Abstract
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.
Highlights
The order Gymnotiformes is a group with high species diversity and about 179 species have been listed so far (Crampton 2011).Karyotype diversity is well known in Gymnotiformes, especially in the genera Gymnotus Linnaeus, 1758 and Eigenmannia Jordan et Evermann, 1896
Apteronotus prope albifrons showed a diploid number of 24 chromosomes and a karyotype formula of 14m+2sm+2st+6a with a fundamental number (FN) of 42 (Fig. 3a)
In addition to the normal chromosome complement, we found that three specimens of Apteronotus prope albifrons had one to three B microchromosomes in their somatic cells, with inter- and intra-individual variations (Fig. 3a, box and Fig. 5a–c)
Summary
The order Gymnotiformes is a group with high species diversity and about 179 species have been listed so far (Crampton 2011). In spite of extensive karyotype variability, a review of the cytogenetic studies of Neotropical freshwater fish shows that, to date, there is no record of B chromosomes (or supernumerary chromosomes) in Gymnotiformes (Carvalho et al 2008, Oliveira et al 2009). These chromosomes are not essential to cell functioning and may be derived from autosomes and sex chromosomes in intra- and interspecies crosses (Camacho et al 2000). Heterochromatic B chromosomes constitute a common situation in many species of Neotropical fishes; they can be completely euchromatic, as in Moenkausia sanctefilomenae (Foresti et al 1989), Steindachnerina insculpta Fernández-Yépez, 1948 (Oliveira and Foresti 1993), Characidium prope zebra (Vênere et al 1999) and Rhamdia quelen (Moraes et al 2009), or partially heterochromatic, as in some populations of “Astyanax scabripinnis complex” (Moreira-Filho et al 2004), Rhamdia hilarii Valenciennes, 1840 (Fenocchio and Bertollo 1990) and Rhamdia quelen (Moraes et al 2009)
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