In the Dongting water system, the Carassius auratus(Crucian carp) complex is characterized by the coexistence of diploid forms(2n=100, 2n CC) and polyploid forms. Chromosomal and karyotypic analyses have suggested that the polyploid C. auratus has a triploid(3n=150, 3n CC) and a tetraploid origin(4n=200), respectively. However, there is a lack of direct genetic evidence to support this conclusion. In this paper, analysis of the 5S r DNA chromosomal locus revealed that the 3n CC is of triploid origin. Analysis of the species-specific chromosomal centromere locus revealed that 3n CC individuals possess three sets of C. auratus-derived chromosomes. Our results provide direct cytogenetic evidence suggesting that individuals with 150 chromosomes are of autotriploid origin within the C. auratus complex. It marks an important contribution to the study of polyploidization and the evolution of vertebrates.
