Heritability of chronic thermal tolerance and genetic correlations with growth traits in the Pacific oyster (Crassostrea gigas)

The increasing seawater temperature during summer months frequently results in severe mortalities in the Pacific oyster Crassostrea gigas around the world, becoming one of the most significant problems challenging the oyster farming industry. In northern China, significant recurrent summer losses of C. gigas have occurred, and its impact on oyster aquaculture has increased in recent years. Selective breeding for improved oyster resistance to high temperature could help to reduce this massive mortality, but the extent of genetic variation underlying this trait is currently unknown. In this study, we constructed 38 full-sib families using the wild C. gigas and estimated the genetic parameters by performing two month-long high-temperature challenge experiments (30 degree celsius). Experiment 1 was performed in March 2022 followed by experiment 2 in June 2022 (spawning season). In both challenge experiments, there were significant differences in survival among families, suggesting that C. gigas has a different ability to survive under heat stress. Notably, significantly greater mortality was observed for experiment 2, which related to reproductive status and may contribute to additional stress. Thermal tolerance was defined using both binary test survival and time of death traits. Heritability estimates for thermal tolerance were low to moderate (0.16-0.36 for experiment 1 and 0.16-0.33 for experiment 2) using both a Bayesian (MCMCglmm) and a likelihood-based (ASReml-R) approach and estimated heritability of the threshold animal model using ASReml-R (0.16) appeared to be lower compared to MCMCglmm (0.31-0.32). Notably, the genetic and phenotypic correlations for thermal tolerance between two experiments were 0.463 (BS) to 0.491 (TD) and 0.510 (family survival), respectively, which suggested a significant re-ranking of the family breeding values in different time periods. Finally, the genetic and phenotypic correlations were low between growth traits (shell height, shell length, and shell width) and thermal tolerance, suggesting that selection for these traits should be conducted separately. This study reports the first estimation of genetic parameters for chronic thermal tolerance in C. gigas and indicates that this trait is heritable and selective breeding for thermal tolerance is a feasible and promising approach to reduce summer mortality.