LAUSR

Abstract The family Parodontidae is a small group of fish that has distinct morphological differences. Despite its conserved diploid number, structural cytogenetic polymorphisms involving sex chromosomes are commonly described in this group. Conversely, reports on numerical polymorphisms owing to the occurrence of supernumerary chromosomes are rare. To better understand the karyotype evolution of this group, we analyzed specimens of Parodon nasus collected in the Três Bocas/PR stream, applying different cytogenetic techniques. All individuals presented 54 meta-submetacentric chromosomes, except for three females, which exhibited structural polymorphism. The NOR occurred on a pair of submetacentric chromosomes. Heterochromatin was distributed in some pericentromeric areas, predominating in the terminal regions. We detected B microchromosomes in four of the collected individuals, which were heterochromatic and bearers of 18S rDNA sites. This work deals with the first description of these chromosomes in the family Parodontidae. The results indicate that, despite the conserved diploid number, this group of fish presents a more divergent karyotypic evolution, involving chromosome rearrangements in its karyotypic structure.