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.
Highlights
Double-Fluorescence In Situ Hybridization (FISH) with 5S and 18S rDNA probes revealed a non-syntenic location for these ribosomal sites
The 18S rDNA sites were exclusively located in the terminal regions on the p arms of the pair No 9, corresponding with the Ag-NOR signals
DNA methylation is one of the epigenetic processes that has modulated the molecular evolution of life, but its influence in karyotype evolution and interaction in the structural chromosome regions are little known, especially for fish species
Summary
Genomes of some representatives of Labriformes families carry preferential chromosomal rearrangements (Sena and Molina 2007; Molina et al 2014; Almeida et al 2017), and singular regional DNA organization (Molina et al 2012; Amorim et al 2016). Particular evolutionary trends in karyotype differentiation, such as pericentric inversions and centric fusions, occur among tribes of this family (Molina and Galetti 2004, Sena and Molina 2007, Molina et al 2014, Almeida et al 2017). Some Atlantic species, such as Bodianus rufus (Linnaeus, 1758), B. pulchellus (Poey, 1860) and B. insularis Gomon & Lubbock, 1980, have been analyzed in detail, and phylogenetically shared particular chromosomal regions have been identified. These regions, located at the p arms of the second subtelocentric chromosome pair, were characterized as exceptionally decondensed, heterochromatic and argentophilic, suggesting the presence of rDNA sites. These regions are neither GC-rich, nor do they display hybridization signals with 18S rDNA probes, indicating the presence of distinct repetitive sequences with unusual organization (Molina et al 2012)
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