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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

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