Successive selection responses of tetraploid Crassostrea angulata: comparison of growth, survival, sex ratio and ploidy composition between selected and unselected groups

Triploid Crassostrea angulata has the advantages of fast growth, poor fertility and high quality. However, due to poor fertility, direct genetic improvement is difficult. Mating between tetraploid males and diploid females has been shown to be the best method for commercial triploid production, so genetic improvement of tetraploids will directly affect the phenotypic traits of triploid offspring. However, to date, few studies have investigated the genetic improvement of tetraploid oysters. Based on the induced tetraploid C. angulata population, this study carried out three successive generations of growth selective breeding research, and systematically compared the growth, survival, ploidy composition and sex ratio of the selected group and the control group. The results showed that there was no significant difference in the survival rate from planktonic larvae to adult between the two groups. However, the growth of the selected group was significantly higher than that of the control group, that was, the shell height and whole weight of S3 were significantly higher than that of C3, and the selection advantage of shell height increased from 7.80% of S1 to 34.68% of S3. The ploidy composition of the two tetraploid groups was relatively stable before the 90th days, but on the 270th day, some tetraploids underwent chromosome loss and transformed into diploids or triploids, which might be related to gametogenesis of tetraploids. Moreover, with the increase of generations, the number of individuals with chromosome loss in the selected group gradually decreased, and their ploidy became more stable than that in the control group. In addition, both groups were fertile, with a sex ratio of more males than females, and a proportion of hermaphrodite individuals. In conclusion, this study successfully selected a fast growth line of tetraploid C. angulata, which can be used for commercial production of fast-growing triploid C. angulata, and provides a basis and reference for the genetic improvement of tetraploid oysters.