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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., (© 2014 S. Karger AG, Basel.)

Approximately 90 species in the genus Leporinus (Characiformes, Anostomidae) are known, and most of them do not present differentiated sex chromosomes. However, there is a group of 7 species that share a heteromorphic ZW sex system. In all of these species, the W chromosome is the largest one in the karyotype and is mostly heterochromatic. We investigated the distribution of several microsatellites in the genome of 4 Leporinus species that possess ZW chromosomes. Our results showed a very large accumulation of mostly microsatellites on the W chromosomes. This finding supports the presence of an interconnection between heterochromatinization and the accumulation of repetitive sequences, which has been proposed for sex chromosome evolution, and suggests that heterochromatinization is the earlier of the 2 processes. In spite of the common origin that has been proposed for W chromosomes in all of the studied species, the microsatellites followed different evolutionary trajectories in each species, which indicates a high plasticity for sex chromosome differentiation., (© 2013 S. Karger AG, Basel.)

Small supernumerary marker chromosomes (sSMC) are known for being present in mosaic form as 47,+mar/46 in >50% of the cases with this kind of extra chromosomes. However, no detailed studies have been done for the mitotic stability of sSMC so far, mainly due to the lack of a corresponding in vitro model system. Recently, we established an sSMC-cell bank (Else Kröner-Fresenius-sSMC-cellbank) with >150 cell lines. Therefore, 93 selected sSMC cases were studied here for the presence of the corresponding marker chromosomes before and after Epstein-Barr virus-induced immortalization. The obtained results showed that dicentric inverted duplicated-shaped sSMC are by far more stable in vitro than monocentric centric minute- or ring-shaped sSMC. Simultaneously, a review of the literature revealed that a comparable shape-dependent mitotic stability can be found in vivo in sSMC carriers. Additionally, a possible impact of the age of the sSMC carrier on mitotic stability was found: sSMC cell lines established from patients between 10-20 years of age were predominantly mitotically unstable. The latter finding was independent of the sSMC shape. The present study shows that in vitro models can lead to new and exciting insights into the biology of this genetically and clinically heterogeneous patient group., (© 2014 S. Karger AG, Basel)

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.

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.

The arrangement of 6 repetitive DNA sequences in the mitotic and meiotic sex chromosomes of 2 Erythrinidae fish, namely Hoplias malabaricus and Erythrinus erythrinus, both with a multiple X(1)X(1)X(2)X(2)/X(1)X(2)Y sex chromosome system, was analyzed using fluorescence in situ hybridization. The distribution patterns of the repetitive sequences were distinct for each species. While some DNA repeats were species-specific, others were present in the sex chromosomes of both species at different locations. These data, together with the different morphological types of sex chromosomes and the distinct chromosomal rearrangements associated with the formation of the neo-Y chromosomes, support the plasticity of sex chromosome differentiation in the Erythrinidae family. Our present data highlight that the sex chromosomes in fish species may follow diverse differentiation patterns, even in the same type of sex chromosome system present in cofamiliar species., (Copyright © 2011 S. Karger AG, Basel.)

Distribution of 12 mono-, di- and tri-nucleotide microsatellites on the chromosomes of 2 karyomorphs with 2 distinct sex chromosome systems (a simple XX/XY - karyomorph B and a multiple X(1)X(1)X(2)X(2)/X(1)X(2)Y - karyomorph D) in Hoplias malabaricus, commonly referred to as wolf fish, was studied using their physical mapping with fluorescence in situ hybridization (FISH). The distribution patterns of different microsatellites along the chromosomes varied considerably. Strong hybridization signals were observed at subtelomeric and heterochromatic regions of several autosomes, with a different accumulation on the sex chromosomes. A massive accumulation was found in the heterochromatic region of the X chromosome of karyomorph B, whereas microsatellites were gathered at centromeres of both X chromosomes as well as in corresponding regions of the neo-Y chromosome in karyomorph D. Our findings are likely in agreement with models that predict the accumulation of repetitive DNA sequences in regions with very low recombination. This process is however in contrast with what was observed in multiple systems, where such a reduction might be facilitated by the chromosomal rearrangements that are directly associated with the origin of these systems., (Copyright © 2010 S. Karger AG, Basel.)

The processes working on sex chromosome differentiation are still not completely understood. However, the accumulation of repetitive DNA sequences has been shown to be one of the first steps in the early stages of such differentiation. In addition, regions with suppressed or no recombination have a potential to accumulate these DNA sequences and, for this reason, the absence of recombination between the sex chromosomes favors, by itself, the accumulation of repetitive sequences on these chromosomes during evolution. The diversity of sex-determining mechanisms in fish, alongside with the absence of heteromorphic sex chromosomes in many species, makes this group a useful model to better understand evolutionary processes of sex chromosomes in vertebrates, considering that fish occupy the basal position in the phylogeny of this group. In this review we draw attention to a preferential accumulation and enrichment in repetitive DNAs in sex chromosomes of many neotropical fish species in comparison with autosomes. This phenomenon has been observed between both morphologically differentiated and nascent sex chromosome systems, which highlight the potential role of these sequences in the differentiation of fish sex chromosomes generating differences in morphology and size between them., (2010 S. Karger AG, Basel.)

Karyotype and chromosomal characteristics from 3 allopatric populations of Hoplias malabaricus, cytogenetically the most studied Erythrinidae taxon, were investigated using different staining techniques (C-, Ag-, and CMA(3) banding) as well as fluorescent in situ hybridization (FISH) to detect 18S rDNA, 5S rDNA, and 5SHindIII satellite DNA sites. The isolation, cloning and characterization of an 18S rDNA probe from H. malabaricus genome were also performed for the first time in order to develop a more specific probe. The 3 populations, named PR, CR, and DR, showed identical karyotypes, with 2n = 42 chromosomes composed of 11 m pairs and 10 sm pairs, without heteromorphic sex chromosomes, which characterize the populations as belonging to karyomorph A. In all populations C-positive heterochromatin was situated in the centromeric/pericentromeric regions of the chromosomes, as well as in the telomeric region of several pairs. A conspicuous proximal heterochromatic block on the long arm of pair No. 16 was the only GC-rich segment in the karyotypes. 5SHindIII satellite DNA was always mapped in the centromeric region of several chromosomes. The 18S rDNA sites were situated on the telomeric or centromeric regions, whereas the 5S rDNA showed an interstitial or proximal location in some pairs. Several chromosomes bearing these repetitive DNA sequences were shared by the 3 populations, alongside with some exclusive chromosomal markers. In this sense, population CR was the most differentiated one, including a syntenic condition for the 18S and 5S rDNA probes, as confirmed by double FISH. Thus, despite their inclusion in the same major karyotypic group, the distinct populations cannot be considered an absolute evolutionary unit, as evidenced by their inner chromosomal differentiations., ((c) 2009 S. Karger AG, Basel.)

Triportheus fish species present 2n = 52 chromosomes. The karyotypes show similar macrostructure and a ZZ/ZW sex chromosome system, which probably represents synapomorphy for the genus. A probe of the Z chromosome was obtained from T. nematurus through microdissection, followed by unspecific amplification via DOP-PCR. This probe was used for WCP (whole chromosome painting) through fluorescent in situ hybridization (FISH) in several other Triportheus species, to analyze the differentiation of the ZZ/ZW system. The homology between this probe and chromosomes of species from other genera, putatively related to Triportheus, was also examined to search for evidence about evolution of their sex chromosomes. Complete homology was found among the Z chromosomes of all Triportheus species, while only small positive signals were found on the W chromosomes. Hybridization signals were absent in species from other genera. The present results reinforce both the conservative nature of Z chromosomes and the hypothesis that the ZZ/ZW sex chromosome system is a synapomorphic feature of Triportheus. On the other hand, besides reduction of size, W chromosomes have undergone accentuated composition changes in relation to Z chromosomes, since only a small region, usually located in the short arm, kept homology with the Z chromosomes., (Copyright 2008 S. Karger AG, Basel.)