Karyotype divergence in Symphodus melops and Symphodus roissali (Labridae, Perciforms): C-banded and Ag-NOR karyotypes

Abstract

A comparative analysis of conventional C-banded and Ag-NOR karyotypes involving the closely related labrid species Symphodus melops and Symphodus roissali showed conspicuous differences between them, reflected mainly in their 2n and fundamental number values. The chromosome rearrangements apparently involved consisted of three centric fusions, pericentric inversions, and (or) translocations, as well as small heterochromatic additions. In spite of this clear divergence, the two species seem to have one larger metacentric pair, the nucleolar organizer region (NOR) carrier chromosomes, and some smaller unidentified pairs in common. Both karyotypes are fully representative of the described tendency in the labrids towards symmetric morphology. The NOR regions were shown to have C-banding and a heteromorphism between homologous regions was detected by conventional and silver staining, but not by the C-banding technique, which leads us to suggest that the NOR regions' heteromorphism is functional rather than structural in nature.Key words: Labridae, heterochromatin, nucleolus organizers.