Abstract
As a native of subtropical regions, the Portuguese oyster (Crassostrea angulata) has not been extensively evaluated under monsoon tropical farming systems for breeding program. This study examined additive and non-additive genetic effects for body weight (g, total live whole weight of individual oysters) and survival rate (%, proportion of oysters alive relative to the initial stocking) in a complete 4 × 4 diallel cross using four geographically different C. angulata strains, tested under monsoon tropical estuarine conditions in Southern Vietnam. After 180 days of grow-out, data on body weight and survival at harvest were collected from 13,173 oysters across four sites with fluctuating salinity (14–30 ppt) and stable temperatures (29–30 °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 body weight and survival rate, 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 the inclusion of genetically distinct strains such as Q and T may help maintain long-term diversity in breeding programs targeting tropical oyster aquaculture.