Comparative cytogenetics among sympatric species of Characidium (Pisces, Characiformes). Diversity analysis with the description of a ZW sex chromosome system and natural triploidy

A comparative cytogenetic analysis was carried out in two sympatric fish sp ecies of the genus Characidium, C. gomesi and C. cf. zebra, from the Parana basin, Sao Paulo State, Brazil. Although both species showed the same dipl oid number (2n = 50) and a similar karyotypic macrostructure, an interspeci fic diversity was detected concerning the distribution of the C-bands and t he nucleolar organizing regions in the karyotypes, as well as the sex chrom osomes. A ZZ/ZW sex chromosome system was detected in C gomesi, where the W chromosome appears fully heterochromatic and with a small size than Z chro mosome. This species also presented a triploid female specimen bearing one W and two Z chromosomes. In C. cf.zebra heteromorphic sex chromosomes were not found. These data are discussed concerning the characterization of the regional ictiofauna and its evolutionary aspects.