Gideão Wagner Werneck Félix da Costa

Unusual Dispersion of Histone Repeats on the Whole Chromosomal Complement and Their Colocalization with Ribosomal Genes in Rachycentron canadum (Rachycentridae, Perciformes)

Rachycentron canadum, the only representative of the family Rachycentridae, has been the focus of biotechnological interest due to its significant potential in marine fish farming. The chromosome set of this species has been widely investigated with respect to the location of genes and multigene families. A FISH analysis was performed using 4 multigene families as probes, represented by 5S and 18S ribosomal genes and histones H2B-H2A and H3. Earlier data suggested that differential replication of heterochromatin could be partially associated with functional genes. Indeed, our results showed that the DNA contained in heterochromatic regions of R. canadum contains 5S and 18S ribosomal genes as well as the gene sequences of histones H2B-H2A and H3, which were colocalized. The distribution of H3 sequences in all heterochromatic regions, except in 13q, could indicate an important evolutionary role for this class of repetitive sequences. Besides, the presence of chromosome regions bearing multifunctional repetitive sequences formed by H2B-H2A/H3/18S rDNA and H2B-H2A/H3/5S rDNA clusters was demonstrated for the first time in fishes. The implications of differential histone gene extension and its functionality in the karyotype of R. canadum remain unknown.

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 diversification of Western Atlantic Bathygobius species based on cytogenetic, morphologic and DNA barcode data

A number of fish groups, such as Gobiidae, are highly diversified and taxonomically complex. Extensive efforts are necessary to elucidate their cryptic diversity, since questions often arise about the phylogenetic aspects of new species. Clarifications about the diversity and phylogeny of the Bathygobius species from the southwestern Atlantic are particularly needed. Evidence has been accumulating on the Brazilian coast regarding the possible presence of new species while doubts remain about the taxonomic status of others. The taxonomic identification of some species of Bathygobius has been problematic, given their generally conservative external morphology, and several species are recognized as cryptic. This situation hinders understanding the real diversity in this taxon. Taken together, genetic, cytogenetic and morphometric analyses have been effective in identifying new species of this genus. Here we describe the karyotypic features and morphological patterns of three Western South Atlantic species of Bathygobius. Furthermore, its cytochrome c oxidase I (COI) gene sequences were compared with those of species from Central America, North America and the Caribbean. The broad analyses performed demonstrated an unsuspected diversity, leading to the identification of an un-described new species (Bathygobius sp.2) and the geographic redefinition of another, Bathygobius sp.1, undoubtedly a branch of B. geminatus, hitherto inaccurately identified as B. mystacium on the coast of Brazil.

Inferring Diversity and Evolution in Fish by Means of Integrative Molecular Cytogenetics

Fish constitute a paraphyletic and profusely diversified group that has historically puzzled ichthyologists. Hard efforts are necessary to better understand this group, due to its extensive diversity. New species are often identified and it leads to questions about their phylogenetic aspects. Cytogenetics is becoming an important biodiversity-detection tool also used to measure biodiversity evolutionary aspects. Molecular cytogenetics by fluorescence in situ hybridization (FISH) allowed integrating quantitative and qualitative data from DNA sequences and their physical location in chromosomes and genomes. Although there is no intention on presenting a broader review, the current study presents some evidences on the need of integrating molecular cytogenetic data to other evolutionary biology tools to more precisely infer cryptic species detection, population structuring in marine environments, intra- and interspecific karyoevolutionary aspects of freshwater groups, evolutionary dynamics of marine fish chromosomes, and the origin and differentiation of sexual and B chromosomes. The new cytogenetic field, called cytogenomics, is spreading due to its capacity to give resolute answers to countless questions that cannot be answered by traditional methodologies. Indeed, the association between chromosomal markers and DNA sequencing as well as between biological diversity analysis methodologies and phylogenetics triggers the will to search for answers about fish evolutionary, taxonomic, and structural features.

Genetic Multipartitions Based on D-Loop Sequences and Chromosomal Patterns in Brown Chromis, Chromis multilineata (Pomacentridae), in the Western Atlantic

Connectivity levels among Brazilian reef fish fauna populations have attracted growing interest, mainly between mainland shores and oceanic islands. The Pomacentridae, whose phylogeographic patterns are largely unknown in the Atlantic, are a family of dominant fish in reef regions. We present data on the variability and population structure of damselfish Chromis multilineata in different areas along the northeast coast of Brazil and in the waters around the oceanic islands of Fernando de Noronha (FNA) and Saint Peter and Saint Paul Archipelago (SPSPA) through analysis of the HVR1 mtDNA sequence of the control region. The remote SPSPA exhibits the highest level of genetic divergence among populations. Conventional and molecular cytogenetic analysis showed similar karyotype patterns (2n = 48 acrocentrics) between these insular areas. Our estimates reveal three genetically different population groups of C. multilineata on the Brazilian coast. The level of genetic structure is higher than previous data suggested, indicating complex panel of interactions between the oceanic island and coastal populations of Brazil.

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

Abstract 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.

Differential hypomethylation of the repetitive Tol2/Alu-rich sequences in the genome of Bodianus species (Labriformes, Labridae)

Abstract Representatives of the order Labriformes show karyotypes of extreme conservatism together with others with high chromosomal diversification. However, the cytological characterization of epigenetic modifications remains unknown for the majority of the species. In the family Labridae, the most abundant fishes on tropical reefs, the genomes of the genus Bodianus Bloch, 1790 have been characterized by the occurrence of a peculiar chromosomal region, here denominated BOD. This region is exceptionally decondensed, heterochromatic, argentophilic, GC-neutral and, in contrast to classical secondary constrictions, shows no signals of hybridization with 18S rDNA probes. In order to characterize the BOD region, the methylation pattern, the distribution of Alu and Tol2 retrotransposons and of 18S and 5S rDNA sites, respectively, were analyzed by Fluorescence In Situ Hybridization (FISH) on metaphase chromosomes of two Bodianus species, B. insularis Gomon & Lubbock, 1980 and B. pulchellus (Poey, 1860). Immunolocalization of the 5-methylcytosine revealed hypermethylated chromosomal regions, dispersed along the entire length of the chromosomes of both species, while the BOD regions exhibited a hypomethylated pattern. Hypomethylation of the BOD region is associated with the precise co-location of Tol2 and Alu elements, suggesting their active participation in the regulatory epigenetic process. This evidence underscores a probable differential methylation action during the cell cycle, as well as the role of Tol2/Alu elements in functional processes of fish genomes.