Karyotypic characterization of Symphurus tessellatus and Symphurus plagusia (Pleuronectiformes, Cynoglossidae) from Brazilian coastal waters
The family Cynoglossidae (tonguefishes) is a speciose group of Pleuronectiformes, encompassing about 20% of described species in this order, distributed among three genera (Cynoglossus, Paraplagusia and Symphurus). Symphurus is the only genus of tonguefish in the Western Atlantic, being characterized by species complexes and cryptic forms; consequently the actual species richness of this genus is likely to be underestimated. Comparative cytogenetic studies have proved to be useful to resolve taxonomic uncertainties in ichthyofauna. Therefore, we carried out the karyotypic characterization of Symphurus tessellatus and Symphurus plagusia from the Brazilian coast based on conventional analysis, C-banding, silver nitrate (Ag-NORs) and base-specific fluorochrome staining. The specimens of S. tessellatus presented 2n = 46, 20 m/sm+26st/a, differing from that previously reported (2n = 46, 22m/sm+24 st/a), with heterochromatic blocks in the centromeric region of most chromosomes, as usually described in this group. We also found interstitial segments on the long arms of the third chromosome; these were coincident to Ag-NORs and GC-rich sites. In S. plagusia, the karyotype is composed of 32m/sm+14st/a (2n=46), with a single nucleolar organizer regions (NORs) system, which is the first cytogenetic data for this species. The unique karyotype formulae of these two species suggest that pericentric inversions played a major role in the chromosomal differentiation of tonguefishes, being useful for cytotaxonomy and diagnosis of evolutionary units in Symphurus.