biarmed elements. We further revealed a single NOR site (probably an ancestral condition for charinids), the presence of a (TTAGG)n telomeric motif localized mostly at the chromosome ends, and an absence of heteromorphic sex chromosomes. Our data collectively suggest a high pace of karyotype repatterning in amblypygids, with probably a high ancestral 2n and its subsequent gradual reduction by fusions, and the action of pericentric inversions, similarly to what has been proposed for neoamblypygids. The possible contribution of fissions to charinid karyotype repatterning, however, cannot be fully ruled out.

Evidence of Interspecific Chromosomal Diversification in Rainbowfishes (Melanotaeniidae, Teleostei)

Rainbowfishes (Melanotaeniidae) are the largest monophyletic group of freshwater fishes occurring in Australia and New Guinea, with 112 species currently recognised. Despite their high taxonomic diversity, rainbowfishes remain poorly studied from a cytogenetic perspective. Using conventional (Giemsa staining, C banding, chromomycin A3 staining) and molecular (fluorescence in situ hybridisation with ribosomal DNA (rDNA) and telomeric probes) cytogenetic protocols, karyotypes and associated chromosomal characteristics of five species were examined. We covered all major lineages of this group, namely, Running River rainbowfish Melanotaenia sp., red rainbowfish Glossolepisincisus, threadfin rainbowfish Iriatherina werneri, ornate rainbowfish Rhadinocentrus ornatus, and Cairns rainbowfish Cairnsichthys rhombosomoides. All species had conserved diploid chromosome numbers 2n = 48, but karyotypes differed among species; while Melanotaenia sp., G. incisus, and I. werneri possessed karyotypes composed of exclusively subtelo/acrocentric chromosomes, the karyotype of R. ornatus displayed six pairs of submetacentric and 18 pairs of subtelo/acrocentric chromosomes, while C. rhombosomoides possessed a karyotype composed of four pairs of submetacentric and 20 pairs of subtelo/acrocentric chromosomes. No heteromorphic sex chromosomes were detected using conventional cytogenetic techniques. Our data indicate a conserved 2n in Melanotaeniidae, but morphologically variable karyotypes, rDNA sites, and heterochromatin distributions. Differences were observed especially in taxonomically divergent species, suggesting interspecies chromosome rearrangements.

Taxonomic Diversity Not Associated with Gross Karyotype Differentiation: The Case of Bighead Carps, Genus Hypophthalmichthys (Teleostei, Cypriniformes, Xenocyprididae)

The bighead carps of the genus Hypophthalmichthys (H. molitrix and H. nobilis) are important aquaculture species. They were subjected to extensive multidisciplinary research, but with cytogenetics confined to conventional protocols only. Here, we employed Giemsa-/C-/CMA3- stainings and chromosomal mapping of multigene families and telomeric repeats. Both species shared (i) a diploid chromosome number 2n = 48 and the karyotype structure, (ii) low amount of constitutive heterochromatin, (iii) the absence of interstitial telomeric sites (ITSs), (iv) a single pair of 5S rDNA loci adjacent to one major rDNA cluster, and (v) a single pair of co-localized U1/U2 snDNA tandem repeats. Both species, on the other hand, differed in (i) the presence/absence of remarkable interstitial block of constitutive heterochromatin on the largest acrocentric pair 11 and (ii) the number of major (CMA3-positive) rDNA sites. Additionally, we applied here, for the first time, the conventional cytogenetics in H. harmandi, a species considered extinct in the wild and/or extensively cross-hybridized with H. molitrix. Its 2n and karyotype description match those found in the previous two species, while silver staining showed differences in distribution of major rDNA. The bighead carps thus represent another case of taxonomic diversity not associated with gross karyotype differentiation, where 2n and karyotype structure cannot help in distinguishing between genomes of closely related species. On the other hand, we demonstrated that two cytogenetic characters (distribution of constitutive heterochromatin and major rDNA) may be useful for diagnosis of pure species. The universality of these markers must be further verified by analyzing other pure populations of bighead carps.

Chromosomal mapping of repetitive sequences in Hyphessobrycon eques (Characiformes, Characidae): a special case of the spreading of 5S rDNA clusters in a genome

Cytogenetic data showed a variation in diploid chromosome number in the genus Hyphessobrycon ranging from 2n = 46 to 52, and studies involving repetitive DNA sequences are scarce in representatives of this genus. The purpose of this paper was the chromosomal mapping of repetitive sequences (rDNA, histone genes, U snDNA and microsatellites) and investigation of the amplification of 5S rDNA clusters in the Hyphessobrycon eques genome. Two H. eques populations displayed 2n = 52 chromosomes, with the acrocentric pair No. 24 bearing Ag-NORs corresponding with CMA₃+/DAPI-. FISH with a 18S rDNA probe identified the NORs on the short (p) arms of the acrocentric pairs Nos. 22 and 24. The 5S rDNA probe visualized signals on almost all chromosomes in genomes of individuals from both populations (40 signals); FISH with H3 histone probe identified two chromosome pairs, with the pericentromeric location of signals; FISH with a U2 snDNA probe identified one chromosome pair bearing signals, on the interstitial chromosomal region. The mononucleotide (A), dinucleotide (CA) and tetranucleotide (GATA) repeats were observed on the centromeric/pericentromeric and/or terminal positions of all chromosomes, while the trinucleotide (CAG) repeat showed signals on few chromosomes. Molecular analysis of 5S rDNA and non-transcribed spacers (NTS) showed microsatellites (GATA and A repeats) and a fragment of retrotransposon (SINE3/5S-Sauria) inside the sequences. This study expanded the available cytogenetic data for H. eques and demonstrated to the dispersion of the 5S rDNA sequences on almost all chromosomes.

Karyotype and chromosomal characteristics of rDNA of Cobitisstrumicae Karaman, 1955 (Teleostei, Cobitidae) from Lake Volvi, Greece

The karyotype of Greek cobitid fish Cobitisstrumicae Karaman, 1955, from Lake Volvi, Greece, a representative of one of its two major intraspecific phylogenetic lineages, was analysed by means of sequential Giemsa-staining, C-banding, silver-staining, CMA3 fluorescence banding and also by in situ hybridization (FISH) with rDNA probe. The diploid chromosome number was 2n = 50, karyotype composed of 10 pairs of metacentric to submetacentric and 15 pairs of subtelocentric to acrocentric chromosomes. The nucleolus organizer regions (NORs) as revealed by Ag- and CMA3 staining and FISH were situated in the telomeric region of the fourth submetacentric chromosome pair. The chromosomes contained very low content of C-positive heterochromatin. No heteromorphic sex chromosomes were detected. This first karyotype report for any species of lineage Bicanestrinia Băcescu, 1962 shows a simple karyotype dominated by acrocentric chromosomes and possessing single NOR-bearing chromosome pair. Cytotaxonomic implications of this finding for the taxonomy of the genus Cobitis Linnaeus, 1758 are further discussed.

From Chromosomes to Genome: Insights into the Evolutionary Relationships and Biogeography of Old World Knifefishes (Notopteridae; Osteoglossiformes)

In addition to its wide geographical distribution, osteoglossiform fishes represent one of the most ancient freshwater teleost lineages; making it an important group for systematic and evolutionary studies. These fishes had a Gondwanan origin and their past distribution may have contributed to the diversity present in this group. However, cytogenetic and genomic data are still scarce, making it difficult to track evolutionary trajectories within this order. In addition, their wide distribution, with groups endemic to different continents, hinders an integrative study that allows a globalized view of its evolutionary process. Here, we performed a detailed chromosomal analysis in Notopteridae fishes, using conventional and advanced molecular cytogenetic methods. Moreover, the genetic distances of examined species were assessed by genotyping using diversity arrays technology sequencing (DArTseq). These data provided a clear picture of the genetic diversity between African and Asian Notopteridae species, and were highly consistent with the chromosomal, geographical, and historical data, enlightening their evolutionary diversification. Here, we discuss the impact of continental drift and split of Pangea on their recent diversity, as well as the contribution to biogeographical models that explain their distribution, highlighting the role of the Indian subcontinent in the evolutionary process within the family.

Dynamics of tandemly repeated DNA sequences during evolution of diploid and tetraploid botiid loaches (Teleostei: Cobitoidea: Botiidae)

Polyploidization has played an important role in the evolution of vertebrates, particularly at the base of Teleostei-an enormously successful ray-finned fish group with additional genome doublings on lower taxonomic levels. The investigation of post-polyploid genome dynamics might provide important clues about the evolution and ecology of respective species and can help to decipher the role of polyploidy per se on speciation. Few studies have attempted to investigate the dynamics of repetitive DNA sequences in the post-polyploid genome using molecular cytogenetic tools in fishes, though recent efforts demonstrated their usefulness. The demonstrably monophyletic freshwater loach family Botiidae, branching to evolutionary diploid and tetraploid lineages separated >25 Mya, offers a suited model group for comparing the long-term repetitive DNA evolution. For this, we integrated phylogenetic analyses with cytogenetical survey involving Giemsa- and Chromomycin A3 (CMA3)/DAPI stainings and fluorescence in situ hybridization with 5S/45S rDNA, U2 snDNA and telomeric probes in representative sample of 12 botiid species. The karyotypes of all diploids were composed of 2n = 50 chromosomes, while majority of tetraploids had 2n = 4x = 100, with only subtle interspecific karyotype differences. The exceptional karyotype of Botia dario (2n = 4x = 96) suggested centric fusions behind the 2n reduction. Variable patterns of FISH signals revealed cases of intraspecific polymorphisms, rDNA amplification, variable degree of correspondence with CMA3+ sites and almost no phylogenetic signal. In tetraploids, either additivity or loci gain/loss was recorded. Despite absence of classical interstitial telomeric sites, large blocks of interspersed rDNA/telomeric regions were found in diploids only. We uncovered different molecular drives of studied repetitive DNA classes within botiid genomes as well as the advanced stage of the re-diploidization process in tetraploids. Our results may contribute to link genomic approach with molecular cytogenetic analyses in addressing the origin and mechanism of this polyploidization event.

Karyotype and Mapping of Repetitive DNAs in the African Butterfly Fish Pantodon buchholzi, the Sole Species of the Family Pantodontidae

The monophyletic order Osteoglossiformes represents one of the most ancestral groups of teleosts and has at least 1 representative in all continents of the southern hemisphere, with the exception of Antarctica. However, despite its phylogenetic and biogeographical importance, cytogenetic data in Osteoglossiformes are scarce. Here, karyotype and chromosomal characteristics of the lower Niger River population of the African butterfly fish Pantodon buchholzi, the sole species of the family Pantodontidae (Osteoglossiformes), were examined using conventional and molecular cytogenetic approaches. All specimens examined had 2n = 46 chromosomes, with a karyotype composed of 5 pairs of metacentric, 5 pairs of submetacentric, and 13 pairs of acrocentric chromosomes in both sexes. No morphologically differentiated sex chromosomes were identified. C-bands were located in the centromeric/pericentromeric region of all chromosomes and were associated with the single AgNOR site. FISH with ribosomal DNA probes revealed that both 5S and 18S rDNA were present in only 1 pair of chromosomes each, but did not colocalize. CMA3+ bands were observed near the telomeres in several chromosome pairs and also at the 18S rDNA sites. The mapping of di- and trinucleotide repeat motifs, Rex6 transposable element, and U2 snRNA showed a scattered distribution over most of the chromosomes, but for some microsatellites and the U2 snRNA also a preferential accumulation at telomeric regions. This study presents the first detailed cytogenetic analysis in the African butterfly fish by both conventional and molecular cytogenetic protocols. This is the first of a series of further cytogenetic and cytogenomic studies on osteoglossiforms, aiming to comprehensively examine the chromosomal evolution in this phylogenetically important fish order.

Karyotype differentiation in 19 species of river loach fishes (Nemacheilidae, Teleostei): extensive variability associated with rDNA and heterochromatin distribution and its phylogenetic and ecological interpretation

BACKGROUND

Loaches of the family Nemacheilidae are one of the most speciose elements of Palearctic freshwater ichthyofauna and have undergone rapid ecological adaptations and colonizations. Their cytotaxonomy is largely unexplored; with the impact of cytogenetical changes on this evolutionary diversification still unknown. An extensive cytogenetical survey was performed in 19 nemacheilid species using both conventional (Giemsa staining, C- banding, Ag- and Chromomycin A3/DAPI stainings) and molecular (fluorescence in situ hybridization with 5S rDNA, 45S rDNA, and telomeric (TTAGGG)n probes) methods. A phylogenetic tree of the analysed specimens was constructed based on one mitochondrial (cytochrome b) and two nuclear (RAG1, IRBP) genes.

RESULTS

Seventeen species showed karyotypes composed of 2n = 50 chromosomes but differentiated by fundamental chromosome number (NF = 68-90). Nemachilichthys ruppelli (2n = 38) and Schistura notostigma (2n = 44-48) displayed reduced 2n with an elevated number of large metacentric chromosomes. Only Schistura fasciolata showed morphologically differentiated sex chromosomes with a multiple system of the XY1Y2 type. Chromomycin A3 (CMA3)- fluorescence revealed interspecific heterogeneity in the distribution of GC-rich heterochromatin including its otherwise very rare association with 5S rDNA sites. The 45S rDNA sites were mostly located on a single chromosome pair contrasting markedly with a pattern of two (Barbatula barbatula, Nemacheilus binotatus, N. ruppelli) to 20 sites (Physoschistura sp.) of 5S rDNA. The cytogenetic changes did not follow the phylogenetic relationships between the samples. A high number of 5S rDNA sites was present in species with small effective population sizes.

CONCLUSION

Despite a prevailing conservatism of 2n, Nemacheilidae exhibited a remarkable cytogenetic variability on microstructural level. We suggest an important role for pericentric inversions, tandem and centric fusions in nemacheilid karyotype differentiation. Short repetitive sequences, genetic drift, founder effect, as well as the involvement of transposable elements in the dispersion of ribosomal DNA sites, might also have played a role in evolutionary processes such as reproductive isolation. These remarkable dynamics of their genomes qualify river loaches as a model for the study of the cytogenetic background of major evolutionary processes such as radiation, endemism and colonization of a wide range of habitats.

Comparative cytogenetics in the genus Hoplias (Characiformes, Erythrinidae) highlights contrasting karyotype evolution among congeneric species

BACKGROUND

The Erythrinidae fish family contains three genera, Hoplias, Erythrinus and Hoplerythrinus widely distributed in Neotropical region. Remarkably, species from this family are characterized by an extensive karyotype diversity, with 2n ranging from 39 to 54 chromosomes and the occurrence of single and/or multiple sex chromosome systems in some species. However, inside the Hoplias genus, while H. malabaricus was subject of many studies, the cytogenetics of other congeneric species remains poorly explored. In this study, we have investigated chromosomal characteristics of four Hoplias species, namely H. lacerdae, H. brasiliensis, H. intermedius and H. aimara. We used conventional staining techniques (C-banding, Ag-impregnation and CMA3 -fluorescence) as well as fluorescence in situ hybridization (FISH) with minor and major rDNA and microsatellite DNAs as probes in order to analyze the karyotype evolution within the genus.

RESULTS

All species showed invariably 2n = 50 chromosomes and practically identical karyotypes dominated only by meta- and submetacentric chromosomes, the absence of heteromorphic sex chromosomes, similar pattern of C-positive heterochromatin blocks and homologous Ag-NOR-bearing pairs. The cytogenetic mapping of five repetitive DNA sequences revealed some particular interspecific differences between them. However, the examined chromosomal characteristics indicate that their speciation was not associated with major changes in their karyotypes.

CONCLUSION

Such conserved karyotypes contrasts with the extensive karyotype diversity that has been observed in other Erythrinidae species, particularly in the congeneric species H. malabaricus. Nevertheless, what forces drive such particularly different modes of karyotype evolution among closely related species? Different life styles, population structure and inner chromosomal characteristics related to similar cases in other vertebrate groups can also account for the contrasting modes of karyotype evolution in Hoplias genus.

Comparative cytogenetics of Neotropical cichlid fishes (Nannacara, Ivanacara and Cleithracara) indicates evolutionary reduction of diploid chromosome numbers

A comparative cytogenetic analysis was carried out in five species of a monophyletic clade of neotropical Cichlasomatine cichlids, namely Cleithracara maronii Steindachner, 1881, Ivanacara adoketa (Kullander & Prada-Pedreros, 1993), Nannacara anomala Regan, 1905, N. aureocephalus Allgayer, 1983 and N. taenia Regan, 1912. Karyotypes and other chromosomal characteristics were revealed by CDD banding and mapped onto the phylogenetic hypothesis based on molecular analyses of four genes, namely cyt b, 16S rRNA, S7 and RAG1. The diploid numbers of chromosomes ranged from 44 to 50, karyotypes were composed predominantly of monoarmed chromosomes and one to three pairs of CMA3 signal were observed. The results showed evolutionary reduction in this monophyletic clade and the cytogenetic mechanisms (fissions/fusions) were hypothesized and discussed.

Molecular cytogenetic study of heterochromatin in Hisonotus leucofrenatus (Teleostei, Loricariidae, Hypoptopomatinae)

The fish species Hisonotus leucofrenatus exhibits a large amount of C-band positive segments with different responses after application of the C-banding technique. Type I class named herein appeared to be heavily stained after C-banding in the terminal position of five chromosome pairs and type II class, weakly stained after C-banding in the interstitial or pericentromeric position in nine chromosome pairs and in the supernumerary chromosomes. No variation was observed in type II C-band positive segments, however, type I segments displayed conspicuous polymorphisms, and six cytotypes were detected among the fish analyzed. Chromosomes were also analyzed by CMA(3) and DAPI staining, which showed that type I C-band positive segments comprised both AT-rich and GC-rich DNA, while type II segments were mainly composed of GC-rich sequences. HindIII-digested genomic DNA exhibits fragments of the ladder-like pattern, characteristic of tandemly arrayed repetitive sequences. Two of those fragments corresponding to monomeric and dimeric units of a 78 bp repetitive DNA sequence were cloned and sequenced. The cloned repetitive DNA was used as probe in fluorescent in situ hybridization experiments. The results revealed that these sequences were located in the same position as the type I C-band positive segments. This satellite DNA did not hybridize with DNA from other species of Hisonotus or from other fish of the family Loricariidae, suggesting that this sequence is specific to H. leucofrenatus. The role of these repetitive sequences in the karyotypic evolution of this species is discussed.

Karyotype and chromosomal characteristics of Ag-NOR sites and 5S rDNA in European smelt, Osmerus eperlanus

Karyotype and cytogenetic characteristics of European smelt Osmerus eperlanus were investigated using different staining techniques (sequential Ag-, CMA3 and DAPI banding) and PRINS to detect 5S rDNA and telomeric sites. The diploid chromosome number was invariably 2n = 56 and karyotype composed of 5 pairs of metacentrics, 9 pairs of subtelocentrics and 14 pairs of subtelo- to acrocentrics. The DAPI-positive heterochromatic regions were found in centromeric positions on bi-armed chromosomes and few acrocentrics. Additionally, some interstitial DAPI-positive bands were identified on three pairs of submetacentric chromosomes. The nucleolar organizer regions (NORs) were detected in the short (p) arms of the largest metacentric pair of chromosomes No. 1. Sequential banding (Giemsa-, AgNO(3) and CMA(3) stainings) revealed NOR sites corresponding to achromatic regions but not associated with CMA(3)-positive blocks of heterochromatin located on either side of NORs. Individuals from the analyzed population had this conspicuous pair of chromosomes always in heterozygous combination. A complex inversion system was hypothesized to be involved in the origin of the observed variation but analysis with telomeric PRINS and PNA-FISH did not reveal any Interstitial Telomeric Sites (ITS). Hybridization signals were confined exclusively to terminal chromosomal regions. The 5S ribosomal sites as revealed by PRINS were found to be invariably located in the short (p) arms of four pairs of subtelocentric chromosomes. Cytotaxonomic comparisons of the present results with the voluminous available cytogenetic data-set from salmoniform and esociformes fishes appear to support the recent view, based on robust molecular-based phylogeny, that salmoniform and osmeriform fishes are not as closely related as previously assumed.

Heterochromatin characterization of four fish species of the family Loricariidae (Siluriformes)

The karyotypic structures and the composition and distribution of the heterochromatin in the karyotypes of four catfish species belonging to four Loricariidae subfamilies were analysed, namely: Neoplecostomus microps (Neoplecostominae) with 2n=54 chromosomes, Harttia loricariformis (Loricariinae) with 2n=56 chromosomes, Hypostomus affinis (Hypostominae) with 2n=66 chromosomes and Upsilodus sp. (Upsilodinae) with 2n=96 chromosomes. The amount and composition of heterochromatin was quite unequal among the studied species, being copious and mainly GC-rich in Upsilodus sp. and scarce and balanced in H. loricariformis. All of the H. affinis heterochromatin is GC-rich and related with nucleolar organizing regions. N. microps show low quantity of interstitial and GC-rich heterochromatin, one of them being related with NORs. Trends in the macrokaryotypic diversification as well as in the distribution pattern of the heterochromatin are discussed.

Lack of correspondence between CMA3-, Ag-positive signals and 28S rDNA loci in two Iberian minnows (Teleostei, Cyprinidae) evidenced by sequential banding

Despite the growing outcome of results that put doubt upon the reliability of silver (Ag) staining and chromomycin A3 (CMA3) fluorescent banding in the detection of major ribosomal gene sites (NORs), these methods have been widely used, especially in fishes. In order to clarify the previous patterns obtained with those techniques, we performed fluorescence in situ hybridisation (FISH) with 28S rDNA probe followed by sequential CMA3 and Ag staining in diploid non-hybrid males of the Squalius alburnoides complex and in Squalius pyrenaicus. The results from all the studied specimens revealed a lack of correlation between classical and molecular techniques. Not just some other regions besides NORs were stained with CMA3 and Ag, but also the majority of the 28S rDNA sites were not detected. Care should then be taken in considering CMA3- and Ag-stained sites as NORs since their accuracy for that purpose may not always correspond to the expectations.

Amplification of a GC-rich heterochromatin in the freshwater fish Leporinus desmotes (Characiformes, Anostomidae)

This is the first description of the karyotype of Leporinus desmotes. The diploid female number was 2n = 54 meta- and submetacentric chromosomes. The nucleolar organizing regions (NORs) were studied by silver nitrate staining and rDNA fluorescence in situ hybridization (FISH) and were found to be located in the telomeric region of the long arm of the 9th pair. C-banding revealed centromeric and telomeric heterochromatin segments in most chromosomes. Intercalar blocks of heterochromatin were observed in the long arm of six chromosome pairs. Besides a NOR-adjacent heterochromatin, all of the intercalar heterochromatic segments were brightly fluorescent by mithramycin staining. These data suggest that a unique amplification of a primordial GC-rich heterochromatin, probably NOR-associated, may have taken place in the karyotype diversification of this Leporinus species.

Heterochromatin patterns and karyotype relationships within and between the genera Brycon and Salminus (Pisces, Characidae)

Chromosomes of two Brycon species (B. lundii and B. microlepis) and Salminus hilarii were analyzed. Based on constitutive heterochromatin distribution patterns, karyotypic relationships within and between Bryconinae and Salmininae were examined. A monophyletic origin for the genus Brycon, comprising at least two chromosome synapomorphies (presence of two large submetacentric bearing paracentromeric and telomeric heterochromatin), is suggested. Moreover, Bryconinae and Salmininae may represent a monophyletic unit among Characidae, as they share several chromosome features.

Heterochromatin analysis in the fish species Liposarcus anisitsi (siluriformes) and Leporinus elongatus (characiformes)

The chromosomes of two neotropical freshwater fish species, namely Liposarcus anisitsi (Siluriformes, Loricariidae) and Leporinus elongatus (Characiformes, Anostomidae), were investigated by means of C-banding, Ag-NORs, fluorochrome staining and banding by hot saline solution (HSS) treatment, to reveal patterns of heterochromatin differentiation. The karyotype of L. anisitsi is described for the first time. Staining with the GC-specific fluorescent antibiotic mithramycin (MM) revealed bright signals in some C-banded blocks in both species, suggesting that these MM+ heterochromatin contains GC-rich DNA. Banding by denaturation employing HSS, followed by Giemsa staining, yielded corresponding results documenting the thermal stability of GC-rich DNA part of heterochromatin positive after C-banding. In L. elongatus the Ag-NOR also followed the above banding patterns. However, in L. anisitsi the Ag-NOR was MM+ but negatively stained after C-banding and HSS treatment. L. elongatus also showed C-banded segments that were negative for mithramycin staining and HSS treatment. The results obtained evidence the heterochromatin heterogeneity in these fish species.

A new method for detecting nucleolus organizer regions in fish chromosomes using denaturation and propidium iodide staining

A rapid method for detecting nucleolus organizer regions (NORs) in fish chromosomes based on thermal denaturation and staining with propidium iodide is described. Under epifluorescence, the NORs of 15 fish species from six families could be detected. This protocol differentiates constitutive heterochromatin in mammalian and avian chromosomes, and in some cases, heterochromatic blocks in fish chromosomes. The staining of NORs of fish chromosomes with propidium iodide following denaturation with formalin is likely the result of differential denaturation of the rDNA due to the thermal characteristics of AT- and GC-rich domains of the rDNA cistron. This technique provides a new useful marker for descriptive fish cytogenetic studies.

Post-zygotic modifications and intra- and inter-individual nucleolar organizing region variations in fish: report of a case involving Leporinus friderici

Silver nitrate staining, a rapid and efficient method, has proven to be excellent for nucleolar organizing region (NOR) studies in fish. Some fish appear to have only two NOR-bearing chromosomes in their karyotype, whereas others probably have several. In the present study we analyzed the NORs of Leporinus friderici, a species that, on the basis of previous studies, has been considered as representative of species with NORs carried by a single chromosome pair. The analyses were performed by a combination of three methods, i.e. silver nitrate staining, staining with the GC-specific fluorochrome chromomycin A3, and in situ hybridization with digoxigenin-labeled probes. The results showed that, although more frequent and conspicuous in a single chromosome pair, the NORs of this species are present in multiple chromosomes. Intra- and inter-individual variations observed by the three methods strongly suggest the occurrence of post-zygotic modifications involving NORs. NOR identification in fish, almost exclusively performed by the silver nitrate method, is currently being re-evaluated by methods such as chromomycin A3 staining and in situ hybridization, which may provide important information leading to a better understanding of chromosome evolution in these animals.

Localisation of NORs and counterstain-enhanced fluorescence studies in Salmo gairdneri and Salmo trutta (Pisces, Salmonidae)

The karyotypes of the rainbow trout (Salmo gairdneri R.) and the brown trout (Salmo trutta L.) were analyzed by means of silver staining and the chromomycin A3/distamycin A/DAPI fluorescence banding technique. The nucleolus organizer regions (NORs) were localized at the secondary constrictions of chromosome no. 14 in S. gairdneri and of chromosome no. 10 in S. trutta. Additional silver positive dots were observed at or close to several centromeres in S. gairdneri. Brilliant chromomycin A3 (CMA3) fluorescence heterochromatin blocks were localized on both sides of the nucleolar constrictions in S. gairdneri. A polymorphic CMA3 positive band was detected close to the NORs of S. trutta. No distamycin A/DAPI intense heterochromatin blocks were detected in the genomes of the two Salmo species investigated.