Comparative study on the growth, survival, gonad development and trait segregation of F2 hybrids and their grandparent species (Crassostrea ariakensis and C. hongkongensis)

In previous studies, we showed that F1 hybrids of Crassostrea ariakensis and C. hongkongensis had faster growth and higher survival rates than the parent species. Further intraspecific crossing of F1 hybrids allowed us to further understand the law of trait segregation and indicated a potential direction for breeding of a new strain with high salinity tolerance and fast growth. In this study, we systematically analyzed the growth, survival, fertility and trait segregation of F2 hybrids and their grandparent species (C. ariakensis and C. hongkongensis) at two sites. Our results confirmed that there were no significant differences in fertilization, cleavage, D larval and survival rates among the three groups of larvae, but the shell heights of F2 hybrids and C. ariakensis were significantly higher than those of C. hongkongensis on the 12th and 16th days. At the high salinity site (Zhulin Pond), the shell heights and survival rates of the F2 hybrids were significantly higher than those of C. hongkongensis from the 90th to the 360th day. However, at the low salinity site (Dafeng River), the F2 hybrids consistently had higher shell heights than C. ariakensis, but there was no significant difference in the survival rates among the three groups. F2 hybrids were classified into three types based on size and number of holes on the gill water tubes, and only F2-A (like C. ariakensis) and F2-H/A (intermediate form) were confirmed to survive at the high salinity site. Moreover, three number of ITS-2 gene fragment were identified in the F2 hybrids, and only A (like C. ariakensis) and HA (like F1 hybrids) existed at the high salinity site. Paraffin sections of gonad tissue confirmed that all three types of F2 hybrids were fertile and could produce mature gametes. In summary, through this comprehensive comparative analysis, we confirmed that F2-A and F2-H/A hybrids had clear advantages in terms of high salinity tolerance and fast growth over their grandparent species, and had potential to be used as the foundations of a new strain.