Transcriptomics Analysis And Re-sequencing Reveal The Mechanism Underlying The Thermotolerance Of An Artificial Selection Population Of The Pacific Oyster(Crassostrea Gigas)

The Pacific oyster（Crassostrea gigas）is one of the major oyster species in the world.It lives in the intertidal environment with great temperature variation.Mass deaths in the summer are one of the challenges facing the oyster industry of C.gigas.This study had started an artificial selection breeding program based on acute heat shock of the parents to select thermal tolerance oysters since 2017.In this study,we compared the differentiation of the respiration rate,summer survival rate,gene expression and gene structure of the selected population F₂and non-selected wild population F₂of C.gigas.The experimental results showed that:1.Transcriptomics and re-squencing analysis of selected and control grooups F₂of C.gigas.In this study,the transcriptional analysis displayed global divergence between the selected and the control groups at the larval stage of C.gigas with 4764 DEGs produced.Among which 79 genes were heat responsive genes.Four of the six genes（Five heat stock genes in the enriched GO and KEGG and one BAG4）were further showed expressional differentiation in one-year-old oysters.The integration of the transcriptomic and the re-sequencing of the selected and the control groups found 1090 gens showing differentiation in both the gene structure and expression,and two SNPs（Single Nucleotide Polymorphism）which may mediated the expression of CGI＿10022585 and CGI＿10024709 were validated.2.The respiration rate and the survival rate analysis of selected and control groups F₂of C.gigas.In this study,the respiration rate of six-months-old oysters varied significantly between the selected population and the wild population of C.gigas at room temperature（20℃）.In addition,the survival rate of selected group was 78.05%and control group was 66.34%after 1 h of heat shock at 42℃in laboratory.The field summer survival rate of selected group was 73.00%and control group was 62.00%.The summer survival rate of the selected group of C.gigas was significantly improved.In this study,we compared the differentiation of the transcriptomic,genetic structure and physiological parameters related to the thermotolerance of oysters for the F₂ selected and non-selected groups of C.gigas.This study shows that:1)Transcriptional divergence between the selected and control groups at the larval stage;2)Two SNPs that may mediate the DEGs were validated;3)The respiration rate varied after the selection for the thermotolerance,and the survival rate post the acute heat stress and the summer survivor in the field were significantly improved in the selected group.Therefore,this study not only shows that artificial selection has a significant impact on the genetic structure and expression of oysters,but also it provides an alternative way to reveal the high-temperature resistance mechanism and predict the oyster’s potential adaptation to climate change.This study will provide empirical basis for molecular breeding of C.gigas in the future.