Induction to tetraploidy in Pacific oysters (Crassostrea gigas)

As an alternative to the use of cytochalasin B (CB), 6-dimethylamino-purine (6-DMAP) and thermal shock (heat shock by increasing the temperature from 25 to 36ºC) could be used to induce tetraploidy in Pacific oyster (Crassostrea gigas) diploids. Induction was performed by applying shocks after elimination of the first polar corpuscle at the end of meiosis I. Ploidy rates were verified using flow cytometry. Tetraploid larvae were obtained using all inductor (6-DMAP, thermal shock and CB) treatments. No difference in the efficiency of tetraploidy induction was noted among 6-DMAP, thermal shock and CB. The number of D-larvae and their yield, determined by calculating the percentage of well-formed D-larvae in relation to the total number of larvae, was similar (p > 0.05) among the evaluated induction methods. We suggest that 6-DMAP and thermal shock should be used in tetraploidy induction protocols, thereby avoiding the use of CB, which is a harmful agent for both humans and the environment.

Tetraploid induction of Crassostrea hongkongensis and C. sikamea by inhibiting the polar body 1 release in diploid fertilized eggs

The production of an all-triploid population by mating tetraploid males with diploid females is the best and most fundamental method for the large-scale production of triploid oysters. Obtaining a stable tetraploid population is essential for guaranteed production in industrialized triploid cultivation. C. hongkongensis and C. sikamea are important oyster breeding species in southern China, and have great economic value. However, there are not any published data on inducing tetraploid C. hongkongensis or C. sikamea. Therefore, we investigated tetraploid induction in these two oyster species by inhibiting the PB1 release in diploid fertilized eggs using Cytochalasin B (CB) under 31 °C, 15 ‰ salinity. The results confirmed that the optimal tetraploid induction conditions for C. hongkongensis were a CB concentration of 0.50 mg/L with induction starting at 9.0 min after fertilization, and stopping at 21.0 min after fertilization; the induction efficiency index reached 0.123 under these conditions. The optimal tetraploid induction conditions for C. sikamea were a CB concentration of 0.50 mg/L, with induction starting at 7.5 min after fertilization and stopping at 18 min after fertilization; the induction efficiency index could be as high as 0.281 under these conditions. However, we confirmed that the tetraploid rate decreased with larval growth, and no tetraploids were detected in the juvenile period of either C. hongkongensis or C. sikamea. This may be attributed to the very low survival of the tetraploid larvae induced by this method, especially as most tetraploid larvae died during the first three days. In summary, it is simple to directly induce tetraploid C. hongkongensis and C. sikamea larvae by inhibiting the PB1 release of diploid zygotes, but the low survival rate makes it challenging to obtain viable juvenile tetraploids.

Toward reference intervals for shellfish: An illustrative case of feeding and respiratory activities in the Pacific cupped oyster, Crassostrea gigas

BACKGROUND

The Quality Assurance and Laboratory Standards Committee of the American Society for Veterinary Clinical Pathology and the guidelines of the Clinical and Laboratory Standards Institute provide a framework for establishing reference intervals of physiological parameters in reputedly healthy individuals, humans, and terrestrial animals, respectively. This framework was applied for the first time to the Pacific cupped oyster, Crassostrea gigas. Reference intervals (RIs) would, first, be of interest for research purposes, including pathophysiology studies. RI determination is the first step before considering the use of RIs for field applications by farmers and marine shellfish health services.

OBJECTIVES

The purpose of this study was to propose reference intervals of feeding and respiration parameters, the clearance rate (CR), and oxygen consumption rate (OCR), in a reference population of hatchery-reared diploid Pacific oysters.

METHODS

A de novo, a priori, and a direct approach were applied. The reference values acquired from 214 healthy diploid C gigas (total wet weight 6.23-83.64 g, DW 0.06-1.87 g) were analyzed using a non-parametric statistical method.

RESULTS

Reference intervals were proposed for CR, 0.7-4.1 L/h/g dry flesh weight (DW), and OCR, 0.4-1.3 mg O2/h/g DW in C gigas in a seawater at a temperature of 22℃ and a salinity of 32‰. Animals were fed 30-40 cells/µL of Isochrysis affinis galbana. The confidence intervals at 90% of the upper limits of the two parameters were found to be higher than those of the Clinical and Laboratory Standards Institute (CLSI) recommendations.

CONCLUSIONS

Obtaining reference intervals is an important step and must be completed by proposed decision limits to facilitate the early detection of health disorders in C gigas.

Aneuploid progeny of the American oyster, Crassostrea virginica, produced by tetraploid × diploid crosses: another example of chromosome instability in polyploid oysters

The commercial production of triploids, and the creation of tetraploid broodstock to support it, has become an important technique in aquaculture of the eastern oyster, Crassostrea virginica. Tetraploids are produced by cytogenetic manipulation of embryos and have been shown to undergo chromosome loss (to become a mosaic) with unknown consequences for breeding. Our objective was to determine the extent of aneuploidy in triploid progeny produced from both mosaic and non-mosaic tetraploids. Six families of triploids were produced using a single diploid female and crossed with three mosaic and non-mosaic tetraploid male oysters. A second set of crosses was performed with the reciprocals. Chromosome counts of the resultant embryos were tallied at 2-4 cell stage and as 6-hour(h)-old embryos. A significant level of aneuploidy was observed in 6-h-old embryos. For crosses using tetraploid males, aneuploidy ranged from 53% to 77% of observed metaphases, compared to 36% in the diploid control. For crosses using tetraploid females, 51%-71% of metaphases were aneuploidy versus 53% in the diploid control. We conclude that somatic chromosome loss may be a regular feature of early development in triploids, and perhaps polyploid oysters in general. Other aspects of chromosome loss in polyploid oysters are also discussed.