Possible origins of B chromosomes in Rineloricaria pentamaculata (Loricariidae, Siluriformes) from the Paraná River basin.

We made a cytogenetic study of Rineloricaria pentamaculata from the Tauá Stream, in the Pirapó River sub-basin in Paraná State, Brazil, focused on the occurrence and origins of the B chromosomes. The diploid number varied from 2n = 56 to 2n = 59, due to the presence of 0 to 3 B microchromosomes of the acrocentric type, which were observed in 92.3% of the specimens (N = 12). These chromosomes were totally heterochromatic, with the C banding technique, and there were inter- and intraindividual numerical differences. Meiotic cells in metaphase I had 28 bivalent chromosomes and 0 to 3 univalent chromosomes. We suggest that the B microchromosomes are centric fragments originated from chromosome rearrangements.