Karyotype description and comparative chromosomal mapping of rDNA and U2 snDNA sequences in Eigenmannialimbata and E.microstoma (Teleostei, Gymnotiformes, Sternopygidae)

The genus Eigenmannia Jordan et Evermann,1896 includes electric fishes endemic to the Neotropical region with extensive karyotype variability and occurrence of different sex chromosome systems, however, cytogenetic studies within this group are restricted to few species. Here, we describe the karyotypes of Eigenmannialimbata (Schreiner et Miranda Ribeiro, 1903) and E.microstoma (Reinhardt, 1852) and the chromosomal locations of 5S and 18S rDNAs (ribosomal RNA genes) and U2 snDNA (small nuclear RNA gene). Among them, 18S rDNA sites were situated in only one chromosomal pair in both species, and co-localized with 5S rDNA in E.microstoma. On the other hand, 5S rDNA and U2 snRNA sites were observed on several chromosomes, with variation in the number of sites between species under study. These two repetitive DNAs were observed co-localized in one chromosomal pair in E.limbata and in four pairs in E.microstoma. Our study shows a new case of association of these two types of repetitive DNA in the genome of Gymnotiformes.

First chromosomal analysis of Gymnorhamphichthys britskii: the remarkable lowest diploid value within the family Rhamphichthyidae (Gymnotiformes)

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.

Studies in two allopatric populations of Hypostomus affinis (Steindachner, 1877): the role of mapping the ribosomal genes to understand the chromosome evolution of the group

Several cytogenetic markers show chromosomal diversity in the fish such as "armoured catfish". Although studies have characterized many species in the major genera representing these Siluridae, particularly in the genus Hypostomus Lacépède, 1803, trends in chromosome evolution of this group remain unclear. The Paraíba do Sul river basin contains the armoured catfish Hypostomus affinis Steindachner, 1877, which is unique because of its distribution of repetitive DNAs, the 5S and 18S rDNA. Identified samples and registered collections in Brazilian museums were identified as the same typological species, while we observed wide variations in the physical location of this gene in the karyotype based on fluorescent in situ hybridization results. In this study, we propose that these species can represent evolutionarily independent units, as these fish frequently undergo processes such as dispersion and vicariance and that the rDNA is associated with DNA that spreads in the genome, such as transposons. Additionally, the absence of gene flow due to the distance of the sample location could intensify evolutionary processes. The phenotypes found for the 18S rDNA showed minor changes in relation to the number of sites between the lower and upper drainage regions of Paraíba do Sul. The large difference in the number of sites found for the 5S rDNA entered the same region (upper drainage of the basin) and the literature data could represent a population dynamics where an expansion of the 5S rDNA sites provides an extinct or non-sampled cytotype in this work.

Comparative Cytogenetics of the Black Ghost Knifefish (Gymnotiformes: Apteronotidae): Evidence of Chromosomal Fusion and Pericentric Inversions in Karyotypes of Two Apteronotus Species

The karyotype and chromosomal characteristics of Apteronotus albifrons and Apteronotus caudimaculosus collected from populations of two different large Brazilian river basins were analyzed by conventional and molecular cytogenetic techniques, to contribute to the differentiation and identification of the species in this genus. The diploid chromosome number was 2n = 24 for A. albifrons, but with difference in the karyotype structure and fundamental number values between two populations under study. In A. caudimaculosus, the diploid chromosome number was 2n = 26, which was classified as 22 metacentric (m), 2 submetacentric (sm), and 2 acrocentric (a) chromosomes. Heterochromatins were preferentially located in pericentromeric regions for both species. However, there are more C-banded chromosomes in A. caudimaculosus than A. albifrons. The sites of 18S DNA as revealed by fluorescence in situ hybridization (FISH) in the karyotypes of both species corresponded to sites revealed by Ag impregnation, although some additional 18S rDNA sites were observed in the genome of A. caudimaculosus. FISH with 5S rDNA-probe revealed interstitial sites on the m pair No. 1 for individuals of both A. albifrons populations, and in pericentromeric regions on the long arm of pair Nos. 5 and 9 in those of A. caudimaculosus. The karyotypes of A. albifrons and A. caudimaculosus indicated a reduction of 2n resulting from chromosomal fusion, as could be hypothesized from the presence of an interstitial telomere sequence in two chromosome pairs in karyotype of A. caudimaculosus. Thus, the present study demonstrated species-specific cytogenetic markers of otherwise morphologically very similar species A. albifrons and A. caudimaculosus.