Crossbreeding of three different shell color lines in the Pacific oyster reveals high heterosis for survival but low heterosis for growth

Heterosis is important for improving productive traits in aquaculture, but the heterosis between selected lines remains unclear in bivalve molluscs. Here, a diallel crosses among black shell (B, 7th generation), white shell (W, 7th generation) and orange shell (O, 6th generation) lines of the Pacific oyster were performed to examine the potential for exploiting heterosis of productive traits by crossbreeding of selected lines. Heterosis and combining abilities for growth and survival of 27 full-sib families were analyzed at both larval and juvenile stages, of which the juvenile stage was examined at two environments (Rongcheng and Rushan) for 10 months. The variance of specific combining ability accounts for the majority of the total variance for larval and juvenile survival and juvenile growth, which revealed the importance of non-additive genetic effects (heterosis) among these lines. Moreover, all reciprocal hybrids showed low or no heterosis for growth at both larval and juvenile stages, but a high heterosis for larval survival. This finding suggested that inter-line crossbreeding can be applied to improve the larval survival in hatcheries. Furthermore, four of six hybrid crosses showed positive HPH in Rushan but negative HPH in Rongcheng. The higher HPH at Rushan suggested that inter-line crossbreeding in C. gigas has more application value in Rushan. In addition, the survival of both black shell and white shell lines have been improved by crossing with the orange shell line. Especially, the hybrid cross WO (W female x O male) can improve the survival rate of purebred WW by nearly 130% at larval stage and nearly 40% at juvenile stage. Overall, this study reveals the high heterosis for survival among selected lines in the Pacific oyster, and will be helpful for the utilization of non-additive genetic effects among existing selected lines in bivalves.