Derived cytogenetic traits, multiple NORs and B chromosomes in the compact karyotype of Canthigaster figueiredoi (Tetraodontiformes)

The Tetraodontiformes are one of the main irradiation lineages of Teleosts representing a highly specialized group. Among its families, Tetraodontidae has the lowest DNA/cell content of the vertebrates, and for this reason has been used as a model group for genomic evolution studies. Seeking to widen the cytogenetic database of this family, we performed a chromosomal analysis of the species Canthigaster figueiredoi using conventional staining, Ag-NORs, C-banding, base-specific fluorochromes DAPI-CMA(3), and in situ hybridization with 18S rDNA probe. This species has 2n=36 (10 m+6sm+20a; FN=52). Multiple Ag-NORs (CMA(3)(+)) sites were detected on the four chromosome pairs. Heterochromatic blocks (CMA(3)(+)) were present in the centromeric regions of most of the pairs, extending on the short arm of some chromosomes. A GC-positive polymorphic heterochromatic region was identified in some of the individuals, in one or both of the homologs of the 6th pair. The presence of heteromorphic B microchromosomes was detected in the karyotype of one female, exhibiting intra-individual variation of 0-3 Bs. The occurrence of heterochromatic polymorphisms, multiple NORs, and B chromosomes in C. figueiredoi are very infrequent events in marine fish. They are probably associated to the accentuated restructuring and genomic reduction suffered by this family.