Analysis on selection effect and growth advantage of a fifth generation 

selected line of red shell colored Meretrix meretrix

Abstract: Meretrix meretrix is widely distributed in coastal areas of China, and Jiangsu Province is the main production area of M. meretrix. In recent years, the eutrophication of aquaculture water in shrimp and crab ponds due to high input of feed has reached the red line of environmental protection, and trapping shellfish has become a necessary solution to improve aquaculture water quality, energy conservation and emission reduction. As the most important shellfish in Jiangsu Province, the culture space of M. meretrix can be further extended to shrimp and crab ponds in addition to the traditional tidal flat and pond culture. Therefore, it is necessary to breed a new species of M. meretrix to better match with other species such as shrimp and crab, and jointly promote the ecological transformation of pond in feeding, catching and other daily aquaculture management, so as to achieve green and highquality development. So far, four new varieties of M. meretrix breeding have been announced by the Ministry of Agriculture and Rural Affairs. However, the lack of local varieties of M. meretrix industry in Jiangsu Province does not match with the highly developed M. meretrix industry. It is urgent to breed new varieties of M. meretrix in Jiangsu Province to support the development of the existing industry. Since 2007, a continuous population breeding process has been carried out by selecting M. meretrix with red color shell from YaoshaLengjiasha sand sea area in Rudong. Until 2018, it had been continuously bred for 5 generations. In 2018, 4 000 seeds were randomly selected from the breeding line G4 to measure the shell length and weight under the indoor conditions in Jiangsu Provincial Fine Breeds of Meretrix. The first 40 seeds were selected as the breeding group of the fifth generation (G5) by sorting the shell length and grain weight, and the seed retention rate was 1% and the selection strength was 2.665. In addition, 40 seeds were randomly selected from the same breeding line G4 as the control group without breeding. The parents of the breeding group and the control group were subjected to induction of spawning. During the breeding process, at larval stage, the shell length and shell height of the selected group and the control group were measured by random sampling every afternoon. After attachment, the shell length and shell height of the selected group and the control group were randomly sampled at 7, 13, 32 days old. 67, 102, 143, 252, 310, 328, 358dayold individuals were randomly sampled and measured. At 310 days of age, the selected group and the control group were placed in the same pond for breeding. The shell length, shell height, shell width and grain weight were measured once a month. In this study, the selection response, realized heritability h2R and genetic gain of the fast growing and breeding line G5 established in 2018 were evaluated and compared with the control group without breeding. Results showed that the shell length, shell height, shell width and grain weight of the selected group were significantly higher than those of the control group at larval stage, intermediate cultivation stage and growout stage in most time (P < 0.05). In harvest, the shell length, shell height, shell width and grain weight of the selected group were (43.020±3.267) mm, (35.948±2.757) mm, (19.838±1.768) mm, (22.008±4.755)g, while the shell length, shell height, shell width and grain weight of the control group were (40.019±3.644) mm, (33.446±3.091) mm, (18.449±1.860) mm, (17.755±4.527) g. The average values of selection effect of shell length and shell height at larval stage were 0.40±0.28 and 0.46±0.31, respectively. The average values of genetic gain were 1.09%±0.68% and 1.44%±0.95%, respectively. The average values of realized heritability were 0.15±0.11 and 0.17±0.11, respectively. The average values of selection effect of shell length and shell height at the intermediate cultivation stage were 0.62±0.20 and 0.63±0.09, respectively. The average values of genetic gain were 11.17%±8.30% and 7.28%±3.36%, respectively. The average values of realized heritability were 0.23±0.07 and 0.24±0.03, respectively. The average values of selection effect of shell length, shell height, shell width and grain weight at growout stage were 0.63±0.22, 0.74±0.14, 0.64±0.21, 0.73±0.19, respectively. The average values of genetic gain were 8.64%±2.78%, 8.18%±1.64%, 6.87%±1.41%, 28.18%±8.12%, and the average values of realized heritability was 0.24±0.08, 0.28±0.05, 0.24±0.08, 0.27±0.07, respectively. In quantitative genetics research, h2R<0.15 is usually defined as low heritability，0.15<h2R<0.3 is defined as medium heritability and h2R>0.3 is defined as high heritability. In this study, although the red shell colored breeding line of M. meretrix has been continuously selected for five generations, the heritability of shell length, shell height, shell width and grain weight is still at a medium level of 0.24. From the genetic gain of grain weight of target traits, there is still large selection space from selection limit. In the study, it was also found that the heritability at the development stage was greater than that at the intermediate culture stage of juvenile M. meretrix, and at the intermediate culture stage juvenile M. meretrix was greater than that at the larval stage, which was consistent with the fact that the growth heritability at the development stage varied greatly and the adult growth heritability was generally greater than that at the larval stage. In this study, the genetic gains of shell length, shell height and shell width in the incubation period were similar, all of which were about 7.50 %, while the genetic gains of grain weight reached 23.96%. This was mainly due to the correlation between the four parameters of shell length, shell height, shell width and grain weight of bivalves, and the grain weight showed a power function relationship with shell length. The existing studies on the phenotypic parameters of bivalves confirmed the close relationship between weight traits and shell shape traits. Therefore, a small increase in shell length, shell height and shell width led to a significant increase in grain weight, and the same findings were found in other studies. The research results can clarify the superiority of selfselected new lines of M. meretrix and provide basic information for new varieties identification and extension.