Unusual occurrence of a ZZ/ZW sex-chromosome system and supernumerary chromosomes in Characidium cf. fasciatum (Pisces, Characiformes, Characidiinae)

Individuals of two populations of the fish Characidium cf. fasciatum were c ytogenetically studied and showed a basic diploid number of 50 chromosomes. Some fishes were found to have 51 to 54 chromosomes due to the presence of one to four small subtelocentric/acrocentric supernumerary chromosomes. Wh en analyzed by conventional Giemsa staining, male and female specimens of C . cf. fasciatum from the Quinta stream and Pardo River presented the same b asic karyotypic macro- and microstructure, consisting of 32 metacentric and 18 submetacentric chromosomes. Ag-NORs were terminally located on the long arms of two submetacentric chro mosome pairs. Constitutive heterochromatin was identified by C-banding as s mall pericentromeric blocks in the majority of the chromosomes, and B-chrom osomes were found to be heterochromatic. The occurrence of one totally hete rochromatic submetacentric chromosome restricted to females and considered as an unusual feature in fish karyotypes led to the identification of a ZZ/ ZW sex-chromosome system. The implications of chromosomic differentiation o bserved in the genus Characidium are discussed.