Chromosome mapping of repetitive sequences in Rachycentron canadum (Perciformes: Rachycentridae): implications for karyotypic evolution and perspectives for biotechnological uses

Abstract

The cobia, Rachycentron canadum, a species of marine fish, has been increasingly used in aquaculture worldwide. It is the only member of the family Rachycentridae (Perciformes) showing wide geographic distribution and phylogenetic patterns still not fully understood. In this study, the species was cytogenetically analyzed by different methodologies, including Ag-NOR and chromomycin A(3) (CMA(3))/DAPI staining, C-banding, early replication banding (RGB), and in situ fluorescent hybridization with probes for 18S and 5S ribosomal genes and for telomeric sequences (TTAGGG)(n). The results obtained allow a detailed chromosomal characterization of the Atlantic population. The chromosome diversification found in the karyotype of the cobia is apparently related to pericentric inversions, the main mechanism associated to the karyotypic evolution of Perciformes. The differential heterochromatin replication patterns found were in part associated to functional genes. Despite maintaining conservative chromosomal characteristics in relation to the basal pattern established for Perciformes, some chromosome pairs in the analyzed population exhibit markers that may be important for cytotaxonomic, population, and biodiversity studies as well as for monitoring the species in question.

Figure 1

Karyotypes of Rachycentron canadum. Conventional staining (a) highlighting the Ag-NOR sites in chromosome 2; (b) C-banding, highlighting the NORs as CMA₃⁺/DAPI⁻ heterochromatic regions; (c) replication bands, showing 18S (pair 2) and 5S (pairs 3 and 13) rDNA sites with early replication; (d) double-FISH with 18S (pink) and 5S (green) rDNA probes, showing the location of 18S rDNA sites in pair 2 and of 5S rDNA sites in pairs 3 and 13; (e) FISH with (TTAGGG)_n sequences showing the location of telomeric sites in the chromosomes (orange). Bar = 5 μm.

Figure 2

Idiogram of the chromosome complement of Rachycentron canadum, exhibiting cytogenetic mapping of ribosomal sequences, Ag-NORs, heterochromatic regions, and chromosome replication bands. Bar = 5 μm.