M.B. Cioffi

The contrasting role of heterochromatin in the differentiation of sex chromosomes: an overview from Neotropical fishes

During the evolutionary process of the sex chromosomes, a general principle that arises is that cessation or a partial restriction of recombination between the sex chromosome pair is necessary. Data from phylogenetically distinct organisms reveal that this phenomenon is frequently associated with the accumulation of heterochromatin in the sex chromosomes. Fish species emerge as excellent models to study this phenomenon because they have much younger sex chromosomes compared to higher vertebrates and many other organisms making it possible to follow their steps of differentiation. In several Neotropical fish species, the heterochromatinization, accompanied by amplification of tandem repeats, represents an important step in the morphological differentiation of simple sex chromosome systems, especially in the ZZ/ZW sex systems. In contrast, multiple sex chromosome systems have no additional increase of heterochromatin in the chromosomes. Thus, the initial stage of differentiation of the multiple sex chromosome systems seems to be associated with proper chromosomal rearrangements, whereas the simple sex chromosome systems have an accumulation of heterochromatin. In this review, attention has been drawn to this contrasting role of heterochromatin in the differentiation of simple and multiple sex chromosomes of Neotropical fishes, highlighting their surprising evolutionary dynamism.

Comparative Chromosomal Mapping of Microsatellites in Leporinus Species (Characiformes, Anostomidae): Unequal Accumulation on the W Chromosomes

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.

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.

Chromosomal Distribution of Repetitive DNA Sequences Highlights the Independent Differentiation of Multiple Sex Chromosomes in Two Closely Related Fish Species

The arrangement of 6 repetitive DNA sequences in the mitotic and meiotic sex chromosomes of 2 Erythrinidae fish, namely Hoplias malabaricus and Erythrinuserythrinus, both with a multiple X1X1X2X2/X1X2Y 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.

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.

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.

Evolutionary dynamics of rDNA genes on chromosomes of the Eucinostomus fishes: cytotaxonomic and karyoevolutive implications

Several chromosomal features of Gerreidae fish have been found to be conserved. In this group, it is unclear whether the high degree of chromosomal stasis is maintained when analyzing more dynamic regions of chromosomes, such as rDNA sites that generally show a higher level of variability. Thus, cytogenetic analyses were performed on 3 Atlantic species of the genus Eucinostomus using conventional banding (C-banding, Ag-NOR), AT- and GC-specific fluorochromes, and fluorescence in situ hybridization mapping of telomeric sequences and 5S and 18S rDNA sites. The results showed that although the karyotypical macrostructure of these species is similar (2n = 48 chromosomes, simple Ag-NORs seemingly located on homeologous chromosomes and centromeric heterochromatin pattern), there are differences in the positions of rDNA subunits 5S and 18S. Thus, the ribosomal sites have demonstrated to be effective cytotaxonomic markers in Eucinostomus, presenting a different evolutionary dynamics in relation to other chromosomal regions and allowing access to important evolutionary changes in this group.

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.

Comparative cytogenetic mapping of rRNA genes among naked catfishes: implications for genomic evolution in the Bagridae family

In the present study, the karyotype and chromosomal characteristics of 9 species of the Bagridae fish family were investigated using conventional Giemsa staining as well as dual-color fluorescence in situ hybridization to detect the 18S and 5S rDNA sites. In addition to describing the karyotype of several Bagridae catfishes, we established molecular cytogenetic techniques to study this group. The 9 species contained a diploid chromosomal number, varying from 50 (Pseudomystus siamensis) to 62 (Hemibagrus wyckii), while none contained heteromorphic sex chromosomes. 18S rDNA sites were detected in only 1 chromosomal pair among all species evaluated. However, 3 different patterns were observed for the distribution of the 5S rDNA: 2 sites were found in the genus Mystus and in P. siamensis, multiple sites were observed in the genus Hemibagrus, and a syntenic condition for the 18S and 5S rDNA sites was identified in H. wyckii. The extensive variation in the number and chromosomal position of rDNA clusters observed among these Bagridae species may be related to the intense evolutionary dynamics of rDNA-repeated units, which generates divergent chromosomal distribution patterns even among closely related species. In summary, the distribution of repetitive DNA sequences provided novel, useful information regarding the evolutionary relationships between Bagridae fishes.

Chromosomal distribution of two multigene families and the unusual occurrence of an X1X1X2X2/X1X2Y sex chromosome system in the dolphinfish (Coryphaenidae): an evolutionary perspective.

Dolphinfishes (Coryphaenidae) are pelagic predators distributed throughout all tropical and subtropical oceans and are very important for commercial, traditional, and sport fishing. This small family contains the Coryphaena hippurus and Coryphaena equiselis species whose chromosomal aspects remain unknown, despite recent advances in cytogenetic data assimilation for Perciformes. In this study, both species were cytogenetically analyzed using different staining techniques (C-, Ag-, and CMA3 banding) and fluorescence in situ hybridization, to detect 18S rDNA and 5S rDNA. C. hippurus females exhibit 2n = 48 chromosomes, with 2m+4sm+42a (NF = 54). In C. equiselis, where both sexes could be analyzed, females displayed 2n = 48 chromosomes (2m+6sm+40a) and males exhibited 2n = 47 chromosomes (3m+6sm+38a) (NF = 56), indicating the presence of X1X1X2X2/X1X2Y multiple sex chromosomes. Sex-chromosome systems are rare in Perciformes, with this study demonstrating the first occurrence in a marine pelagic species. It remains unknown as to whether this system extends to other populations; however, these data are important with respect to evolutionary, phylogenetic, and speciation issues, as well as for elucidating the genesis of this unique sex system.