Study on karyotype and occurrence of B chromosomes in two endemic species of the genus Pimelodus (Siluriformes, Pimelodidae) from the river Iguaçu.

The current work describes, for the first time, the occurrence of B chromosomes in the genus Pimelodus. Cytogenetic analysis in P. ortmanni and Pimelodus sp.

A natural triploid in Trichomycterus davisi (Siluriformes, Trichomycteridae): mitotic and meiotic characterization by chromosome banding and synaptonemal complex analyses.

Within a total of 50 analyzed specimens a male individual of Trichomycterus davisi has been recorded with 81 chromosomes including 60 metacentric, 18 submetacentric and three subtelocentric chromosomes. When compared with diploid individuals (2n = 54) and the morphological standard of chromosomes, this male is a triploid with 3n = 81 chromosomes. Since staining with silver nitrate indicates three active nucleolar organizer regions (NORs), the three NOR-bearing chromosomes in this individual are genetically active.