Additive and non-additive genetic effects in complete diallel cross of Crassostrea angulata strains tested in tropical farming environments

As a native of subtropical regions, the Portuguese oyster ( Crassostrea angulata ) has not been extensively evaluated under tropical farming systems. This study examined additive and non-additive genetic effects for body weight and survival in a complete 4 × 4 diallel cross using four geographically distinct C. angulata strains, tested under tropical estuarine conditions in Southern Vietnam. After 180 days of grow-out, performance data were collected from 13,173 oysters across four sites with fluctuating salinity (14–30 ppt) and stable temperatures (27–32°C). Our results revealed significant additive genetic variation and genotype-by-environment interactions across environment. The Vung Tau (V) and Khanh Hoa (K) strains exhibited the strongest additive genetic effects (AGE) for both traits, with V showing consistent contributions across hybrid combinations. In contrast, Quang Ninh (Q) and Taiwan (T) strains displayed negative additive genetic performance, reflecting limited adaptation to tropical estuarine conditions. Reciprocal effects contributed to phenotypic variation and showed asymmetry in certain crosses, suggesting maternal and environmental interactions. Heterosis was generally negative or negligible for body weight, but modest positive values for survival were detected in crosses involving Q (e.g., QK, QT), indicating limited dominance effects. Collectively, additive genetic variance was the primary driver of trait improvement, underscoring the importance of strain selection. Strains V and K are promising candidates for developing a synthetic base population, while inclusion of genetically distinct strains such as Q and T may help maintain long-term diversity in breeding programs targeting tropical oyster aquaculture.