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Large-scale production of triploid oysters requires tetraploids as broodstock; hence, tetraploidy is a key asset for the triploid oyster industry. To explore the possibility of obtaining new allotetraploid strains with improved traits through hybridization and polyploidization, we conducted a comparative study for growth, survival, and gonadal development of tetraploid Crassostrea gigas, tetraploid C. angulata, and four allotetraploids. Firstly, allotriploid GGA (C. gigas ♀ × C. angulata ♂) and AAG (C. angulata ♀ × C. gigas ♂) were obtained from the hybridization between diploid C. gigas and diploid C. angulata by inhibiting the polar body II. Subsequently, allotetraploid GGGA (triploid C. gigas ♀ × diploid C. angulata ♂), allotetraploid AAAG (triploid C. angulata ♀ × diploid C. gigas ♂), allotetraploid GGAG (allotriploid GGA ♀ × diploid C. gigas ♂), allotetraploid AAGG (allotriploid AAG ♀ × diploid C. gigas ♂), autotetraploid GGGG (triploid C. gigas ♀ × diploid C. gigas ♂), and autotetraploid AAAA (triploid C. angulata ♀ × diploid C. angulata ♂) were obtained by inhibiting polar body I. The growth, survival, and gonad development of six groups (GGGA, AAAG, GGAG, AAGG, GGGG, and AAAA) were assessed. There were no detectable differences in fertilization and hatching rates among the six groups. Flow cytometry analysis showed that the tetraploid ratios of the four allotetraploid groups exceeded 88% on day 20. At larval stage, the allotetraploids GGGA and GGAG groups had higher growth rates and cumulative survival rates than the autotetraploid groups. Field-based phenotyping at three sites indicated that certain allotetraploid groups exhibited higher growth and survival compared to the autotetraploid groups. In particular, the best group over 9 months of testing was the allotetraploid GGAG, which significantly outperformed the autotetraploid groups in growth and survival. Furthermore, most of the individuals from the four allotetraploids were fully mature, with morphologically normal oocytes or sperm. The establishment of these four allotetraploids offers new germplasm resources for oyster genetic improvement and commercial production. [ABSTRACT FROM AUTHOR]

Tetraploid oysters are of rising aquacultural importance because they can be used to produce 100% triploid seeds. Nevertheless, notable gaps in the understanding of the environmental preferences of tetraploid oyster larvae still exist. To investigate the combined effects of temperature and salinity on the growth, survival and metamorphosis of larvae of tetraploid Crassostrea gigas (GGGG), tetraploid C. angulata (AAAA) and two allotetraploid oysters (GGAG and AAGG), a total of 192 independent experiments were established, including 4 temperatures (21, 24, 27 and 30 ℃) × 4 salinities (22, 25, 28 and 31) × 4 populations (GGGG, AAAA, GGAG and AAGG) treatment combinations. Results showed that both temperature and salinity significantly affected growth, survival and metamorphosis of tetraploid oyster larvae and there was a significant interaction between salinity and temperature on survival and metamorphosis. The highest specific growth rate (SGR) of AAAA (7.61%), GGAG (8.47%) and AAGG (8.33%) occurred at a salinity-temperature combination of 25/27 °C, and that of GGGG (8.07%) occurred at a salinity-temperature combination of 25/24 °C. The highest accumulative survival rate (ASR) of AAAA (22.72%), GGAG (33.88%) and AAGG (31.10%) occurred at a salinity-temperature combination of 25/24 °C, and that of GGGG (27.51%) occurred at a salinity-temperature combination of 25/21 °C. The largest metamorphosis rate (MR) of AAAA (25.50%), GGAG (33.27%) and AAGG (32.73%) were found at a salinity-temperature combination of 28/27 °C, and that of GGGG (27.60%) were found at a salinity-temperature combination of 28/24 °C. In general, the larval performance of allotetraploid oysters was superior to that of autotetraploid oysters, with an allotetraploid superiority ranging from 3.57 to 9.39% in SGR, 32.82 to 103.26% in ASR and − 12.08 to 28.79% in MR. The optimum larval rearing salinity-temperature condition recommended for GGGG, GGAG and AAGG is 25–28 at 21–24 °C, while for AAAA is 25–28 at 24–27 °C; a temperature range of 24–27 °C and a salinity range of 28–31 are suggested to improve spat production of these four tetraploid oysters. [ABSTRACT FROM AUTHOR]

The sterility advantages of triploids make them paramount for oyster aquaculture. Tetraploids are always used as male parents for triploid industrial breeding in current hatcheries, thus they are the core asset for triploid oyster culture. The oyster Crassostrea angulata is the most productive oyster species in China. However, the optimal conditions for tetraploid induction have never been examined. In this study, we investigated the effectiveness of three induction methods (Method I, 2n ♀ × 2n ♂ + inducer abbreviated as DD + ind; Method II, 3n ♀ × 2n ♂ + inducer abbreviated as TrD + ind; Method III, 2n ♀ × 4n ♂ + inducer abbreviated as DT + ind) and three inducers (salinity; cytochalasin B, CB; and 6-dimethylaminopurine, 6-DMAP) for tetraploid induction in C. angulata. The optimal induction conditions for the three induction methods were a CB concentration of 0.5 mg/L treated for 20 min, 0.75 mg/L CB for a constant induction treatment of 20 min and 6-DMAP at a concentration of 100 mg/L for 20 min, respectively. When the optimal induction conditions were applied to a single pair of oysters, the D-larval tetraploid rate for the three methods were 18.93 ± 1.46%, 58.32 ± 1.03% and 51.61 ± 1.37%, respectively. After 30 days, the survival rates of the experimental groups were only 10.36 ± 2.51%, 28.07 ± 2.03% and 21.99 ± 1.27%, respectively, at day 30. The tetraploid rates in DD + ind and DT + ind were both decreased significantly over time, but they still reached 11.81% and 21.99% at 30 days, and 8% and 16% at day 180, respectively. Tetraploid rates in TrD + ind were 58.32 − 71.28% at the larval stage and 80% at day 180. In summary, despite very different induction rates, surviving tetraploids of C. angulata could be obtained in all three methods. Method II (TrD + inducer) was regarded as the optimal method to induce C. angulata tetraploids, and the optimal induction condition was a CB concentration of 0.75 mg/L and treated for 20 min under this method. [ABSTRACT FROM AUTHOR]