Study On Breeding And Growth Of Triploid Pacific Oysters (Crassostrea Gigas)

The results testified that Chinese population (diploid♀)×American population (tetraploid♂) was possessed of advantage of triploidy and heterosis by growth comparison of induced and mated triploid Pacific oysters (Crassostrea gigas) and heterosis and advantage of triploidy studied on hybridization of the different geographic population of Pacific oysters. It was evident that mated triploids were superior over triploids induced by chemical or physical treatment. But the obtainment of tetraploid was different. Hypotonic treatment as a new method of polyploidy induction was developed. The superiority of hypotonic treatment was low price and high induced ratio of triploids. Triploid Pacific oysters (Crassostrea gigas) were induced by inhibiting polar bodyⅡ(PB2) releasing with hypotonic treatment. The suitable induction parameters were studied. Then four different triploid induced methods were compared including hypotonic treatment, cold shock, heat shock and 6-DMAP. The research is necessary to rich the breeding theory of the polyploidy and accelerate the development of the aquiculture.The main contents consist of the following three parts.1. Growth Comparison of Induced and Mated Triploid Pacific Oysters (Crassostrea gigas)Performance of induced and mated triploid Pacific oysters (Crassostrea gigas) was studied and compared for traits such as fertilization and hatching success, survival, larval and juvenile growth. The results showed that mated triploids were superior over induced triploids, and the difference in hatching success and D-larval size was significant (P<0.05). Larval survival of the mated triploids was significantly higher than that of induced triploids. Juvenile shell height and shell length of mated triploids were larger than that of induced triploids, showing their superiority in growth. Flow cytometry analysis showed that the mated group contained 100% triploids, and the induced group had only (51.0±11.8)% triploids. It is evident that mated triploids are superior over triploids induced by chemical treatment.2. Heterosis and triploid advantage between Chinese and American populations of Pacific oysters (Crassostrea gigas)Hybrid crosses of Pacific oysters between diploids from China and diploids as well as tetraploids from America were conducted. Four groups were produced: Chinese diploid♀×Chinese diploid♂, American diploid♀×American diploid♂, Chinese diploid♀×American diploid♂and Chinese diploid♀×American tetraploid♂. Heterosis and triploid advantage of experimental groups were studied for traits such as fertilization and hatching success, survival, larval and juvenile growth. Our results show that heterosis in diploid hybrid progeny varied among traits and developmental stages: 1.21% for larval growth, 34.47% for larval survival and 20.39% for juvenile growth. The triploid advantage was significant for all traits and stages, except D larval size (5.19%) and larval growth (4.00%). Triploid advantages for larval survival, larval growth and juvenile survival were 19.92%, 30.18% and 54.43%, respectively. The highest triploid advantage (202.96%) was observed in wet weight of juveniles at Day 200. Heterosis and the triploid advantage were stronger at juvenile stages than at larvae stages. Our results suggest that the hybrid triploid Pacific oysters between Chinese diploid females and American tetraploid males have greatly improved growth and survival compared to all diploid groups. Production of triploids through interstrain crossing may be the best way for producing triploids for aquaculture. However, at Day 200 when the triploid advantage was 202.96% in growth and 73.6% in survival, the diploid heterosis was only 3.75% and -26.25%, respectively. This finding suggests that the superior performance of the triploids may not be caused by the same heterosis observed in diploids. It is possible that heterosis in triploids are different from heterosis in diploids. We did not have pure-line triploids as controls in this study. Further studies are needed to determine to what degree that heterosis contributes to the superior performance of hybrid triploids.In conclusion, we show again that mating diploids and tetraploids is an ideal approach to triploid production. These triploids are 100% pure, and free of any artificial treatments and toxic substances. Our results also demonstrate that hybrid triploids between diploids from China and tetraploids from US offer vastly superior in growth and survival over diploids, although the possible effects of heterosis require further studies.3. Triploid induction in Pacific oyster (Crassostrea gigas) by hypotonic treatment and comparison with other treatment methodsTriploid Pacific oysters (Crassostrea gigas) were induced by inhibiting polar bodyⅡ(PB2) releasing with hypotonic treatment. Different hypotonic treatments, including salinity (4, 6, 8, 10, 12, 14 and 16‰), initial treatment time (at the time when the first PB1, 30 percents of PB1, 40 percents of PB1, 50 percents of PB1 and the fist PB2 were observed) and duration time (10, 15, 20 and 25min), were tested at 23-25℃water temperature. Flow cytometry analysis showed that the highest triploidy rate was (89.16±1.39)﹪, when the fertilized eggs were treated 15 min with hypotonic solution of 8‰at the time when 40 percents of PB1 were appeared. The results also showed that shell height of triploids were larger than that of diploids. And triptoid larvae appeared much advantageous in growth. Then four different triploid induced methods were compared including hypotonic treatment, cold shock (2℃), heat shock (32℃) and 6-DMAP (450μmol/L).The triploids induced by hypotonic treatment were superior over those by cold shock, heat shock and 6-DMAP, and the difference in hatching success and D-larval size was significant (P<0.05). The induced ratio of the triploids induced by hypotonic treatment was lower than that by 6-DMAP (the difference was not significant (P>0.05)), but higher than that by cold and heat shock (the difference was significant (P<0.05)). Comparison of general evaluation index (Ie), hypotonic treatment is superior over other three treatments of triploid induction. Because of the low price, the high induced ratio of triploids, and innocuity, it is feasible and worth while to induced triploid by hypotonic treatment.