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

BACKGROUND: Crucian carp (abbreviated CC) belongs to the genus of Carassius within the family of Cyprinidae. It has been one of the most important freshwater species for Chinese aquaculture and is especially abundant in the Dongting water system of Hunan province. CC used to be considered as all diploid forms. However, coexistence of diploid (abbreviated 2nCC), triploid (abbreviated 3nCC) and tetraploid crucian carp (abbreviated 4nCC) population of the Dongting water system was first found by our recently researches. RESULTS: 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. CONCLUSIONS: 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.

[Cytogenetic and molecular genetic analysis of gynogenesis in Megalobrama amblycephala using spermatozoa of Erythroculter ilishaeformis].

In the present study we used both cytogenetics (measurement of DNA content, detection of chromosome number, observation of gonadal development)and molecular genetics(microsatellite analysis)to analyze the biological characteristics of gynogenetic M. amblycephala, which werecreatedthrough gynogenesis induced via UV-irradiated E. ilishaeformis spermatozoa to fertilize M. amblycephala eggs. The maternal genome was duplicated by cold shock in 0~4℃ cold water to form a population of M. amblycephala with 48 chromosomes whose DNA content was identical to the diploid maternal parent. Morphologically, this group of gynogenetic M. amblycephala was similar to the control group. All gynogenetic M. amblycephala were female, and no males were found in any of the examined gynogenetic M. amblycephala, providing cytogenetic evidence that our gynogenetic M. amblycephala are type XY. At the same time, microsatellite analysis showed that 63 alleles were amplified in the three test groups of gynogenetic M. amblycephala. Overall, the population of gynogenetic M. amblycephala observed heterozygosity average, and the expected average was significantly lower than the parental averages, demonstrating that after generation gynogenesis the gene homozygosity of M. amblycephala was significantly higher than the ordinary bream and E. ilishaeformis, making it a pure line. The genetic proximity of gynogenetic M. amblycephala to M. amblycephala demonstrates that gynogenesis passes on maternal DNA. Gynogenetic groups developed in this study may provide good genetic material for future breeding projects of M. amblycephala.