Comparative cytogenetics of giant trahiras Hoplias aimara and H. intermedius (Characiformes, Erythrinidae): chromosomal characteristics of minor and major ribosomal DNA and cross-species repetitive centromeric sequences mapping differ among morphologically identical karyotypes.

Karyotype and cytogenetic characteristics of 2 species of giant trahiras, Hopliasintermedius, São Francisco river basin, and Hopliasaimara, Arinos river (Amazon basin), were examined by conventional (C-banding, Ag-NOR, DAPI/CMA(3) double-staining) and fluorescent in situ hybridization (FISH) with 5S, 18S rDNA probes and cross-species Cot-1 DNA probing. Both species invariably had diploid chromosome number 2n = 50 and identical karyotypes composed of 10 pairs of metacentric and 15 pairs of submetacentric chromosomes. On the other hand, staining with base-specific fluorochromes (CMA(3), DAPI) and FISH mapping of repetitive DNA sequences showed extensive interspecific differences: while the genome of H. aimara had one submetacentric pair bearing CMA(3)-positive (DAPI-negative) sites, that of H. intermedius had 4 such pairs; while FISH with a 5S rDNA probe showed one (likely homologous) signal-bearing pair, that with 18S rDNA displayed one signal-bearing pair in H. intermedius and 2 such pairs in H. aimara. Cross-species FISH probing with Cot-1 DNA prepared from total DNA of both species showed no signals of Cot-1 DNA from H. aimara on chromosomes of H. intermedius but reciprocally (Cot-1 DNA from H. intermedius on chromosomes of H. aimara) displayed signals on at least 4 chromosome pairs. Present findings indicate (i) different composition of repetitive sequences around centromeres, (ii) different NOR phenotypes and (iii) distinct taxonomic status of both giant trahira species.