The Impact Of Selection Of Heat Stress On Economic Characters And Its Mechanism For Pacific Oyster Crassostrea Gigas

Oyster is one of the most important worldwide economic species.Mass mortality was happened during summer and has become a serious threat to the global oyster aquaculture industry.Elevating temperature is considered as one of the most important abiotic factors causing the summer mortality of oysters.Therefore,it is necessary to explore the mechanism for thermal adaptation and thermotolerance of oysters.We initiated an artificial selective breeding program aimed at increasing survival rate of C.gigas during the summer since 2017.we collected phenomics and genomics evidence to identify discrepant responses to temperature elevation between thermal tolerance oysters after one-generation of selection,and to elucidate the potential molecular mechanisms for heat tolerance in oysters.1.Differences in thermotolerance and physiological responses between selected F1 population and natural population.Parental selection is based on acute thermal tolerance under controlled heat stress（42℃,1h）to ensure selection intensity（37%）.We compared the difference in ability of thermal adaptation between the selected F₁ and natural population.Specifically,the growth,respiration rate,heat tolerance and physiological indicators was measured between 6-month-old selected F₁ and natural population.The summer mortality and growth was evaluated to verify practical significance for aquaculture industry of artificial selection breeding in thermotolerance when the oysters was 19-month-old.The results showed that there was no significant difference in growth after one-generation selection between selected and control population.The thermal tolerance of the progeny post exposure to acute heat stress in the selected population was significantly higher than that in the natural population even only after one-generation selection,suggesting the thermotolerance of selected oysters was improved.This result was in line with the results under field conditions.The respiration rate of the selected population at 38℃was significantly higher than that of 20℃,while the respiration rate of the natural population was significantly increased at 35℃,indicating that the natural population was more sensitive to high temperature stress,selected population has a greater capacity of oxygen supply.The results of physiological indicators showed that the selected oysters had higher enzyme activities related to metabolism and anti-oxidation（PK,SOD）during the heat stress,indicating that selected population had stronger antioxidant capacity.It has been proved by various means that the thermotolerance of oyster has been significantly improved after one generation selection of acute heat stress,which proves that the breeding strategy is effective.2.Genetic evolution analysis of thermotolerant population and heat-sensitive populationIn order to analyze the molecular mechanism of oysters’thermal adaptability,the survival individuals of selected population were considered as thermotolerant population,the dead individuals of natural population after heat stress were considered as heat-sensitive population（control）.Specific-locus amplified fragment sequencing（SLAF-seq）was used to analysis the genetic structure of the two population（50 individuals per population）.We also identified the selected region and candidate genes.The gene expression under heat shock was analyzed.After sequencing and quality control,379002 SNPs were obtained.Based on high quality SNPs,phylogenetic tree divides the two populations into 3 clusters.The results of PCA verified the results of phylogenetic tree and population structure analysis.These results suggested that the restricted genetic divergence between the oysters with one-generation selection and un selected oysters.The 115 selected region and 472 genes was determined based on F_ST andθπvalues,and 18 candidate genes were obtained by GO enrichment（BAG4 was selected according previous studies that suggests it may play important role in temperature adaptation）.Eight genes（IF4A2,IF6,EIF3A,MANBA,DDX43,RECS,CAT2,and BAG4）diverged in transcriptional expression response to heat stress.Six of genes showed lower basal expression and higher induced expression under heat stress in thermotolerant population compared with control population.which suggested higher plasticity of thermotolerant population.The heat tolerance of organisms plays an important role in prediction the potential ability to adapt climate change.Our results provide a molecular basis for researches on thermotolerance of oysters.