Corrigendum: Evolutionary tracks of chromosomal diversification in surgeonfishes (Acanthuridae: Acanthurus) along the world's biogeographic domains

[This corrects the article DOI: 10.3389/fgene.2021.760244.].

Comparative cytogenetic patterns in Carangidae fishes in association with their distribution range

Carangidae are an important and widespreaded family of pelagic predatory fishes that inhabit reef regions or open ocean areas, some species occupying a vast circumglobal distribution. Cytogenetic comparisons among representatives of its different tribes help to understand the process of karyotype divergence in marine ecosystems due to the variable migratory ability of species. In this sense, conventional cytogenetic investigations (Giemsa staining, Ag-NORs, and C-banding), GC base-specific fluorochrome staining and FISH mapping of ribosomal DNAs were performed. Four species, Elagatisbipinnulata (Quoy et Gaimard, 1825) and Seriolarivoliana (Valenciennes, 1883) (Naucratini), with circumtropical distributions, Gnathanodonspeciosus (Forsskål, 1775) (Carangini), widely distributed in the tropical and subtropical waters of the Indian and Pacific oceans, and Trachinotuscarolinus (Linnaeus, 1766) (Trachinotini), distributed along the western Atlantic Ocean, were analyzed, thus encompassing representatives of three out its four tribes. All species have diploid chromosome number 2n = 48, with karyotypes composed mainly by acrocentric chromosomes (NF = 50-56). The 18S rDNA/Ag-NORs/GC+ and 5S rDNA loci were located on chromosomes likely homeologs. Karyotypes showed a pattern considered basal for the family or with small variations in their structures, apparently due to pericentric inversions. The migratory capacity of large pelagic swimmers, in large distribution areas, likely restricts the fixation of chromosome changes in Carangidae responsible for a low level of karyotype diversification.

A new view on the scenario of karyotypic stasis in Epinephelidae fish: Cytogenetic, historical, and biogeographic approaches

Epinephelidae (groupers) is an astonishingly diverse group of carnivorous fish widely distributed in reef environments around the world, with growing economic importance. The first chromosomal inferences suggested a conservative scenario for the family. However, to date, this has not been validated using biogeographic and phylogenetic approaches. Thus, to estimate karyotype diversification among groupers, eight species from the Atlantic and Indian oceans were investigated using conventional cytogenetic protocols and fluorescence in situ hybridization of repetitive sequences (rDNA, microsatellites, transposable elements). Despite the remarkable persistence of some symplesiomorphic karyotype patterns, such as all species sharing 2n=48 and most preserve a basal karyotype (2n=48 acrocentrics), the chromosomal diversification in the family revealed an unsuspected evolutionary dynamic, where about 40% of the species escape from the ancestral karyotype pattern. These karyotype changes showed a relation with the historical biogeography, likely as a byproduct of the progressive occupancy of new areas (huge diversity of adaptive and speciation conditions). In this context, oceanic regions harboring more recent clades such as those of the Indo-Pacific, exhibited a higher karyotype diversity. Therefore, the karyotype evolution of Epinephelidae fits well with the expansion and geographic contingencies of its clades, providing a more complex and diverse scenario than previously assumed.

 New records of two teleost species from two insular areas of the western equatorial Atlantic

We report the first record of two teleost species from two archipelagos in the western equatorial Atlantic. We recorded the occurrence of Cantherhines pullus (Ranzani, 1842) (Monacanthidae, Tetraodontiformes) from the Fernando de Noronha Archipelago, a group of volcanic islands 345 km off the northeastern coast of Brazil. We also report the first regional record of Hemiramphus brasiliensis (Linnaeus, 1758) (Hemiramphidae, Beloniformes) from the Saint Peter and Saint Paul’s Archipelago, which is a small and isolated group of rocky islands 520 km from the Fernando de Noronha Archipelago on the Mid-Atlantic Ridge.

Evolutionary Tracks of Chromosomal Diversification in Surgeonfishes (Acanthuridae: Acanthurus) Along the World's Biogeographic Domains

Fishes of the genus Acanthurus (Acanthuridae) are strongly related to reef environments, in a broad biogeographic context worldwide. Although their biological aspects are well known, cytogenetic information related to this genus remains incipient. In this study, Acanthurus species from populations inhabiting coastal regions of the Southwest Atlantic (SWA), South Atlantic oceanic islands (Fernando de Noronha Archipelago and Trindade Island), Greater Caribbean (GC), and Indo-Pacific Ocean (the center of the origin of the group) were analyzed to investigate their evolutionary differentiation. For this purpose, we employed conventional cytogenetic procedures and fluorescence in situ hybridization of 18S rDNA, 5S rDNA, and H3 and H2B-H2A histone sequences. The Atlantic species (A. coeruleus, A. chirurgus, and A. bahianus) did not show variations among them, despite their vast continental and insular distribution. In contrast, A. coeruleus from SWA and GC diverged from each other in the number of 18S rDNA sites, a condition likely associated with the barrier created by the outflows of the Amazonas/Orinoco rivers. The geminate species A. tractus had a cytogenetic profile similar to that of A. bahianus. However, the chromosomal macrostructures and the distribution of rDNA and hisDNA sequences revealed moderate to higher rates of diversification when Acanthurus species from recently colonized areas (Atlantic Ocean) were compared to A. triostegus, a representative species from the Indian Ocean. Our cytogenetic data covered all Acanthurus species from the Western Atlantic, tracked phylogenetic diversification throughout the dispersive process of the genus, and highlighted the probable diversifying role of ocean barriers in this process.

High chromosomal evolutionary dynamics in sleeper gobies (Eleotridae) and notes on disruptive biological factors in Gobiiformes karyotypes (Osteichthyes, Teleostei)

The order Gobiiformes is made up of more than 2200 species, representing one of the most diverse groups among teleost fishes. The biological causes for the tachytelic karyotype evolution of the gobies have not yet been fully studied. Here we expanded cytogenetic data for the Eleotridae family, analyzing the neotropical species Dormitator maculatus, Eleotris pisonis, Erotelis smaragdus, and Guavina guavina. In addition, a meta-analytical approach was followed for elucidating the karyotype diversification versus biological aspects (habitat and egg type) of the Gobiiformes. The species E. smaragdus and E. pisonis present 2n = 46 acrocentric chromosomes (NF = 46), D. maculatus 2n = 46 (36sm + 4st + 6a; NF = 86), and G. guavina, the most divergent karyotype, with 2n = 52 acrocentric chromosomes (NF = 52). Besides numeric and structural diversification in the karyotypes, the mapping of rDNAs and microsatellites also showed noticeable numerical and positional variation, supporting the high chromosomal evolutionary dynamism of these species. In Gobiiformes, karyotype patterns which are more divergent from the basal karyotype (2n = 46a) are associated with characteristics less effective to dispersion, such as the benthic habit. These adaptive characteristics, connected with the organization of the repetitive DNA content in the chromosomes, likely play a synergistic role in the remarkable karyotype diversification of this group.

Molecular cytogenetics insights in two pelagic big-game fishes in the Atlantic, the tarpon, Megalops atlanticus (Elopiformes: Megalopidae), and the sailfish, Istiophorus platypterus (Istiophoriformes: Istiophoridae)

Abstract Some pelagic and usually large sized fishes are preferential targets for sport and commercial fishing. Despite their economic importance, cytogenetic data on their evolutionary processes and management are very deficient, especially due to logistical difficulties. Here, information for two of such charismatic species, the tarpon, Megalops atlanticus (Elopiformes: Megalopidae), and the sailfish, Istiophorus platypterus (Istiophoriformes: Istiophoridae), both with a wide Atlantic distribution, were provided. Cytogenetic data were obtained using conventional methods (Giemsa staining, Ag-NORs technique, and C-banding), base-specific fluorochrome staining and fluorescence in situ hybridization (FISH) with rDNA probes. Megalops atlanticus has 2n = 50 chromosomes, all acrocentric ones (NF = 50), while Istiophorus platypterus has 2n = 48 chromosomes, 2m + 2st + 44a (NF = 52). Megalops atlanticus populations from the South Atlantic and Caribbean share identical karyotypic patterns, likely associated with gene flow between them. In turn, I. platypterus presents karyotype similarities with phylogenetically close groups, such as Carangidae. The chromosomal characteristics of these species highlight their independent evolutionary paths. Additionally, the current data contribute to knowledge of new aspects of pelagic fish fauna and will support further comparative studies with congeneric species, clarifying evolutionary karyotype trends of these fish groups.

Comparative chromosomal mapping of microsatellite repeats reveals divergent patterns of accumulation in 12 Siluridae (Teleostei: Siluriformes) species

The freshwater family Siluridae occurs in Eurasia and is especially speciose in South and Southeast Asia, representing an important aquaculture and fishery targets. However, despite the restricted cytogenetic data, a high diploid number variation (from 2n=40 to 92) characterizes this fish group. Considering the large genomic divergence among its species, silurid genomes have experienced an enormous diversification throughout their evolutionary history. Here, we aim to investigate the chromosomal distribution of several microsatellite repeats in 12 Siluridae species and infer about their possible roles in the karyotype evolution that occurred in this group. Our results indicate divergent patterns of microsatellite distribution and accumulation among the analyzed species. Indeed, they are especially present in significant chromosome locations, such as the centromeric and telomeric regions, precisely the ones associated with several kinds of chromosomal rearrangements. Our data provide pieces of evidence that repetitive DNAs played a direct role in fostering the chromosomal differentiation and biodiversity in this fish family.

Extensive Chromosomal Reorganization in Apistogramma Fishes (Cichlidae, Cichlinae) Fits the Complex Evolutionary Diversification of the Genus

Neotropical cichlid fishes are one of the most diversified and evolutionarily successful species assemblages. Extremely similar forms and intraspecific polychromatism present challenges for the taxonomy of some of these groups. Several species complexes have a largely unknown origin and unresolved evolutionary processes. Dwarf cichlids of the genus Apistogramma, comprising more than a hundred species, exhibit intricate taxonomic and biogeographic patterns, with both allopatric and sympatric distributions. However, karyotype evolution and the role of chromosomal changes in Apistogramma are still unknown. In the present study, nine South American Apistogramma species were analyzed using conventional cytogenetic methods and the mapping of repetitive DNA sequences [18S rDNA, 5S rDNA, and (TTAGGG)n] by fluorescence in situ hybridization (FISH). Our results showed that Apistogramma has unique cytogenetic characteristics in relation to closely related groups, such as a reduced 2n and a large number of bi-armed chromosomes. Interspecific patterns revealed a scenario of remarkable karyotypic changes, including a reduction of 2n, the occurrence of B-chromosomes and evolutionary dynamic of rDNA tandem repeats. In addition to the well-known pre-zygotic reproductive isolation, the karyotype reorganization in the genus suggests that chromosomal changes could act as postzygotic barriers in areas where Apistogramma congeners overlap.

Genomic Organization of Repetitive DNA Elements and Extensive Karyotype Diversity of Silurid Catfishes (Teleostei: Siluriformes): A Comparative Cytogenetic Approach

The catfish family Siluridae contains 107 described species distributed in Asia, but with some distributed in Europe. In this study, karyotypes and other chromosomal characteristics of 15 species from eight genera were examined using conventional and molecular cytogenetic protocols. Our results showed the diploid number (2n) to be highly divergent among species, ranging from 2n = 40 to 92, with the modal frequency comprising 56 to 64 chromosomes. Accordingly, the ratio of uni- and bi-armed chromosomes is also highly variable, thus suggesting extensive chromosomal rearrangements. Only one chromosome pair bearing major rDNA sites occurs in most species, except for Wallago micropogon, Ompok siluroides, and Kryptoterus giminus with two; and Silurichthys phaiosoma with five such pairs. In contrast, chromosomes bearing 5S rDNA sites range from one to as high as nine pairs among the species. Comparative genomic hybridization (CGH) experiments evidenced large genomic divergence, even between congeneric species. As a whole, we conclude that karyotype features and chromosomal diversity of the silurid catfishes are unusually extensive, but parallel some other catfish lineages and primary freshwater fish groups, thus making silurids an important model for investigating the evolutionary dynamics of fish chromosomes.

Emerging patterns of genome organization in Notopteridae species (Teleostei, Osteoglossiformes) as revealed by Zoo-FISH and Comparative Genomic Hybridization (CGH)

Notopteridae (Teleostei, Osteoglossiformes) represents an old fish lineage with ten currently recognized species distributed in African and Southeastern Asian rivers. Their karyotype structures and diploid numbers remained conserved over long evolutionary periods, since African and Asian lineages diverged approximately 120 Mya. However, a significant genetic diversity was already identified for these species using molecular data. Thus, why the evolutionary relationships within Notopteridae are so diverse at the genomic level but so conserved in terms of their karyotypes? In an attempt to develop a more comprehensive picture of the karyotype and genome evolution in Notopteridae, we performed comparative genomic hybridization (CGH) and cross-species (Zoo-FISH) whole chromosome painting experiments to explore chromosome-scale intergenomic divergence among seven notopterid species, collected in different African and Southeast Asian river basins. CGH demonstrated an advanced stage of sequence divergence among the species and Zoo-FISH experiments showed diffuse and limited homology on inter-generic level, showing a temporal reduction of evolutionarily conserved syntenic regions. The sharing of a conserved chromosomal region revealed by Zoo-FISH in these species provides perspectives that several other homologous syntenic regions have remained conserved among their genomes despite long temporal isolation. In summary, Notopteridae is an interesting model for tracking the chromosome evolution as it is (i) ancestral vertebrate group with Gondwanan distribution and (ii) an example of animal group exhibiting karyotype stasis. The present study brings new insights into degree of genome divergence vs. conservation at chromosomal and sub-chromosomal level in representative sampling of this group.

Comparative cytogenetics in three Sciaenid species (Teleostei, Perciformes): evidence of interspecific chromosomal diversification

Background

Species belonging to the Sciaenidae family present a karyotype composed by 48 acrocentric chromosomes and are thus considered a striking example of chromosomal conservation. In this family, three species are extensively studied including Larimichthys crocea, Larimichthys polyactis and Nibea albiflora due to their importance in fishery and aquaculture in East Asia. Despite abundant data of population genetics available for some of them, cytogenetic information on these species is still scarce and obtained by conventional cytogenetic protocols. Therefore, a more detailed cytogenomic investigation was performed in these species to analyze their karyotype differentiation using conventional staining techniques and fluorescence in situ hybridization to map several repetitive DNAs.

Results

The three species showed a slight karyotype differentiation with 4sm + 2st + 42a in L. polyactis, 20st + 28a in L. crocea and 48a in N. albiflora. Additionally, the mapping of repetitive sequences further revealed a number of interspecific differences among them. Particularly, 18S and 5S rDNA sites showed syntenic arrangements in N. albiflora and non-syntenic arrangements in both Larimichthys species. The microsatellites (CA)₁₅ and (GA)₁₅ showed conspicuous terminal clusters in some chromosomes of all species. On the other hand, (CGG)₁₀ repeats, Rex6 elements and U2 snRNA displayed a scattered distribution on the chromosomes.

Conclusions

Although the three Sciaenid species examined displayed a general pattern of karyotypic conservatism, we explored chromosomal diversification among them. The diversificated karyotypic macrostructure is followed by intergeneric evolutionary diversification of the repetitive sequences. The data indicate some degree of intergeneric evolutionary diversification at chromosomal level, and suggest the evolutionary dynamics among Sciaenid species, higher than previously thought. The present cytogenetic data provide new insight into the chromosomal diversification in Sciaenidae, and contribute to inferring the chromosomal rearrangements and trends of karyotype evolution in this fish group.

Contrasting Evolutionary Paths Among Indo-Pacific Pomacentrus Species Promoted by Extensive Pericentric Inversions and Genome Organization of Repetitive Sequences

Pomacentrus (damselfishes) is one of the most characteristic groups of fishes in the Indo-Pacific coral reef. Its 77 described species exhibit a complex taxonomy with cryptic lineages across their extensive distribution. Periods of evolutionary divergences between them are very variable, and the cytogenetic events that followed their evolutionary diversification are largely unknown. In this respect, analyses of chromosomal divergence, within a phylogenetic perspective, are particularly informative regarding karyoevolutionary trends. As such, we conducted conventional cytogenetic and cytogenomic analyses in four Pomacentrus species (Pomacentrus similis, Pomacentrus auriventris, Pomacentrus moluccensis, and Pomacentrus cuneatus), through the mapping of repetitive DNA classes and transposable elements, including 18S rDNA, 5S rDNA, (CA)₁₅, (GA)₁₅, (CAA)₁₀, Rex6, and U2 snDNA as markers. P. auriventris and P. similis, belonging to the Pomacentrus coelestis complex, have indistinguishable karyotypes (2n = 48; NF = 48), with a peculiar syntenic organization of ribosomal genes. On the other hand, P. moluccensis and P. cuneatus, belonging to another clade, exhibit very different karyotypes (2n = 48, NF = 86 and 92, respectively), with a large number of bi-armed chromosomes, where multiple pericentric inversions played a significant role in their karyotype organization. In this sense, different chromosomal pathways followed the phyletic diversification in the Pomacentrus genus, making possible the characterization of two well-contrasting species groups regarding their karyotype features. Despite this, pericentric inversions act as an effective postzygotic barrier in many organisms, which appear to be also the case for P. moluccensis and P. cuneatus; the extensive chromosomal similarities in the two species of P. coelestis complex suggest minor participation of chromosomal postzygotic barriers in the phyletic diversification of these species.

Interregional cytogenetic comparisons in Halichoeres and Thalassoma wrasses (Labridae) of coastal and insular regions of the southwestern Atlantic

The distribution patterns of marine biodiversity are complex, resulting from vicariant events and species dispersion, as well as local ecological and adaptive conditions. Furthermore, the wide geographic distribution of some species may be hindered by biogeographical barriers that can interfere in the gene flow. Cytogenetic analyses in marine fishes, especially those involving populations in small remote insular environments, remain scarce. In the Western Atlantic, species of wrasses from the genera Halichoeres and Thalassoma occur in biogeographic arrangements that make it possible to analyze cytogenetic patterns between coastal and widely separated island populations. Species of these genera were punctually analyzed in some Atlantic regions. In this study, we compared several chromosomal features, such as karyotype macrostructure, heterochromatic patterns, patterns of base-specific fluorochromes, Ag-NORs, and 18S and 5S ribosomal sites in Thalassoma noronhanum, Halichoeres poeyi, and Halichoeres radiatus individuals from distinct coastal or insular regions of Atlantic. Notably, all of them are characterized by multiple 18S and 5S rDNA sites with syntenic arrangements in some chromosome pairs. Individuals of T. noronhanum (between the insular regions of Rocas Atoll and Fernando de Noronha Archipelago - FNA) and H. poeyi (coastal areas from Northeastern Brazil) show no detectable differences among their cytogenetic patterns. On the other hand, H. radiatus from FNA and São Pedro and São Paulo Archipelago exhibit differences in the frequency of rDNA sites that could suggest some level of population structuring between these insular regions. Interregional cytogenetic inventories of marine species with wide geographic distribution need to be rapidly expanded. These data will allow a better understanding of the level of chromosomal stability between vast oceanic spaces, which may be less than previously thought.

Chromosomal evolution in large pelagic oceanic apex predators, the barracudas (Sphyraenidae, Percomorpha)

Sphyraena (barracudas) represents the only genus of the Sphyraenidae family and includes 27 species distributed into the tropical and subtropical oceanic regions. These pelagic predators can reach large sizes and, thus, attracting significant interest from commercial and sport fishing. Evolutionary data for this fish group, as well its chromosomal patterns, are very incipient. In the present study, the species Sphyraena guachancho, S. barracuda, and S. picudilla were analyzed under conventional (Giemsa staining, C-banding, and Ag-NOR) and molecular (CMA₃ banding, and in situ hybridization with 18S rDNA, 5S rDNA, and telomeric probes) cytogenetic methods. The karyotypic patterns contrast with the current phylogenetic relationships proposed for this group, showing by themselves to be distinct among closely related species, and similar among less related ones. This indicates homoplasic characteristics, with similar karyotype patterns originating at least twice, independently. Although still cytogenetically poor investigated, our data were enough to put in evidence a variety of ancient conserved traits and evolutionary novelties for the Sphyraena genus. In this sense, it is fundamental that a larger number of Sphyraenidae species, as well as of other phylogenetically related families, be also investigated. This will solidify the knowledge of their karyotypic patterns, and the evolutionary path followed by the species of this particular fish family.

Co-located 18S/5S rDNA arrays: an ancient and unusual chromosomal trait in Julidini species (Labridae, Perciformes)

Wrasses (Labridae) are extremely diversified marine fishes, whose species exhibit complex interactions with the reef environment. They are widely distributed in the Indian, Pacific and Atlantic oceans. Their species have displayed a number of karyotypic divergent processes, including chromosomal regions with complex structural organization. Current cytogenetic information for this family is phylogenetically and geographically limited and mainly based on conventional cytogenetic techniques. Here, the distribution patterns of heterochromatin, GC-specific chromosome regions and Ag-NORs, and the organization of 18S and 5S rDNA sites of the Atlantic species Thalassoma noronhanum (Boulenger, 1890), Halichoeres poeyi (Steindachner, 1867), Halichoeres radiatus (Linnaeus, 1758), Halichoeres brasiliensis (Bloch, 1791) and Halichoeres penrosei Starks, 1913, belonging to the tribe Julidini were analyzed. All the species exhibited 2n=48 chromosomes with variation in the number of chromosome arms among genera. Thalassoma noronhanum has 2m+46a, while species of the genus Halichoeres Rüppell, 1835 share karyotypes with 48 acrocentric chromosomes. The Halichoeres species exhibit differences in the heterochromatin distribution patterns and in the number and distribution of 18S and 5S rDNA sites. The occurrence of 18S/5S rDNA syntenic arrangements in all the species indicates a functionally stable and adaptive genomic organization. The phylogenetic sharing of this rDNA organization highlights a marked and unusual chromosomal singularity inside the family Labridae.

Chromosome mapping of repetitive DNAs in sergeant major fishes (Abudefdufinae, Pomacentridae): a general view on the chromosomal conservatism of the genus

Species of the Abudefduf genus (sergeant-majors) are widely distributed in the Indian, Pacific and Atlantic oceans, with large schools inhabiting rocky coastal regions and coral reefs. This genus consists of twenty recognized species are of generalist habit, showing typical characteristics of colonizers. Some populations maintain gene flow between large oceanic areas, a condition that may influence their cytogenetic features. A number of species have been shown to be invaders and able to hybridize with local species. However, cytogenetic data in this genus are restricted to few species. In this way, the present study includes the chromosomal investigation, using conventional (Giemsa staining, Ag-NOR and C-banding) and molecular (in situ mapping of six different repetitive DNA classes) approaches in four Abudefduf species from different oceanic regions (A. bengalensis and A. sexfasciatus from the Indo-Pacific, A. vaigiensis from the Indian and A. saxatilis from the Atlantic oceans, respectively), to investigate the evolutionary events associated with the chromosomal diversification in this group. All species share a similar karyotype (2n = 48; NF = 52), except A. sexfasciatus (2n = 48; NF = 50), which possesses a characteristic pericentric inversion in the NOR-bearing chromosomal pair. Mapping of repetitive sequences suggests a chromosomal conservatism in this genus. The high karyotypic similarity between allopatric species of Abudefduf may be related to the success of natural viable hybrids among species with recent secondary contact.

The Evolutionary Dynamics of Ribosomal Genes, Histone H3, and Transposable Rex Elements in the Genome of Atlantic Snappers

Lutjanidae is a family of primarily marine and carnivorous fishes distributed in the Atlantic, Indian, and Pacific oceans, with enormous economic and ecological importance. In order to better clarify the conservative chromosomal evolution of Lutjanidae, we analyzed the evolutionary dynamics of 5 repetitive DNA classes in 5 Lutjanus and in 1 Ocyurus species from the Western Atlantic. The ribosomal 18S sites were generally located in a single chromosome pair, except for L. jocu and L. alexandrei where they are found in 2 pairs. In turn, the 5S rDNA sites are unique, terminal and nonsyntenic with the 18S rDNA sites. In 3 species analyzed, H3 hisDNA genes were found in 1 chromosomal pair. However, while L. jocu presented 2 H3 sites, O. chrysurus showed a noteworthy dispersion of this gene in almost all chromosomes of the karyotype. Retrotransposons Rex1 and Rex3 do not exhibit any association with the explosive distribution of H3 sequences in O. chrysurus. The low compartmentalization of Rex elements, in addition to the general nondynamic distribution of ribosomal and H3 genes, corroborate the karyotype conservatism in Lutjanidae species, also at the microstructural level. However, some "disturbing evolutionary waves" can break down this conservative scenario, as evidenced by the massive random dispersion of H3 hisDNA in the genome of O. chrysurus. The implication of the genomic expansion of H3 histone genes and their functionality remain unknown, although suggesting that they have higher evolutionary dynamics than previously thought.

Genomic organization of repetitive DNAs highlights chromosomal evolution in the genus Clarias (Clariidae, Siluriformes)

BACKGROUND

The genus Clarias (Clariidae, Siluriformes) contains at least 61 species naturally spread over vast regions of Asia, India and Africa. However, Clarias species have also been introduced in many different countries and represent the most widespread catfishes in the world. These fishes are also known as "walking catfishes" due to their ability to move over land. A large degree of chromosomal variation has been previously found in this family, mainly using conventional cytogenetic investigations, with diploid chromosome numbers ranging between 48 and 100. In this study, we analyzed the karyotype structure and distribution of four repetitive DNA sequences (5S and 18S rDNAs and (CA)15 and (GA)15 microsatellites) in three Clarias species (C. batrachus, C. gariepinus, C. macrocephalus), as well as in a probable natural hybrid of the two latter species from different Thailand river basins.

RESULTS

Clarias gariepinus and C. macrocephalus had 2n = 56 and 2n = 54, respectively, as well as karyotypes composed mainly by metacentric and submetacentric chromosomes. Their karyotypes differed in the number and location of 5S and 18S rDNA sites and in the degree of microsatellite accumulation. An intermediate chromosomal pattern incorporating those of the parental species was found in the probable hybrid, confirming its interspecific origin. Clarias batrachus had 2n = 104 chromosomes and its karyotype was dominated by mainly acrocentric elements, indicating that unusual multiple centric fissions were involved in its karyotype differentiation. The karyotype of this species presented an unexpected dispersion of ribosomal DNAs, possessing 54 and 12 sites of 5S and 18S rDNAs, respectively, as well as a high accumulation and differential distribution of both microsatellite repeats, representing 'hot spots' for chromosomal rearrangement.

CONCLUSION

Both conventional and molecular cytogenetic markers were useful tools for demonstrating remarkable evolutionary dynamism and highlighting multiple chromosomal rearrangements and hybridization events correlated with the notable karyotypic diversity of these walking catfishes.

Physical mapping of 18S and 5S genes in pelagic species of the genera Caranx and Carangoides (Carangidae)

In Carangidae, Caranx is taxonomically controversial because of slight morphological differences among species, as well as because of its relationship with the genus Carangoides. Cytogenetic data has contributed to taxonomic and phylogenetic classification for some groups of fish. In this study, we examined the chromosomes of Caranx latus, Caranx lugubris, and Carangoides bartholomaei using classical methods, including conventional staining, C-banding, silver staining for nuclear organizer regions, base-specific fluorochrome, and 18S and 5S ribosomal sequence mapping using in situ hybridization. These 3 species showed chromosome numbers of 2n = 48, simple nuclear organizer regions (pair 1), and mainly centromeric heterochomatin. However, C. latus (NF = 50) and C. bartholomaei (NF = 50) showed a structurally conserved karyotype compared with C. lugubris (NF = 54), with a larger number of 2-armed chromosomes. The richness of GC-positive heterochromatic segments and sites in 5S rDNA in specific locations compared to the other 2 species reinforce the higher evolutionary dynamism in C. lugubris. Cytogenetic aspects shared between C. latus and C. bartholomaei confirm the remarkable phylogenetic proximity between these genera.

Karyotype divergence and spreading of 5S rDNA sequences between genomes of two species: darter and emerald gobies ( Ctenogobius , Gobiidae)

Karyotype analyses of the cryptobenthic marine species Ctenogobius boleosoma and C. smaragdus were performed by means of classical and molecular cytogenetics, including physical mapping of the multigene 18S and 5S rDNA families. C. boleosoma has 2n = 44 chromosomes (2 submetacentrics + 42 acrocentrics; FN = 46) with a single chromosome pair each carrying 18S and 5S ribosomal sites; whereas C. smaragdus has 2n = 48 chromosomes (2 submetacentrics + 46 acrocentrics; FN = 50), also with a single pair bearing 18S rDNA, but an extensive increase in the number of GC-rich 5S rDNA sites in 21 chromosome pairs. The highly divergent karyotypes among Ctenogobius species contrast with observations in several other marine fish groups, demonstrating an accelerated rate of chromosomal evolution mediated by both chromosomal rearrangements and the extensive dispersion of 5S rDNA sequences in the genome.

Genomic organization of repetitive DNAs and its implications for male karyotype and the neo-Y chromosome differentiation in Erythrinus erythrinus (Characiformes, Erythrinidae)

Studies have demonstrated the effective participation of repetitive DNA sequences in the origin and differentiation of the sex chromosomes in some biological groups. In this study several microsatellites and retrotranposable sequences were cytogenetically mapped in the Erythrinus erythrinus (Bloch & Schneider, 1801) male genome (karyomorph C), focusing on the distribution of these sequences in the sex chromosomes and in the evolutionary processes related to their differentiation. Males of E. erythrinus - karyomorph C - present 2n = 51 chromosomes (7m + 2sm + 6st + 36a), including the X1X2Y sex chromosomes. The C-positive heterochromatin has a predominant localization on the centromeric region of most chromosome pairs, but also in some telomeric regions. The 5S rDNA sites are located in the centromeric region of 27 chromosomes, including 26 acrocentric ones and the metacentric Y chromosome. The retrotransposons Rex 1 and Rex 6 show a dispersed pattern in the karyotype, contrasting with the Rex 3 distribution which is clearly co-localized with all the 27 5S rDNA sites. The microsatellite sequences show a differential distribution, some of them restricted to telomeric and/or interstitial regions and others with a scattered distribution on the chromosomes. However, no preferential accumulation of these elements were observed in the neo-Y chromosome, in contrast to what usually occurs in simple sex chromosome systems.

Transposable elements in fish chromosomes: a study in the marine cobia species

Rachycentron canadum, a unique representative of the Rachycentridae family, has been the subject of considerable biotechnological interest due to its potential use in marine fish farming. This species has undergone extensive research concerning the location of genes and multigene families on its chromosomes. Although most of the genome of some organisms is composed of repeated DNA sequences, aspects of the origin and dispersion of these elements are still largely unknown. The physical mapping of repetitive sequences on the chromosomes of R. canadum proved to be relevant for evolutionary and applied purposes. Therefore, here, we present the mapping by fluorescence in situ hybridization of the transposable element (TE) Tol2, the non-LTR retrotransposons Rex1 and Rex3, together with the 18S and 5S rRNA genes in the chromosome of this species. The Tol2 TE, belonging to the family of hAT transposons, is homogeneously distributed in the euchromatic regions of the chromosomes but with huge colocalization with the 18S rDNA sites. The hybridization signals for Rex1 and Rex3 revealed a semi-arbitrary distribution pattern, presenting differentiated dispersion in euchromatic and heterochromatic regions. Rex1 elements are associated preferentially in heterochromatic regions, while Rex3 shows a scarce distribution in the euchromatic regions of the chromosomes. The colocalization of TEs with 18S and 5S rDNA revealed complex chromosomal regions of repetitive sequences. In addition, the nonpreferential distribution of Rex1 and Rex3 in all heterochromatic regions, as well as the preferential distribution of the Tol2 transposon associated with 18S rDNA sequences, reveals a distinct pattern of organization of TEs in the genome of this species. A heterogeneous chromosomal colonization of TEs may confer different evolutionary rates to the heterochromatic regions of this species.

Independent sex chromosome evolution in lower vertebrates: a molecular cytogenetic overview in the Erythrinidae fish family

The Erythrinidae fish family is an excellent model for analyzing the evolution of sex chromosomes. Different stages of sex chromosome differentiation from homomorphic to highly differentiated ones can be found among the species of this family. Here, whole chromosome painting, together with the cytogenetic mapping of repetitive DNAs, highlighted the evolutionary relationships of the sex chromosomes among different erythrinid species and genera. It was demonstrated that the sex chromosomes can follow distinct evolutionary pathways inside this family. Reciprocal hybridizations with whole sex chromosome probes revealed that different autosomal pairs have evolved as the sex pair, even among closely related species. In addition, distinct origins and different patterns of differentiation were found for the same type of sex chromosome system. These features expose the high plasticity of the sex chromosome evolution in lower vertebrates, in contrast to that occurring in higher ones. A possible role of this sex chromosome turnover in the speciation processes is also discussed.

Discriminatory profile of rDNA sites and trend for acrocentric chromosome formation in the genus Trachinotus Lacépède, 1801 (Perciformes, Carangidae)

Chromosomal traits have provided valuable information for phylogeny and taxonomy of several fish groups. Three Atlantic Carangidae species of the genus Trachinotus Lacépède, 1801 (Trachinotus goodei Jordan et Evermann, 1896, Trachinotus carolinus (Linnaeus, 1766)and Trachinotus falcatus (Linnaeus, 1758)) were investigated, having 2n=48 chromosomes but different chromosomal arms (FN number), i.e., 52, 56 and 58, respectively, in view of the different number of two-armed chromosomes found in their karyotypes. Thus, Trachinotus goodei, Trachinotus carolinus and Trachinotus falcatus present a progressive distancefrom the probable basal karyotype proposed for Perciformes (2n=48 acrocentrics, FN=48). At first sight, these findings do not agree with the phylogenetic hypothesis based on mitochondrial sequences, where Trachinotus goodei appear as the most derived species, followed by Trachinotus falcatus and Trachinotus carolinus, respectively. However, the chromosomal mapping of ribosomal DNAs was informative for clarifying this apparent conflict. Indeed, the multiple 5S and 18S rDNA sites found in Trachinotus goodei corroborate the most derived condition for this species. In this sense, the occurrence of the unexpected number of two-armed chromosomes and FN value for this species, as well as for Trachinotus carolinus, must be due to additional rounds of acrocentric formation in these species, modifying the macrostructure of their karyotypes.

The key role of repeated DNAs in sex chromosome evolution in two fish species with ZW sex chromosome system

Despite substantial progress, there are still several gaps in our knowledge about the process of sex chromosome differentiation. The degeneration of sex-specific chromosome in some species is well documented, but it is not clear if all species follow the same evolutionary pathway. The accumulation of repetitive DNA sequences, however, is a common feature. To better understand this involvement, fish species emerge as excellent models because they exhibit a wide variety of sex chromosome and sex determining systems. Besides, they have much younger sex chromosomes compared to higher vertebrates, making it possible to follow early steps of differentiation. Here, we analyzed the arrangement of 9 repetitive DNA sequences in the W chromosomes of 2 fish species, namely Leporinus reinhardti and Triportheus auritus, which present well-differentiated ZZ/ZW sex system, but differ in respect to the size of the sex-specific chromosome. Both W chromosomes are almost fully heterochromatic, with accumulation of repeated DNAs in their heterochromatic regions. We found that microsatellites have strongly accumulated on the large W chromosome of L. reinhardti but not on the reduced-size W chromosome of T. auritus and are therefore important players of the W chromosome expansion. The present data highlight that the evolution of the sex chromosomes can diverge even in the same type of sex system, with and without the degeneration of the specific-sex chromosome, being more dynamic than traditionally appreciated.

Karyoevolutionary aspects of Atlantic hogfishes (Labridae-Bodianinae), with evidence of an atypical decondensed argentophilic heterochromatin

Fish from the family Labridae elicit considerable ecological interest, especially due to their complex interactions with the reef environment. Different karyoevolutionary tendencies have been identified in the subfamilies Bodianinae, Corinae and Cheilinae. Chromosomal analyses conducted in the Atlantic species Bodianus rufus (2n=48; 6m+12sm+14st+16a, FN=80), Bodianus pulchellus (2n=48; 4m+12sm+14st+18a, FN=78) and Bodianus insularis (2n=48; 4m+12sm+14st+18a, FN=78) identified Ag-NOR/18SrDNA sites located only in the terminal region of the short arm (p) of the largest subtelocentric pair. The 5S rDNA genes were mapped in the terminal region of the long arm (q) of the largest acrocentric pair and the p arm of chromosome 19 in B. insularis. The karyotype of the three species shows an extensive heterochromatic and argentophilic region, exceptionally decondensed, located in the p arm of the second subtelocentric pair. This region does not correspond to a NOR site, since it is not hybridized with 18S rDNA probes, and is not GC-rich, as generally occurs with nucleolus organizer regions of lower invertebrates. Heterochromatin in the three species is reduced and distributed over the centromeric and pericentromeric regions of chromosomes. The elevated number of two-armed chromosomes in species of Bodianus, in relation to other Labridae, shows karyotype diversification based on pericentric inversions, differentiating them markedly in terms of evolutionary tendencies that occur in subfamilies Corinae and Cheilininae. Structural cytogenetic similarities between B. pulchellus and B. insularis, in addition to the conserved chromosomal location pattern of ribosomal multigenic families, indicate phylogenetic proximity of these species.

Chromosomal complements of some Atlantic Blennioidei and Gobioidei species (Perciformes)

A remarkable degree of chromosomal conservatism (2n=48, FN=48) has been identified in several families of Perciformes. However, some families exhibit greater karyotypic diversity, although there is still scant information on the Atlantic species. In addition to a review of karyotypic data available for representatives of the suborders Blennioidei and Gobioidei, we have performed chromosomal analyses on Atlantic species of the families Blenniidae, Ophioblennius trinitatis Miranda-Ribeiro, 1919 (2n=46; FN=64) and Scartella cristata (Linnaeus, 1758)(2n=48; FN=50), Labrisomidae, Labrisomus nuchipinnis (Quoy & Gaimard, 1824)(2n=48; FN=50) and Gobiidae, Bathygobius soporator (Valenciennes, 1837)(2n=48; FN=56). Besides variations in chromosome number and karyotype formulas, Ag-NOR sites, albeit unique, were located in different positions and/or chromosome pairs for the species analyzed. On the other hand, the heterochromatic pattern was more conservative, distributed predominantly in the centromeric/pericentromeric regions of the four species. Data already available for Gobiidae, Blenniidae and Labrisomidae show greater intra- and interspecific karyotypic diversification when compared to other groups of Perciformes, where higher uniformity is found for various chromosome characteristics. Evolutionary dynamism displayed by these two families is likely associated with population fractionation resulting from unique biological characteristics, such as lower mobility and/or specific environmental requirements.

First record of Scartella itajobi Rangel and Mendes, 2009 (Perciformes: Blenniidae) from São Pedro e São Paulo Archipelago, Brazil

This article presents the first record of the blenniid Scartella itajobi in the São Pedro e São Paulo Archipelago, located off the state of Rio Grande do Norte, northeastern Brazil. As a result, the species distribution is extended 610 km northeast. The number of species of Blenniidae known to occur in São Pedro e São Paulo Archipelago is three, whereas the number of endemic fish species shared by São Pedro e São Paulo, Fernando de Noronha, and Rocas Atoll now stands at two. 

Comparative cytogenetics and heterochromatic patterns in two species of the genus Acanthostracion (Ostraciidae: Tetraodontiformes)

Some groups of fish, such as those belonging to the Order Tetraodontiformes, may differ significantly in the amount and location of heterochromatin in the chromosomes. There is a marked variation in DNA content of more than seven-fold among the families of this Order. However, the karyoevolutionary mechanisms responsible for this variation are essentially unknown. The largest genomic contents are present in species of the family Ostraciidae (2.20-2.60pg). The present study cytogenetically characterized two species of the family Ostraciidae, Acanthostracion polygonius and A. quadricornis, using conventional staining, C-bandings, Ag-NOR, CMA(3)/DAPI, AluI, PstI, EcoRI, TaqI and HinfI restriction enzymes (REs) and double FISH with 18S and 5S rDNA probes. The karyotypes of both species showed 2n=52 acrocentric chromosomes (FN=52; chromosome arms) and pronounced conserved structural characteristics. A significant heterochromatic content was observed equilocally distributed in pericentromeric position in all the chromosome pairs. This condition is unusual in relation to the karyotypes of other families of Tetraodontiformes and probability is the cause of the higher DNA content in Ostraciidae. Given the role played by repetitive sequences in the genomic reorganization of this Order, it is suggested that the conspicuous heterochromatic blocks, present in the same chromosomal position and with apparently similar composition, may have arisen or undergo evolutionary changes in concert providing clues about the chromosomal mechanisms which led to extensive variation in genomic content of different Tetraodontiformes families.

Cytogenetic characterization of three Balistoidea fish species from the Atlantic with inferences on chromosomal evolution in the families Monacanthidae and Balistidae

The Tetraodontiformes are the most derived group of teleostean fish. Among other apomorphies, they are characterized by a high degree of fusions or significant bone loss in the head and body. In the early phylogenetic proposals presented for this order, the families Balistidae and Monacanthidae have been unanimously considered to be closely related. Although they have moderate species diversity, they are scarcely known in cytogenetic aspect and chromosomal pattern comparisons between these groups have yet to be established. The species Cantherhines macrocerus (Hollard,1853), Cantherhines pullus (Ranzani, 1842) (Monacanthidae) and Melichthys niger (Bloch, 1786) (Balistidae) were cytogenetically analyzed using conventional (Ag-impregnation, C-banding, CMA3- and DAPI-fluorescence) and molecular (FISH with an 18S rDNA probe) cytogenetic protocols. The karyotypes of all three species were very similar possessing diploid chromosome numbers 2n = 40 and composed exclusively of acrocentric chromosomes. Single NOR-bearing pair as well as positive heterochromatic blocks at pericentromeric regions were identified in the karyotypes of the three species studied. NOR-bearing sites were positively labeled after Ag-impregnation, C-banding, CMA3-fluorescence and FISH with an 18S rDNA probe but were negative after DAPI-fluorescence. Such remarkable shared conspicuous chromosomal characters corroborate either close phylogenetic relationship of these families, previously established by morphological and molecular data, or rather conservative nature of karyotype differentiation processes. The later hypothesis, however, appears less probable due to centric or in tandem fusions documented for another Balistoidea species.

Ploidy manipulation and polyploid detection in the white shrimp Litopenaeus vannamei (Boone 1931) (Decapoda, Penaeidae)

Ploidy manipulation has been rarely used in the genetic improvement of cultured marine shrimps. Although polyploid induction has been proven to be successful in Penaeids, including the species Litopenaeus vannamei, the methodology still requires some improvements. In the present work, different thermal shock treatments on ploidy manipulation were tested and a protocol for detecting polyploid individuals was also established. Fertilized eggs were treated by cold (10°C) and heat (38°C) thermal shocks for 8, 12, 15, 18, 20, and 22 min to induce polyploidy. Nuclear measurements within distinct treatments revealed a significant deviation in relation to the mean diameter of nuclei in the control individuals. Triploid and tetraploid metaphases were observed within treated individuals, confirming the increase of interphasic nuclear diameter. The cold thermal shock was more efficient than the hot ones, besides leading to a higher and more homogeneous hatchery rate. A mean number of three nucleoli per nucleus were observed in diploid individuals, while treated samples usually presented up to five nucleoli per nucleus. The standardization of protocols to obtain and detect polyploid products allows further utilization of such methods on a commercial scale in order to evaluate the performance of polyploid individuals in the genetic improvement of L. vannamei.

Karyoevolution of the toadfish Thalassophryne nattereri (Batrachoidiformes: Batrachoididae)

The Batrachoididae includes some venomous brackish and marine fish found in the Atlantic, Indian and Pacific oceans. This family is composed of 69 species, distributed among 19 genera. Species of the genus Thalassophryne have been reported along the coast of Rio Grande do Norte (Brazil); T. nattereri has been responsible for a large number of human injuries. Little is known about the cytogenetic features of this family. We made a karyotypic characterization of T. nattereri collected from the estuary of the Apodi/Mossoró River, using conventional Giemsa staining, C-banding and silver nitrate-nucleolar organizer region technique. There was a modal diploid value of 2n = 46 chromosomes (8m + 8sm + 24st + 6a; fundamental number = 86). Single ribosomal sites were detected in the terminal region on short arms of a subtelocentric pair (19th). Heterochromatin segments were preferentially located over centromeric regions in some chromosome pairs. Pericentric inversions and Robertsonian rearrangements seem to have played a major role in karyotype evolution within this genus of toadfish.

Assessing genetic diversity of Brazilian reef fishes by chromosomal and DNA markers

Little is known on genetics of Brazilian coral reef fish and most of this information is limited to chromosome characterization of major representative species. The diploid chromosome number in marine fish varies from 2n= 22-26 to 2n = 240-260. Despite of this apparent diversity, most studied marine species have a diploid complement with 48 acrocentric chromosomes. This latter trend is mostly observed among Perciformes, an important major taxon of coral reef fishes. Studies in the families Pomacentridae, Pomacanthidae and Chaetodontidae, for example, have shown a common karyotype pattern entirely formed by 48 uniarmed chromosomes. However, rare numerical and structural chromosome polymorphisms and cryptic chromosome rearrangements involving heterochromatin segments and/or nucleolar organizing sites have been reported among such fishes. Although new chromosome forms can contribute to the establishment of genetically isolated populations, their role in reef fish speciation at marine realm still is an open question. More recently, genomic DNA analyses using RAPD and microsatellites, and sequencing and RFLP of mitochondrial DNA have increasingly been used in Atlantic reef fish species. Genetic homogeneity over wide geographical ranges has been reported for different fish groups, in contrast to several cases of population substructuring related to environmental constraints or evolutionary history. Amazonas outflow and upwelling on the Southeastern coast of Brazil are believed to be strong barriers to dispersal of some reef species. Moreover, it is suggested that the pattern of speciation and population structure at South Atlantic is quite distinctive from Pacific Ocean, even when comparing closely related taxa. Further genetic studies are strongly encouraged in Brazilian reef fishes in order to provide a reliable scenario of the genetic structure in this important and diverse fish group.

Cytogenetical studies in five Atlantic Anguilliformes fishes

The order Anguilliformes comprises 15 families, 141 genera and 791 fish species. Eight families had at least one karyotyped species, with a prevalence of 2n = 38 chromosomes and high fundamental numbers (FN). The only exception to this pattern is the family Muraenidae, in which the eight species analyzed presented 2n = 42 chromosomes. Despite of the large number of Anguilliformes species, karyotypic reports are available for only a few representatives. In the present work, a species of Ophichthidae, Myrichthys ocellatus (2n = 38; 8m+14sm+10st+6a; FN = 70) and four species of Muraenidae, Enchelycore nigricans (2n = 42; 6m+8sm+12st+16a; FN = 68), Gymnothorax miliaris (2n = 42; 14m+18sm+10st; FN = 84), G. vicinus (2n = 42; 8m+6sm+28a; FN = 56) and Muraena pavonina (2n = 42; 6m+4sm+32a; FN = 52), collected along the Northeastern coast of Brazil and around the St Peter and St Paul Archipelago were analyzed. Typical large metacentric chromosomes were observed in all species. Conspicuous polymorphic heterochromatic regions were observed at the centromeres of most chromosomes and at single ribosomal sites. The data obtained for Ophichthidae corroborate the hypothesis of a karyotypic diversification mainly due to pericentric inversions and Robertsonian rearrangements, while the identification of constant chromosome numbers in Muraenidae (2n = 42) suggests a karyotype diversification through pericentric inversions and heterochromatin processes.

Lack of interpopulation genetic structure in the genus Stegastes (Perciformes) with indication of local introgression

The family Pomacentridae comprises about 326 species belonging to 28 genera. The genus Stegastes is composed of nearly 33 species, and 8 are endemic to the Brazilian Province, inhabiting the Brazilian coast (Stegastes fuscus, S. variabilis, S. leucosticus, S. uenfi, and S. pictus) or Western Atlantic oceanic islands (S. trindadensis, S. rocasensis and S. sanctipauli). Stegastes species play a major role in the reef ecosystem since they interfere significantly with the composition of benthonic organisms. Studies about population genetics and speciation of Neotropical ichthyofauna are scarce, particularly at insular areas from the Western Atlantic. Random amplified polymorphic DNA markers were used to analyze the population genetic structure of the continental species S. fuscus and S. variabilis (Northeastern Brazil) as well as the insular species S. sanctipauli (Saint Paul's Rocks). Analysis of population parameters revealed a high index of intrapopulation genetic variability in the species, except for S. sanctipauli, which showed low values. The phiST values in samples of S. fuscus and S. variabilis obtained at distinct collection sites 35 km apart from each other indicated a lack of population genetic structure. An intermediary profile of species-specific markers was detected in some individuals of S. fuscus and S. variabilis from Santa Rita, Rio Grande do Norte, suggesting a putative introgression event between the two species. The genetic profiles observed in Stegastes populations indicate a higher genetic variability along the shoreline than at oceanic sites, related to a reduced effective population size on islands. The lack of genetic differentiation among coastal populations suggests that, despite some biological features such as non-migratory behavior and territoriality, the pelagic larval phase of these species is able to promote an interpopulation homogeneity among sampled areas.

Robertsonian rearrangements and pericentric inversions in Scaridae fish (Perciformes)

The parrotfishes (family Scaridae) are comprised of the subfamilies Sparisomatinae and Scarinae. They are important agents of marine bioerosion, which rework the substrate with their beaklike jaws. Despite their importance, there are no published cytogenetic data on this group. We made cytogenetic analyses of Sparisoma axillare (Sparisomatinae) and Scarus trispinosus [corrected] (Scarinae) from the Brazilian coast. Differentiation in the diploid number in S. axillare compared to the basal karyotype of the Perciformes apparently occurred due to a Robertsonian fusion, combined with pericentric inversions. S. trispinosus [corrected] presented a conserved diploid number, but showed considerable structural karyotypic changes, resulting mainly from pericentric inversions. The Ag-NOR sites were unique and located on the short arm of the 1st subtelocentric pair in both species (possibly homeologous), corresponding to the 11th pair in S. axillare and the 9th pair in S. trispinosus [corrected] The constitutive heterochromatin is reduced in these species and is distributed in centromeric and pericentromeric regions in most of the chromosomes. The low fundamental number compared to the Scarus genus suggests a more basal condition for Sparisoma. The chromosome formula in S. trispinosus [corrected] was more diversified, deriving from large-scale pericentric inversions. Karyotypic evolution patterns observed for these representatives of the Sparisomatinae and Scarinae subfamilies, added to new data from a larger number of species, would allow us to determine if there is a tendency among the Sparisomatinae for centric fusion events.

Early replication banding in Leporinus species (Osteichthyes, Characiformes) bearing differentiated sex chromosomes (ZW)

There are few examples of differentiated sex chromosomes in fishes. In the genus Leporinus, seven species present a highly differentiated ZW system, derived from heterochromatinization process. Cytogenetic analyses carried out in three of these fish species, Leporinus obtusidens, L. elongatus and L. reinhardti, through RBG-banding, showed late replication bands, coincident with heterochromatic regions in both Z and W chromosomes. A similar interstitial early replication segment was observed in the complex heterochromatic region along the Wq arms in the three species, which might correspond to a pseudoautosomal segment (SD, sex determining locus). Asynchrony related to the replication pattern among different Z chromosomes was not observed. When the identification of nuclear organizer regions by silver nitrate was performed over chromosomal preparations previously exposed to 5-bromo-2'-deoxyuridine (BrdU), remarkable positive signals at interstitial and telomeric position were observed on the q arms of W chromosomes in the species L. elongatus and L. reinhardti. The absence of 18S ribosomal RNA gene loci in this region, formerly demonstrated by FISH, indicates that this argentophilic behavior is putatively due to heterochromatin decondensation caused by BrdU incorporation, favoring such Ag+ reaction. Early and late replication bands were also observed in the heterochromatic portions of Z and W chromosomes, indicating that euchromatic and heterochromatic regions are interspersed. The present data suggest a significant level of heterochromatic complexity in the sex chromosomes of each species. On the other hand, the replication pattern shared by them supports a monophyletic origin.

Chromosome evolution in the erythrinid fish, Erythrinus erythrinus (Teleostei: Characiformes)

The genus Erythrinus belongs to the family Erythrinidae, a neotropical fish group. This genus contains only two described species, Erythrinus erythrinus being the most widely distributed in South America. Six samples of this species from five distinct Brazilian localities and one from Argentina were studied cytogenetically. Four groups were identified on the basis of their chromosomal features. Group A comprises three samples, all with 2n = 54 chromosomes, a very similar karyotypic structure, and the absence of chromosome differentiation between males and females. One sample bears up to four supernumerary microchromosomes, which look like 'double minute chromosomes' in appearance. Groups B-D comprise the three remaining samples, all sharing an X(1)X(1)X(2)X(2)/X(1)X(2)Y sex chromosome system. Group B shows 2n = 54/53 chromosomes in females and males, respectively, and also shows up to three supernumerary microchromosomes. Groups C and D show 2n=52/51 chromosomes in females and males, respectively, but differ in the number of metacentric, subtelocentric, and acrocentric chromosomes. In these three groups (B-D), the Y is a metacentric chromosome clearly identified as the largest in the complement. The present results offer clear evidence that local samples of E. erythrinus retain exclusive and fixed chromosomal features, indicating that this species may represent a species complex.

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.

Cytogenetics of the genus Leporinus (Pisces, Anostomidae). 1. Karyotype analysis, heterochromatin distribution and sex chromosomes

Cytogenetic analyses (Giemsa staining, C-banding, AgNO3 labelling of nucleolus organizer regions (NORs) and staining with base-specific fluorochromes) were performed on the South American fish species Leporinus friderici, L. obtusidens and L. elongatus. The overall karyotypic structure, position of NORs, as well as the amount, distribution and composition of constitutive heterochromatin were determined. Particular attention was given to the highly differentiated ZZ/ZW sex chromosome system of L. obtusidens and L. elongatus. Sharing the apparently ancient macroscopic karyotype of Anostomidae, all three species have 2n = 54 meta- or submetacentric chromosomes. NORs were found exclusively on chromosome pair 2, which may represent the ancestral NOR-bearing chromosome of the anostomid karyotype. Observed differences in the relative position of NORs along chromosome 2 and variations in the amount and distribution of constitutive heterochromatin throughout the karyotype were most probably caused by heterochromatin-mediated chromosome rearrangements. Detailed analysis of the morphologically similar heteromorphic ZZ/ZW sex chromosomes of L. obtusidens and L. elongatus allowed detection of differences in the DNA composition of the largely heterochromatic W chromosomes. However, since these and the W chromosomes of three other Leporinus species exhibit homologies with respect to their relative size, centromere position and amount and distribution of heterochromatin, it is concluded that they evolved from the same ancestral W chromosome.