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Abstract
ABSTRACT Gymnorhamphichthys britskii is a Neotropical electric fish of family Rhamphichthyidae described from the Paraná-Paraguay system. This study reports the first karyotypic description of G. britskii collected from the upper Paraná river basin, which presented 2n=38 chromosomes, karyotype composed of 14 metacentric, 8 submetacentric, 2 subtelocentric and 14 acrocentric chromosomes, and fundamental number as 62 for both sexes. Heteromorphic sex chromosomes were absent. A single pair of nucleolar organizing regions (NORs) was detected in the submetacentric chromosome pair number 9 by silver staining and confirmed by the 18S rDNA probe. The 5S rDNA was located in a single chromosome pair. Heterochromatic regions were clearly observed in the short arms of the NOR-bearing chromosome pair and in the telomeric positions of most acrocentric chromosomes. Besides the present data are valuable to help in understanding karyotypic evolution in Rhamphichthyidae, data from NORs confirmed the tendency of this family in presenting simple NORs sites, similar to the other Gymnotiformes clades. Yet, the presence of a large heterochromatic block in the NOR-bearing chromosome can be used as cytogenetic markers for G. britskii, and that centric fusions appear to be an important mechanism in the karyotype evolution and differentiation among Gymnotiformes species.
Highlights
The Gymnorhamphichthys Ellis, 1912 is a representant of the clade Rhamphichthyidae, a group of Neotropical electric fishes, widely distributed through the cis-Andean drainages of tropical South America, from Venezuela to northeast Argentina (Nijssen et al, 1976)
The genus is composed of five valid species: G. rondoni (Miranda Ribeiro, 1920), G. rosamariae Schwassmann, 1989, G. bogardusae Lundberg, 2005, G. hypostomus Ellis, 1912, and G. britskii Carvalho, Ramos & Albert, 2011 (Fricke et al, 2019)
All the twenty-two individuals of G. britskii had 2n = 38 chromosomes and karyotype composed of 14 m, 8 sm, 2 st and 14 a, and fundamental number (FN) = 62 for both sexes (Fig 3a)
Summary
The Gymnorhamphichthys Ellis, 1912 is a representant of the clade Rhamphichthyidae, a group of Neotropical electric fishes, widely distributed through the cis-Andean drainages of tropical South America, from Venezuela to northeast Argentina (Nijssen et al, 1976). The genus is composed of five valid species: G. rondoni (Miranda Ribeiro, 1920), G. rosamariae Schwassmann, 1989, G. bogardusae Lundberg, 2005, G. hypostomus Ellis, 1912, and G. britskii Carvalho, Ramos & Albert, 2011 (Fricke et al, 2019). Gymnorhamphichthys britskii is distributed from tributaries of the La Plata River system, and share with. G. hypostomus a distinct color pattern of the dorsum composed of a few large, dark saddles, which is not present in any other species of Rhamphichthyidae (Carvalho et al, 2011). There were no records of G. britskii from the upper Paraná River before the construction of the Itaipu Dam. The distribution of G. britskii
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