Asymmetry Of Left-right Shells And Left-right Mantles In Pacific Oyster

Crassostrea gigas is a model organism for bivalve research,which has important economic and research value.The aquaculture industry of Crassostrea gigas is widely valued throughout the world.Beautiful shape and large size are always two important traits to breed in C.gigas.However,unlike most bivalves,C.gigas has a very irregular shape,which seriously affects its product value.Studies have shown that the left and right symmetrical organs that affect appearance of C.gigas differ not only in their shapes but also in functions.The left and right shells are large on the left and small on the right.Once the left shell is fixed,it cannot move permanently,leaving the right shell in charge of opening and closing the shell.The mantle membrane with a symmetrical shape may show differences in many aspects such as shell formation,pheromone,nerve conduction and enzymatic activity in terms of molecular function.This study strives to understand the asymmetry in shell and mantle tissues of C.gigas through multi-level comparative analysis,and provide a new direction for the further improvement of C.gigas breeding.The main results are as follows:1.There are significant differences between the left and right shells of C.gigas in the basic indicators such as shell length,shell width,shell height and shell weight(p <0.05).Among them,the most significant differences are shell height and shell weight(p< 0.01).The correlation analysis between the shell index of the left and right shells and the weight of the soft body was carried out.It was found that the left shell weight of C.gigas had the highest correlation with the weight of the soft body,and the correlation coefficient was 0.918.This suggested that left shell weight is the most influential of the shell shape traits with the weight of the soft body of C.gigas.2.There are certain differences in the content of muscle components and the activities of antioxidant enzymes between the left and right mantles of C.gigas.Both histological sections and creatinine content showed a higher muscle composition in the left mantle than on the right.And the transcription factor Cg Tcf21 gene involved in myogenesis is consistent with this trend,which indicates that the contractility of the left mantle tissue might be stronger.The contents of three oxygen radicals and Malondialdehyde in right side were significantly higer(p < 0.05)than left,indicating that right mantle had more accumulated oxidative damage.Superoxide smutase was significantly higer(p < 0.05)in the right mantle,indicating that the right side is more active in scavenging reactive oxygen radicals.3.Pitx2 is an important left-right asymmetric developmental regulated gene that can continue to be expressed in adults.Through the bioinformatics analysis of the Cg Pitx2 gene sequence of C.gigas,it is found that the gene sequence of Cg Pitx2 is relatively conservative,with an open reading frame of 873 bp,encoding a total of 290 amino acids,and the predicted protein molecular weight is 32.45 k Da,which has high similarity to the Pitx2 gene of Patinopecten yessoensis,Scapharca broughtonii and Danio rerio.Fluorescence quantitative results showed that the right mantle expression quantity of the Pitx2 gene was significantly higher than that of the left mantle(p < 0.01)in C.gigas,significantly higer(p < 0.05)in the left mantle of P.yessoensis,but basically the same in both left and right mantles of S.broughtonii.Such trends of expression quantity are all related to the shape of the shell.The expression quantity of the Pitx2 gene is higher in the mantle of the smaller side of the shell,and that of protein in the left and right mantles of C.gigas has the same variation trend as the gene differences.The above results show that Cg PITX2 may be an important molecule regulating the left-right asymmetry of C.gigas.By analyzing the difference in its expression,the difference between the left and right asymmetry of C.gigas can be obtained;and regulating the genetic expression can carry out molecular breeding for the shell traits of C.gigas.In this study,the differences between the left and right shells of the C.gigas,the differences between the left and right mantles and the differences between Pitx2 in the left and right mantles were investigated respectively.The reasons for the asymmetrical differences between the left and right sides of the C.gigas were analysed,providing basic data for the further breeding of new varieties of C.gigas.