Polymorphism of 5S and 18S ribosomal DNA in a conserved karyotype of the two Hypostomus strigaticeps (Siluriformes: Loricariidae) populations from the upper Paraná River basin

Abstract Hypostomus is a remarkable group within Siluriformes due to its high species richness, pronounced morphological and chromosomal variability, which complicates the process of species identification. In this context, the aim of this study was to identify possible interpopulational chromosomal markers that could significantly contribute to our understanding of the differentiation in H. strigaticeps of distinct tributaries. Two H. strigaticeps populations from the upper Paraná River basin were analyzed using cytogenetic methods.Our results revealed interpopulation variations in H. strigaticeps, despite limited sampling of the Ivaí River population,particularly in the number of 18S and 5S rDNA sites. Moreover, the individuals from both populations exhibited the same chromosome count (2n = 72) and fundamental number (FN = 108), as well as identical chromosome morphology and similarities in the distribution of constitutive heterochromatin. The transposable elements (TEs) Rex1 and Rex3 were found scattered along the chromosomes, with a higher concentration of Rex3 elements in pairs carrying rDNA. Our findings provide novel insights into a possible involvement of TEs in the variability of rDNA distribution patterns, contributing to karyotypic diversification in H. strigaticeps.