Diverse plant and animal species have B chromosomes, also known as accessory, extra or supernumerary chromosomes. Despite being widely distributed among different taxa, the genomic nature and genetic behavior of B chromosomes are still poorly understood. In this study we describe the occurrence of B chromosomes in the African cichlid fish Haplochromis obliquidens. One or two large B chromosome(s) occurring in 39.6% of the analyzed individuals (both male and female) were identified. To better characterize the karyotype and assess the nature of the B chromosomes, fluorescence in situ hybridization (FISH) was performed using probes for telomeric DNA repeats, 18S and 5S rRNA genes, SATA centromeric satellites, and bacterial artificial chromosomes (BACs) enriched in repeated DNA sequences. The B chromosomes are enriched in repeated DNAs, especially non-active 18S rRNA gene-like sequences. Our results suggest that the B chromosome could have originated from rDNA bearing subtelo/acrocentric A chromosomes through formation of an isochromosome, or by accumulation of repeated DNAs and rRNA gene-like sequences in a small proto-B chromosome derived from the A complement.

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The B chromosomes of the African cichlid fish Haplochromis obliquidens harbour 18S rRNA gene copies.

A single NOR-bearing chromosome pair was identified by silver nitrate staining in a previous study of the fish Prochilodus argenteus from the S a o Francisco River (MG, Brazil), with a third metacentric chromosome sporadically bearing active NOR. The present study focused on an analysis of the chromosomal localization of both the major (45S) and the minor (5S) rRNA genes using FISH. The use of the 18S rDNA probe confirmed the previous Ag-NOR sites interstitially located in a large metacentric pair and also identified up to three other sites located in the telomeric regions of distinct chromosomes, characterizing an interindividual variation of these sites. In addition, the 5S rDNA site was revealed adjacent to the major NOR site, identified at the end of the large Ag-NOR bearing metacentric chromosome. In a few metaphases, an additional weak hybridization signal was observed in a third chromosome, possibly indicating the presence of another 5S rDNA cluster. Despite a lower karyotype diversification (2n=54 and FN=108) often observed among species of Prochilodontidae, variations involving both 45S and 5S rRNA genes could play an important role in their chromosome diversification.

Mapping of the 18S and 5S ribosomal RNA genes in the fish Prochilodus argenteus Agassiz, 1829 (Characiformes, Prochilodontidae).

The Neotropical freshwater fish fauna is very rich—according to the most recent catalogue 71 families and 4,475 species have been described. However, only a small amount of general information is available on the composition of Neotropical marine fishes. In Brazil, 1,298 marine species have been recorded. General analysis of available cytogenetic and population genetic data clearly indicates research has been mainly concentrated on freshwater fishes. Thus, today, cytogenetic information is available for 475 species of Characiformes, 318 species of Siluriformes, 48 species of Gymnotiformes, 199 freshwater species that do not belong to the superorder Ostariophysi, and only 109 species of marine fishes. For the species studied, only about 6% have sex chromosomes and about 5% have supernumerary or B chromosomes. A review of the cytogenetic studies shows that these data have provided valuable information about the relationships between fish groups, the occurrence of cryptic species and species complexes, the mechanism of sex determination and sex chromosome evolution, the distribution of nucleolus organizer regions, the existence supernumerary chromosomes, and the relationship between polyploidy and evolution. In relation to populations in Neotropical marine waters, the studies have shown the presence of cryptic species, which has important implications for fishery management. Different levels of genetic structuring can be found among Neotropical freshwater migratory fish species. This raises important implications for fish population genetic diversity and consequently its sustainable utilization in inland fisheries and aquaculture, specifically for conservation of ichthyo-diversity and survival.

Genetics of neotropical fish: from chromosomes to populations.

A karyotypic analysis was carried out using conventional staining and C-banding in 32 specimens of Astyanax scabripinnis (Pisces, Characidae) from Campos do Jordao (Sao Paulo State, Brazil). Twenty-eight individuals (87.5 per cent of the sample studied) showed 1–2 extra B-chromosomes, similar in size and shape (metacentrics) to pair 1 of the karyotype, with a high intra-individual constancy. Two types of B could be identified on the basis of their constitutive heterochromatin patterns. The hypothesis of the origin of the B-chromosome from non-disjunction of chromosome 1, followed by a heterochromatinization process, is postulated.

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B chromosomes in Astyanax scabripinnis (Pisces, Characidae)

Constitutive heterochromatin represents a substantial portion of the eukaryote genome, and it is mainly composed of tandemly repeated DNA sequences, such as satellite DNAs, which are also enriched by other dispersed repeated elements, including transposons. Studies on the organization, structure, composition and in situ localization of satellite DNAs have led to consistent advances in the understanding of the genome evolution of species, with a particular focus on heterochromatic domains, the diversification of heteromorphic sex chromosomes and the origin and maintenance of B chromosomes. Satellite DNAs can be chromosome specific or species specific, or they can characterize different species from a genus, family or even representatives of a given order. In some cases, the presence of these repeated elements in members of a single clade has enabled inferences of a phylogenetic nature. Genomic DNA restriction, using specific enzymes, is the most frequently used method for isolating satellite DNAs. Recent methods such as C(0)t-1 DNA and chromosome microdissection, however, have proven to be efficient alternatives for the study of this class of DNA. Neotropical ichthyofauna is extremely rich and diverse enabling multiple approaches with regard to the differentiation and evolution of the genome. Genome components of some species and genera have been isolated, mapped and correlated with possible functions and structures of the chromosomes. The 5SHindIII-DNA satellite DNA, which is specific to Hoplias malabaricus of the Erythrinidae family, has an exclusively centromeric location. The As51 satellite DNA, which is closely correlated with the genome diversification of some species from the genus Astyanax, has also been used to infer relationships between species. In the Prochilodontidae family, two repetitive DNA sequences were mapped on the chromosomes, and the SATH 1 satellite DNA is associated with the origin of heterochromatic B chromosomes in Prochilodus lineatus. Among species of the genus Characidium and the Parodontidae family, amplifications of satellite DNAs have demonstrated that these sequences are related to the differentiation of heteromorphic sex chromosomes. The possible elimination of satellite DNA units could explain the genome compaction that occurs among some species of Neotropical Tetraodontiformes. These topics are discussed in the present review, showing the importance of satellite DNA analysis in the differentiation and karyotype evolution of Actinopterygii.

/pdf/satellite-dna-and-chromosomes-in-neotropical-fishes-methods-4191kfl35i.pdf

Satellite DNA and chromosomes in Neotropical fishes: methods, applications and perspectives

SUMMARYBoth males and females of the fish Prochilodus scrofa have a diploid number of 2n = 54. However, with the exception of a single female, all 59 specimens (25 ♀, 34 ♂) studied had 1–5 minutes accessory chromosomes. The unstable behavior of these chromosomes during mitosis and meiosis as well as their heterochromatic nature, as detected by C-banding, are characteristics that emphasize their supernumerary nature. This is the first report of chromosomes of this type in the Osteichthyes (Pisces).

https://www.tandfonline.com/doi/pdf/10.1080/00087114.1983.10797671

Evidence for a System of Supernumerary Chromosomes in Prochilodus Scrofa Steindachner, 1881 (Pisces, Prochilodontidae)

A cytogenetic study on eight species of the genus Prochilodus, family Prochilodontidae, from different hydrographic basins of Brazil is reported. All species had a diploid number of 2n=54 and NF=108. Supernumerary heterochromatic chromosomes of variable frequency were detected in Prochilodus scrofa and in P. cearensis. To determine the occurrence and frequency of the supernumerary chromosomes in P. scrofa, specimens from different localities belonging to the basin of the Parana river were analysed. The high karyotypic similarity detected demonstrated that, at least in these eight species, evolution occurred without marked changes in chromosome macrostructure.

Distribution of a supernumerary chromosome system and aspects of karyotypic evolution in the genus Prochilodus (Pisces, Prochilodontidae)

A standard protocol for obtaining fish chromosomes under post-mortem conditions.

Three perciform species associated to coral reefs were, for the first time, investigated under a cytogenetic focus: Holacanthus ciliaris and H. tricolor from family Pomacanthidae and Chaetodon striatus from family Chaetodontidae. Individuals were collected at different sites along Atlantic coast of Brazil. All of them showed similar chromosomal features, considered primitive for the order. These characteristics are 2n=48, NF=48, NORs located at interstitial position on long arms of a single chromosomal pair and discrete heterochromatic blocks at centromeres and NORs.

Cytogenetic Analysis of Coral Reef Fishes from Brazil (Families Pomacanthidae and Chaetodontidae)

Several cytogenetical studies on marine Perciformes have demonstrated a conservative chromosomal pattern. Despite of some exceptions, even more specialized groups such as those from coral reefs have followed this trend. Nevertheless, the number of karyotypical information on fish from this environment is relatively reduced. In this work, cytogenetical data were obtained from two species of family Pomacanthidae (Perciformes), widespread throughout the Brazilian coast and associated to reefs: Pomacanthus paru and P. arcuatus. Both species have the same chromosomal constitution. The respective karyotypes are composed by one subtelocentric pair and 23 acrocentric pairs (2n=48). Single NORs are located on short arms of the subtelocentric pair. Heterochromatic blocks are restricted to centromeres and NORs. An accentuated NOR size heteromorphism between homologous was detected and analyses by chromomycin A3 demonstrated that it can be attributed to structural chromosome differences. A model of NOR evolut...

http://www.caryologia.unifi.it/past_volumes/55_4/Caryologia_Vol_55_323-329.pdf

Close karyotypical relationship between two species of marine angelfishes from South Atlantic: Pomacanthus arcuatus and P. paru (Perciformes, Pomacanthidae)

E. Pauls

Papers

Evidence for a System of Supernumerary Chromosomes in Prochilodus Scrofa Steindachner, 1881 (Pisces, Prochilodontidae)

Distribution of a supernumerary chromosome system and aspects of karyotypic evolution in the genus Prochilodus (Pisces, Prochilodontidae)

A standard protocol for obtaining fish chromosomes under post-mortem conditions.

Cytogenetic Analysis of Coral Reef Fishes from Brazil (Families Pomacanthidae and Chaetodontidae)

Close karyotypical relationship between two species of marine angelfishes from South Atlantic: Pomacanthus arcuatus and P. paru (Perciformes, Pomacanthidae)