Karyotype, C-banding, and Ag-NOR analysis in Diplodus bellottii (Sparidae, Perciforms). Intra-individual polymorphism involving heterochromatic regions

Abstract

A karyotype analysis was carried out in nine specimens of the Sparid species Diplodus bellottii using conventional staining, as well as C-banding and Ag-NOR banding techniques, showing, respectively, 2n = 46 and fundamental number (FN) = 54, and scarce heterochromatic areas irregularly distributed and up to four NOR active regions that were C positive. When compared with the karyotypes of other related species, one centric fusion giving rise to a large metacentric pair and several pericentric inversions seem to have been involved in the karyotype evolution. An intra-individual polymorphism was detected in one specimen, resulting in two karyotypic forms in roughly identical proportion, owing to a larger C-band by the NOR regions, appearing either in a terminal position of the short arms of pair 2 or in telomeric position of pair 3. These findings suggest that the extra heterochromatic segment responsible for the heteromorphism apparently only involves associated heterochromatin and not the NORs themselves. This C-positive block seems to have eventually been transferred between heterologous NOR chromosomes by a somatic event, facilitated by the physical proximity of NOR pairs in the nucleolus.

Conventional karyotype and nucleolar organizer regions of the toadfish <i>Halobatrachus didactylus</i> (Schneider, 1801) (Pisces: Batrachoididae)

“…It shows that the most common chromosome number in the group is 2n = 48 telocentric elements, a condition considered as ancestral for teleosts (Gold, 1979;Ohno, 1974;Vitturi et al, 1995). Assuming that the primitive karyotype for actual teleosteans derives from an ancestral diploid number of 48 telocentric chromosomes, the evolution of the karyotype of Halobatrachus didactylus can be interpreted as the result of at least the central fusion of two pairs of uniarmed chromosomes to form large metacentric or submetacentric elements, resulting in the reduction of the diploid number from 48 to 46, as well as a series of pericentric inversions, translocations and/or simple heterochromatin additions giving rise to the 20 biarmed chromosomes during the species evolution It has been argued that any real difference in the FN between closely related species can be explained as the result of pericentric inversions, whereas the differences in the diploid number (2n) presumably represent Robertsonian changes (fusions, fissions) (LeGrande, 1981;López et al, 1988;Amores et al, 1993); therefore, the FN could be used as an indicator of the accumulation of chromosome reorganisations. The fundamental number (FN) is a common descriptor of the number of chromosome arms, but the methods for calculating it vary among different reports.…”

Cytogenetic analysis in western Atlantic snappers (Perciformes, Lutjanidae)

“…Cases of heterochromatin polymorphism might range from subtle to hypervariable in fish. They can be related to the size of the heterochromatic blocks (Martinez et al, 1991;Jankun et al, 1998) or to associations between heterochromatin and NORs (Hartley, 1988;Amores et al, 1993). Some groups, particularly the Characiformes, present a remarkable karyotype variability influenced by heterochromatinization processes, as extensively seen in the species complex Astyanax scabripinnis (Souza et al, 1996;Mantovani et al, 2000;Mantovani et al, 2004).…”

Cytogenetic studies in Brazilian marine Sciaenidae and Sparidae fishes (Perciformes)

“…Therefore, species bearing a single NOR (e.g., A. probatocephalus) or multiple NORs (Diplodus bellottii; Amores et al, 1993;D. puntazzo;Vitturi et al, 1996) have been identified (Table 2).…”