Chromosome spreading of the retrotransposable Rex-1 and Rex-3 elements and 5S rDNA clusters in the karyotype of Eigenmannia catira (Gymnotiformes: Sternopygidae)

Abstract

Eigenmannia exhibits marked variability in its chromosome structure, including the presence or absence of sex chromosomes. In this study, we present the karyotype and chromosomal localization of four repetitive DNA classes in E. catira from the upper Paraná River basin. The observed diploid number for this population was 2n = 36, with a karyotype comprising 2 metacentric + 10 submetacentric + 8 subtelocentric + 16 acrocentric chromosomes, and a fundamental number (FN) of 56 for both sexes. Using silver nitrate staining and Fluorescence in situ Hybridization (FISH) with the 18S rDNA probe, we detected a single pair of nucleolar organizing regions (NORs) on the subtelocentric chromosome pair 10. Multiple 5S rDNA sites were identified by FISH, located on as many as 21 chromosomes. Additionally, we observed heterochromatic regions in the pericentromeric region of most chromosomes, and the presence of Rex-1 and Rex-3 retroelements in small clusters dispersed across the chromosomes. Our NOR data confirmed the characteristic tendency of this genus to display simple NORs and a significant amount of heterochromatin associated with transposable elements, which may account for the dispersion of 5S rDNA within the genome of this species. These findings provide valuable insights into the karyotypic evolution of Eigenmannia.