Coexistence of diploid, triploid and tetraploid crucian carp (Carassius auratus) in natural waters

We examined the ploidy level and compared biological characteristics in different ploidy CC. In reproductive mode, 2nCC was bisexual generative and 4nCC generated all-female offspring by gynogenesis. However, 3nCC generated progenies in two different ways. 3nCC produced bisexual triploid offspring fertilized with 3nCC spermatozoa, while it produced all-female triploid offspring by gynogenesis when its ova were activated by heterogenous spermatozoa. The complete mitochondrial DNA of three different ploidy fishes was sequenced and analyzed, suggesting no significant differences. Interestingly, microchromosomes were found only in 3nCC, which were concluded to be the result of hybridization. Allogenetic DNA fragments of Sox genes were obtained in 3nCC and 4nCC, which were absent in 2nCC. Phylogenetics analysis based on Sox4 gene indicated 3nCC and 4nCC formed a separate group from 2nCC.In summary, this is the first report of the co-existence of three types of different ploidy crucian carps in natural waters in China. It was proved that the coexistence of different ploidy CC was reproductively maintained. We further hypothesized that 3nCC and 4nCC were allopolyploids that resulted from hybridization. The different ploidy CC population we obtained in this study possesses great significance for the study of polyploidization and the evolution of vertebrates.Taxonomically, crucian carp (Carassius auratus L., abbreviated CC) belongs to the genus of Carassius within the family of Cyprinidae. It is widely bred across Eurasia and America, because of its good survival rate, high reproduction rate and good disease resistance. In China, the species has been found in most provinces, except for the western plateau. It is one of the most important freshwater species for Chinese aquaculture especially in the Dongting water system of Hunan province. Although Carassius auratus gibelio (superspecies auratus) has been reported co-existence of three types of different ploidy [1-4], CC (Cara