Cytogenetic characterization in <i>Hoplias brasiliensis</i> (Characiformes, Erythrinidae) from an isolated river basin in northeastern South America

The genus Hoplias (trahiras) represents a species-rich group of controversial taxonomy in the family Erythrinidae. Even though cytogenetic analyses have been useful to discriminate cryptic fish species, most chromosomal data in Erythrinidae are focused on Hoplias malabaricus (common trahira), considered a species complex based on the presence of reproductively isolated karyomorphs. In turn, the giant trahiras (Hoplias gr. lacerdae and H. aimara complexes) seem to be more cytogenetically conserved even though there are few karyotypic reports in this group. In order to increase the chromosomal information in the giant trahiras, we provided the first cytogenetic characterization in a population of H. brasiliensis from an isolated river basin in Northeastern Brazil. We confirmed the general karyotype trend observed in this fish group, i.e., a diploid number (2n) of 50 distributed into 20 metacentric (m) and 30 submetacentric (sm) chromosomes, pericentromeric heterochromatin segments and active nucleolar organizer regions (Ag-NORs) at interstitial region on long arms of a sm pair. On the other hand, heteromorphic GC-rich (CMA3+) sites were observed at the pericentromeric region of up to 10 chromosomal pairs in H. brasiliensis, diverging from the usual pattern reported in other populations and species of giant trahiras. The present findings indicate that, in spite of the apparent chromosomal conservativeness in this fish group, some genomic regions have undergone distinct evolutionary pathways, even within a same nominal taxon. Therefore, the utilization of refined analyses of chromosomal microstructure is recommended as a powerful tool to resolve the taxonomic uncertainties in giant trahiras.