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The Pacific oyster Crassostrea gigas is the most commonly farmed mollusks worldwide, while its production has been hindered by massive mortalities linked to high temperatures. Selective breeding focusing on thermal tolerance is an attractive option for reducing the impact of massive mortalities, but the genetic basis underlying this trait is currently unknown. Hybridization between C. gigas and C. angulata was conducted and followed by one generation of family selection for thermal tolerance. The genetic parameters for thermal resistance, summer survival and growth and selection response were investigated by using 50 full-sibling families. The mean survival of all families ranged from 30.2 to 69.5%, reflecting the large variation of thermal tolerance in the hybrid population. The estimates of heritability for thermal tolerance were low to moderate, ranging from 0.19 ± 0.03 to 0.27 ± 0.05, confirmed that there is genetic basis for thermal tolerance. The phenotypic (r = 0.537, P < 0.01) and genetic correlations (r = 0.546, P < 0.01) between thermal tolerance and summer survival were positive and significant, while the phenotypic and genetic correlations between growth and survival (including thermal tolerance and summer survival) were positive but low (P > 0.05). High response to selection (ΔG = 36.33%) was observed after one generation of selection for thermal tolerance, and there was also corresponding response (ΔG = 14.46%) for summer survival. These results demonstrated that genetic selection to improve summer survival of oyster may be facilitated by selecting highly heat-tolerant lines.

Oysters are one of the most commercially important shellfish species and have been cultured for thousands of years. Oyster aquaculture supports the major aquaculture industries in many countries. Over the last few decades, the oyster breeding and aquaculture industries have developed rapidly to meet the continually growing demand. Many researchers have made significant efforts toward the genetic improvement of commercially important traits in oysters. Some strains with fast‐growing, disease‐resistant, and stable shell‐colours have been developed through selective breeding. Some hybrid varieties have been developed by crossing different geographical populations or cultivated strains. Several hybrids exhibit considerable genetic variation and improved productive performance. Additionally, polyploid induction technologies have been applied in the oyster aquaculture industry, which provides a useful tool for performance improvement and genetic containment of cultured stocks. At present, the development of molecular breeding also provides a great opportunity for oyster genetic improvement. These advances in oyster breeding have improved the quality of oysters, brought great economic benefits, and been conducive to the sustainability of oyster production. Nonetheless, there are still some limitations and obstacles in oyster breeding, such as infectious diseases, summer mortality, conservation of germplasm resources, environmental contamination, and climate change. The present review provides an overview of the current status, challenges, and prospects in oyster breeding. [ABSTRACT FROM AUTHOR]

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]