Genetic diversity and genetic structure in successive mass selected generations of tetraploid Pacific oysters (Crassostrea gigas), diploid Portuguese oysters (C. angulata) and their allotriploid oysters

In bivalve aquaculture, tetraploids are commonly used to generate sterile triploids for commercial aquaculture. We obtained the allotriploid population (tetraploid C. gigas ♂ × diploid C. angulata ♀) through the hybridization of tetraploid male C. gigas with diploid female C. angulata for the first time. However, the genetic diversity and genetic structure in successive mass selected generations of tetraploid C. gigas, diploid C. angulata and their allotriploid offspring are currently unknown. In this study, nine polymorphic microsatellite markers were used to evaluate their genetic diversity and genetic structure. The trend of reduced genetic diversity was recorded in successive generations of mass selected tetraploid C. gigas and diploid C. angulata. However, the diversity of the allotriploid population was significantly higher than that of the male parent population and the female parent population. This indicates that the hybridization technique of tetraploid C. gigas + diploid C. angulata to produce triploids is beneficial for increasing the genetic diversity of the hybrid triploid. Little genetic differentiation was found within tetraploid C. gigas and diploid C. angulata (except A3, the third generation of C. angulata), which is most likely caused by the insignificant genetic structure of the selected lines. The large genetic differences between tetraploid C. gigas and diploid C. angulata are caused by subspecies differences and ploidy difference. In addition, allotriploids were found to be more closely related to the female population, suggesting that this genetic relationship is more consistent with phenotypic traits. The findings of this study offer useful information for further genetic improvement of oyster polyploidy breeding.