Mapping of Ribosomal DNA by FISH, EcoRI Digestion and Replication Bands in the Cardinalfish Apogon americanus (Perciformes)

Reef fishes of the order Perciformes represent an interesting group for both genetic and cytogenetic studies as their evolutionary pattern, usually unclear, might range from dynamic to quite stable. A karyotype with 2n=48 acrocentric chromosomes is considered basal for this group, whereas karyotypes with other formulae are regarded as derived. Some Perciformes families present a high karyotypic diversification such as Apogonidae, although cytogenetic reports in this family along South Atlantic were absent so far. In the present work, cytogenetic analyses were carried out in Apogon americanus from Northeastern Brazil, revealing a modal diploid number of 2n=36 (12m+6sm+16st+2a; FN=70) and a remarkable presence of large metacentric pairs. The Ag-NORs were located on short arms of the 8th pair, at telomeric position, coincident with hybridization signals obtained by FISH using 18S rDNA probes. The 5S minor ribosomal genes are distributed in 2 chromosomal pairs, not syntenic to the major ribosomal subunits. Heterochromatic blocks are located mainly at pericentromeric position in most of chromosomal pairs. The early replication pattern showed longitudinal bands with both early and late replication in the heterochromatin segments, being very similar to the pattern obtained after chromosomal digestion with EcoRI restriction enzyme. The present data, coupled with previous reports in other species, suggest a karyotypic evolution mainly driven by Robertsonian fusions (Rb) and pericentric inversions. In spite of the structural rearrangements, some features in the DNA composition and specific regions, such as 5S and 18S rDNA sites, reflect symplesiomorphic karyotype traits, shared by other clades in Perciformes.