First record of supernumerary (B) chromosomes in electric fish (Gymnotiformes) and the karyotype structure of three species of the same order from the upper Paraná River basin.

Cytogenetic studies were performed on the species Apteronotus prope albifrons Linnaeus, 1766, Rhamphichthys hahni Meinken, 1937 and Brachyhypopomus gauderio Giora & Malabarba, 2009, collected in the upper Paraná River floodplain, Porto Rico (PR), Brazil. Apteronotus prope albifrons showed a diploid number of 2n=24 chromosomes for both sexes and a karyotype formula of 14m+2sm+2st+6a (FN=42). Besides the standard karyotype, three specimens had one to three extra microchromosomes with inter- and intra-individual variations, which suggested the occurrence of B chromosomes in the species. The chromosomal data of Rhamphichthys hahni, described here for the first time, consists of 50 chromosomes and a formula comprised of 20m+24sm+6a (FN=94). Brachyhypopomus gauderio specimens demonstrated 2n=42 chromosomes infemales, all acrocentric, and 2n=41 chromosomes in males, with 40 acrocentric and 1 medium-sized metacentric chromosome. These differences concern with a multiple system of sexchromosome determination X1X1X2X2/X1X2Y (FN=42) in Brachyhypopomus gauderio. The analysis of nucleolar organizer regions by Ag-NOR and FISH 18S banding revealed a simple NOR system in Apteronotus prope albifrons and Rhamphichthys hahni and a multiple NOR system in Brachyhypopomus gauderio, that is unusual for Gymnotiformes fishes. Constitutive heterochromatin was mainly found in the pericentromere region in most of the chromosomes of the three species, although each species had its own peculiarities. The B chromosomes in Apteronotus prope albifrons demonstrated heterochromatin positioned in the centromeric and telomeric regions whereas Rhamphichthys hahni presented conspicuous blocks of heterochromatin on the long arms in three submetacentric pairs. Brachyhypopomus gauderio showed blocks of heterochromatin on the long arm in the interstitial and telomere positions. The finding of B chromosomes in Apteronotus prope albifrons represents the first description of these elements in the Gymnotiformes order. Although the karyotype of this species is similar with that described for populations in the Amazon basin, the presence of B chromosomes could represent a specific characteristic of this population. A comparative analysis of karyotypes of Rhamphichthys hahni with other species of the genus showed a relatively conservative structure suggesting 2n=50 as a common number in this group. The karyotype of Brachyhypopomus gauderio, a new species, provides an important reference for future chromosome studies of the Brachyhypopmus Mago-Lecia, 1994, and it might be also significant for cytotaxonomy in this group. The cytogenetic data also demonstrate the need of more comparative cytogenetic studies in the families of the highly diversified and taxonomically difficult complex Gymnotiformes.