Structure and variability of nucleolar organizer regions in Parodontidae fish

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Abstract
Nucleolar organizer regions (NOR) were studied in mitotic chromosomes of 4 species of fish of family Parodontidae: Parodon tortuosus, Apareiodon affinis, A. ibitiensis and A. piracicabae. All 4 spp. exhibited only a single nucleolar chromosome pair in their karyotypes. Intraspecific differences were observed in the size of these chromosomes; these were not very clear for A. affinis and A. piracicabae. A. piracicabae exhibited 2 clearly visible NOR in each of the nucleolar chromosomes, which was the only configuration practically found in this species. This trait therefore predominates in a homozygous condition in the population investigated. Regions of constitutive heterochromatin adjacent to the 2 NOR were detected. Possible mechanisms that may have originated the 2 NOR are discussed.