Eliana Feldberg

Comparative cytogenetic analysis of the genus symphysodon (discus fishes, cichlidae): chromosomal characteristics of retrotransposons and minor ribosomal DNA.

As part of a genetic screening program for wild Discus fishes, we analyzed karyotypes and cytogenetic characteristics of Symphysodon aequifasciatus, S. discus and S. haraldi using C-banding and fluorescent in situ hybridization (FISH) with the Rex3 retrotransposon and 5S rDNA probes in mitotic and meiotic chromosomes. In the 3 species, diploid chromosome number was 2n = 60 and karyotypes contained predominantly meta-submetacentric chromosomes. C-banding showed blocks of constitutive heterochromatin mainly in the pericentromeric region. Physical mapping of repetitive 5S rDNA sequences and Rex3 retrotransposons in mitotic and meiotic chromosomes showed partial colocalization of constitutive heterochromatin and repetitive elements. Correlations among the accumulation of repetitive elements, heterochromatinization and chromosome rearrangements have been hypothesized to explain the karyotype differentiation in the Symphysodon genus. The role of repetitive elements in adaptation to highly diverse habitats, as well as in the generation of the phenotypic and genetic variability found in wild Discus populations, needs to be further investigated.

Variability of 18S rDNA locus among Symphysodon fishes: chromosomal rearrangements

Three species of cichlids belonging to the genus Symphysodon have demonstrated interspecific and intraspecific variation in nucleolus organizer regions (NOR) detected with silver nitrate. In order to understand the evolution of this marker in the genus, the structural variability of these sequences in mitotic chromosomes from Symphysodon aequifasciatus, Symphysodon discus and Symphysodon haraldi was investigated using both silver nitrate impregnation and hybridization of the 18S rRNA gene probe. For the three species, the two markers were intraspecifically and interspecifically variable both in the number and in the size of the sites. This polymorphism may stem from duplications and translocations, which suggests that structural chromosome rearrangements effectively act in the karyoevolution of wild Symphysodon species and may have favoured the adaptability of these fishes to diverse aquatic environments in the Amazon.

Karyotypes of 10 species of Neotropical cichlids (Pisces, Perciformes).

SUMMARYTen neotropical species of family Cichlidae (Pisces: Perciformes) were analyzed cytogenetically. The results obtained also support the indications that the chromosomal evolution of this group was more conservative than divergent from the numerical point of view. All species showed a chromosome number of 2n = 48, although differences in chromosome morphology were observed. The species Chaetobranchopsis australe exhibited a karyotype consisting of 48 subtelo-acrocentric chromosomes, which is possibly of a more primitive type in relation to the karyotypes of the remaining species investigated. No chromosome differences were detected between males and females, indicating the absence of morphologically differentiated sex chromosomes.

Chromosomal polymorphism in Serrasalmus spilopleura Kner, 1858 (Characidae, Serrasalminae) from Central Amazon Basin

Abstract Cytogenetic studies were performed in Serrasalmus spilopleura, a piranha species, from three sampling sites in the Central Amazon basin: 1. Catalão; 2. Manacapuru, 3. Itacoatiara. All specimens displayed 2n = 60 chromosomes and no sexual chromosomal heteromorphism was found. However, by considering the karyotypic formula, three cytotypes were identifyed: cytotype A consisted of 24M+20SM+4ST+12A, and was the most common being found in all three sampling sites; cytotype C consisted of 23M+21SM+4ST+12A, was found only in Catalão and differ from cytotype A by the presence of a heteroziguous inversion in the first chromosome pair and a distal heterocromatinization in the long arm of this same pair, probably caused by the rearrangement or due to a chromosomal adjustment; cytotype D consisted of 24M+20SM+4ST+12A, was found only in Manacapuru and differ from cytoytpe A by a heteroziguous size polymorphism on the second chromosome pair, probably due to a translocation. C-banding revealed the same constitutive heterochromatin pattern for the three cytotypes except for the chromosome pair that underwent pericentric inversion (cytotype C) which possessed different heterochromatic blocks. Analysis in the number of active nucleolar organizing regions (NORs) revealed no differentiationamong cytotypes and NOR-bearing chromosomes varied from 10–12 acrocentric chromosomes.

Evolutionary dynamics of retrotransposable elements Rex1, Rex3 and Rex6 in neotropical cichlid genomes

BackgroundTransposable elements (TEs) have the potential to produce broad changes in the genomes of their hosts, acting as a type of evolutionary toolbox and generating a collection of new regulatory and coding sequences. Several TE classes have been studied in Neotropical cichlids; however, the information gained from these studies is restricted to the physical chromosome mapping, whereas the genetic diversity of the TEs remains unknown. Therefore, the genomic organization of the non-LTR retrotransposons Rex1, Rex3, and Rex6 in five Amazonian cichlid species was evaluated using physical chromosome mapping and DNA sequencing to provide information about the role of TEs in the evolution of cichlid genomes.ResultsPhysical mapping revealed abundant TE clusters dispersed throughout the chromosomes. Furthermore, several species showed conspicuous clusters accumulation in the centromeric and terminal portions of the chromosomes. These TE chromosomal sites are associated with both heterochromatic and euchromatic regions. A higher number of Rex1 clusters were observed among the derived species. The Rex1 and Rex3 nucleotide sequences were more conserved in the basal species than in the derived species; however, this pattern was not observed in Rex6. In addition, it was possible to observe conserved blocks corresponding to the reverse transcriptase fragment of the Rex1 and Rex3 clones and to the endonuclease of Rex6.ConclusionOur data showed no congruence between the Bayesian trees generated for Rex1, Rex3 and Rex6 of cichlid species and phylogenetic hypothesis described for the group. Rex1 and Rex3 nucleotide sequences were more conserved in the basal species whereas Rex6 exhibited high substitution rates in both basal and derived species. The distribution of Rex elements in cichlid genomes suggests that such elements are under the action of evolutionary mechanisms that lead to their accumulation in particular chromosome regions, mostly in heterochromatins.

Mechanisms of chromosomal evolution and its possible relation to natural history characteristics in Ancistrus catfishes (Siluriformes: Loricariidae)

Ancistrus is the most speciose genus of the tribe Ancistrini, with 58 valid species and many yet to be described. Cytogenetic studies were conducted on five apparently undescribed species from the Amazon basin, which showed different diploid numbers: Ancistrus sp. Purus (2n = 34); Ancistrus sp. Macoari (2n = 46); Ancistrus sp. Dimona (2n = 52); Ancistrus sp. Vermelho (2n = 42) and Ancistrus sp. Trombetas (2n = 38). All species possessed only one pair of NOR-carrying chromosomes, but with extensive variation in both the location on the chromosome as well as in the position of the ribosomal sites on the karyotype. The karyotypic evolution of Ancistrus species seems to be based on chromosomal rearrangements, with a tendency to a reduction of the diploid number. Two new instances of XX/XY sex chromosomes for Ancistrus species, based on the heteromorphism in the male karyotype, were also recorded. The large karyotypic diversity among Ancistrus species may be related to biological and behavioural characteristics of these fish that include microhabitat preferences, territoriality and specialized reproductive tactics. These characteristics may lead to a fast rate of fixation of chromosomal mutations and eventually speciation across the basin.

Nucleolar Organizing Regions in Some Species of Neotropical Cichlid Fish (Pisces, Perciformes)

SUMMARYIn the present study, 10 neotropical species of family Cichlidae (Pisces, Perciformes) were submitted to analysis of the nucleolar organizing regions (NORs). In 8 of these species, the NORs were located in the first pair in the complement and coincided with the secondary constrictions observed there, whereas in Cichlasoma facetum and Geophagus brasiliensis the NORs were located in another chromosome pair. The possibility that the predominant location of NORs in the 1st pair in the karyotype indicates a more primitive condition of this group of fish is discussed. Variations in NOR size between homologous chromosomes of some species were also observed.

Comparative cytogenetics of cichlid fishes through genomic in-situ hybridization (GISH) with emphasis on Oreochromis niloticus.

Cichlidae is the most species-rich freshwater family of Perciformes and has attracted the attention of aquarium hobbyists, aquaculturists, and sport fisherman. Oreochromis niloticus is very important in aquaculture today and is currently used in varied areas of study as an ‘experimental model’. Oreochromis niloticus has been characterized using classical and molecular cytogenetic techniques, with special attention paid to heterochromatin structure and the identification of sex chromosomes. In this study, we compare the genome of O. niloticus with that of other cichlids from Africa and South America using genomic in-situ hybridization (GISH). Our results show that at least some elements comprising the pericentromeric heterochromatin of Nile tilapia are species-specific and that the sequence of the majority of the long arm of the largest chromosome pair is conserved within the tilapiine group, which is composed of the genera Tilapia, Oreochromis, and Sarotherodon. It is suggested that the extensive regions of repeated DNA in the largest chromosome pair of O. niloticus resulted from chromosome rearrangement or accumulation caused by recombination suppression during the evolutionary history of the tilapiines.

Repetitive sequences associated with differentiation of W chromosome in Semaprochilodus taeniurus.

The possible origins and differentiation of a ZZ/ZW sex chromosome system in Semaprochilodus taeniurus, the only species of the family Prochilodontidae known to possess heteromorphic sex chromosomes, were examined by conventional (C-banding) and molecular (cross-species hybridization of W-specific WCP, Fluorescence in situ hybridization (FISH) with telomere (TTAGGG)n, and Rex1 probes) cytogenetic protocols. Several segments obtained by W-specific probe were cloned, and the sequences localized on the W chromosome were identified by DNA sequencing and search of nucleotide collections of the NCBI and GIRI using BLAST and CENSOR, respectively. Blocks of constitutive heterochromatin in chromosomes of S. taeniurus were observed in the centromere of all autosomal chromosomes and in the terminal, interstitial, and pericentromeric regions of the W chromosome, which did not demonstrate interstitial telomeric sites with FISH of the telomere probe. The Rex1 probe displayed a compartmentalized distribution pattern in some chromosomes and showed signs of invasion of the pericentromeric region in the W chromosome. Chromosomal painting with the W-specific WCP of S. taeniurus onto its own chromosomes showed complete staining of the W chromosome, centromeric sites, and the ends of the Z chromosome, as well as other autosomes. However, cross-species painting using this WCP on chromosomes of S. insignis, Prochilodus lineatus, and P. nigricans did not reveal a proto-W element, but instead demonstrated scattered positive signals of repetitive DNAs. Identification of the W-specific repetitive sequences showed high similarity to microsatellites and transposable elements. Classes of repetitive DNA identified in the W chromosome suggested that the genetic degeneration of this chromosome in S. taeniurus occurred through accumulation of these repetitive DNAs.

A comparative cytogenetic study of five piranha species (Serrasalmus, Serrasalminae) from the Amazon basin

Cytogenetic studies were conducted on five piranha species belonging to the genus Serrasalmus, subfamily Serrasalminae (Serrasalmus altispinis, S. compressus, S. elongatus, S. manuelli, and S. spilopleura). All the species were collected in the Amazon basin: confluence of Negro and Solimõoes Rivers (CatalãoLake), Solimões River (Marchantaria Island – Camaleão Lake), Uatumã River (Hydroelectric Power Station of Balbina), and Pitinga River (Hydroelectric Power Station of Pitinga). All the five species possess 2n = 60 chromosomes with 5–12 subtelo- and acrocentric chromosomes bearing nucleolar organizer regions. A proximal C-band positive heterochromatin block was evident on the long arms of a medium-sized metacentric chromosome pair in all the analized species, thus making it a cytogenetic marker for the genus. It is hypothesized that 2n = 60 chromosomes represents a derived feature in terms of the chromosomal evolution of piranhas because the basal lineages possess 2n = 62. Both Robertsonian centric fusion and non-Robertsonian rearragements such as pericentric inversions seem implicated in the chromosomal evolution of this group.