Pimelodidae has a high number of species, but cytogenetic studies are generally restricted to classical chromosomal characterization and in situ localization of ribosomal DNA (rDNA) genes. This study was developed to compare Pimelodus microstoma and Pimelodus pohli focusing on chromosomal diversification provided by the transposition of DNA sequences containing multigene families. Both species share 56 chromosomes, with centromeric and terminal heterochromatic blocks. The silver nucleolus organizer regions (Ag-NORs)/45S rDNA was located in the chromosome pair 24 for both species. The 5S rDNA sites were evidenced in the pair 8 of P. microstoma, and in the pairs 1, 17, and 18 in P. pohli. The U1 small nuclear RNA (snRNA) was located at terminal site in the first subtelocentric pair in both species. The U2 snRNA site was syntenic to 5S rDNA in non-homeologue chromosomes between analyzed species. The histones H3 and H4 were clustered in chromosome pairs 19 and 23 in P. microstoma, and 21 and 22 in P. pohli. Our study proposes that the movement of DNA sequences carrying multigene families has been driven on the chromosomal diversification of Pimelodidae. These multigene location in the genomes can explain most of the visualized chromosomal rearrangements in Pimelodidae and it is useful to understand the chromosomal changes and their distinctive karyotype formulae.
Keywords: cytogenetics; in situ localization; karyotype evolution; repetitive DNAs.