Karyotypic changes and diversification time in Epinephelidae groupers (Perciformes). Implications on reproductive isolation.

Groupers (Epinephelidae and Serranidae) have attracted special attention to fish farming, and their species offer good opportunities for successful hybridizations. Cytogenetic data allow a better understanding of the role of karyotypic diversification in the acquisition of post-zygotic reproductive isolation (RI). Thus, chromosomal analyses were performed on E. striatus (Caribbean Sea), E. coioides and E. tauvina (Indo-Pacific Region), using standard procedures and mapping of six repetitive DNA classes by the in situ hybridization. The three species have 2n=48 chromosomes. The karyotypes of E. coioides and E. striatus are composed only of acrocentric chromosomes (FN=48), while E. tauvina has 8 submetacentric chromosomes (FN=56). Heterochromatin has a preferential centromeric distribution, and the microsatellite repeats are dispersed throughout the chromosomes of all species. The 18S and 5S rDNA sites are unique but show a colocalization arrangement in E. tauvina and E. striatus. The chromosomal organization suggests that the three species still maintain a significant amount of syntenic regions. The range of the karyotype divergence and the RI levels showed low, but goes turn proportionally greater in relation to the divergence time between the parental species. The slow acquisition of postzygotic RI is consistent with the high karyotype homogeneity presented by Epinephelidae family.

Paracentric Inversions Differentiate the Conservative Karyotypes in Two Centropomus Species (Teleostei: Centropomidae).

Centropomus is the sole genus of the Centropomidae family (Teleostei), comprising 12 species widely distributed throughout the Western Atlantic and Eastern Pacific, with 6 of them occurring in the Western Atlantic in extensive sympatry. Their life history and phylogenetic relationships are well characterized; however, aspects of chromosomal evolution are still unknown. Here, cytogenetic analyses of 2 Centropomus species of great economic value (C. undecimalis and C. mexicanus) were performed using conventional (Giemsa, Ag-NOR, and fluorochrome staining, C- and replication banding) and molecular (chromosomal mapping of 18S and 5S rDNA, H2A-H2B and H3 hisDNA, and (TTAGGG)n repeats) approaches. The karyotypes of both species were composed of 48 solely acrocentric chromosomes (2n = 48; FN = 48), but the single ribosomal site was located in varying positions in the long arms of the second largest chromosome pair. Replication bands were generally similar, although conspicuous differences were observed in some chromosome regions. In both species, the histone H3 genes were located on 3 apparently homeologous chromosome pairs, but the exact position of these clusters differed slightly. Interspecific hisDNA and rDNA site displacements can indicate the occurrence of multiple paracentric inversions during the evolutionary diversification of the Centropomus genomes. Although the karyotypes remained similar in both species, our data demonstrate an unsuspected microstructural reorganization between them, driven most likely by a series of paracentric inversions.

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

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.