Abstract
Conventional (Giemsa, C-banding, Ag – NORs) and molecular [5S rDNA, 18S rDNA, (TTAGGG)n] cytogenetic techniques were employed to study six species of the genus Eigenmannia Jordan & Evermann, 1896. They exhibited diploid chromosome numbers ranging from 2n=28 (Eigenmannia sp.1) to 2n=38 (Eigenmannia virescens (Valenciennes, 1836)). The C-banding results revealed that species with the lowest 2n have less heterochromatin content and that morphologically differentiated sex chromosomes observed in two species showed distinct patterns of heterochromatin. While the X1, X2 and Y-chromosomes of Eigenmannia sp.2 showed only centromeric heterochromatin, the XY sex chromosomes of Eigenmannia virescens possessed large heterochromatic blocks in the terminal position, particularly on the X chromosome. The nucleolus organizer regions (NORs) were located in different positions when compared to the 5S rDNA sites. Additionally, the presence of minor ribosomal gene sites on the sex chromosome pair of Eigenmannia virescens represented a new type of the sex chromosomes in this group. The telomeric probe (TTAGGG)n hybridized to the terminal portion of all chromosomes in all species examined however, interstitial telomeric sites were found in the metacentric pair No. 2 in Eigenmannia sp.1. The analyzes confirmed some hypotheses about karyotype evolution in the genus Eigenmannia, and brought new information about the distribution of the genetic material in the chromosomes of the samples analyzed providing new insights for understanding the process differentiation in the genomes of species under study.
Highlights
Fishes of the Gymnotiformes order, known as “electric knifefishes”, constitute an endemic group in Neotropical freshwaters (Albert and Crampton 2003)
Diploid chromosome numbers ranged from 2n=28 chromosomes in Eigenmannia sp.1 to 2n=38 in Eigenmannia virescens (Table 1)
The constitutive heterochromatin was preferentially located in the pericentromeric regions of all chromosomes of the analyzed species
Summary
Fishes of the Gymnotiformes order, known as “electric knifefishes”, constitute an endemic group in Neotropical freshwaters (Albert and Crampton 2003). This group comprises more than 100 species classified into five families, namely Gymnotidae, Rhamphichthyidae, Hypopomidae, Sternopygidae, and Apteronotidae (Reis et al 2003). The genus Eigenmannia Jordan & Evermann, 1896, family Sternopygidae, is represented by eight widely distributed species (Albert 2001). The actual taxonomic diversity of this genus is still unclear, mainly because presently recognized species very likely include other undescribed species, i.e. they represent catch-all taxa. Studies on the distribution of repetitive sequences are scarce and still restricted to chromosomes of a single species – Eigenmannia virescens (Valenciennes, 1836) (Silva et al 2009)
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