Genotype × environment interactions for weight in Pacific oysters (Crassostrea gigas) on five Australian farms

Genotype × environment (G × E) interactions affecting weight in Pacific oysters grown in five environments, comprising two farms in South Australia (SA1 and SA2), and three farms in Tasmania (Tas1, Tas2 and Tas3), were estimated. Body weights were measured at five time points between approximately 1 and 2 years of age (T0, T1, T2, T3, and T4) in a four generation population, from a breeding program based mainly on full sibling matings. Families were grown in the same rearing environment until T0 when they were selected at random into family batches of size 100 and distributed to the five farms. Data analysed were average animal weight derived from total batch weight. G × E interactions were present as scale effects, with greater variation between families in high growth environments in South Australia relative to Tasmania. These effects increased with time, such that by T4 the estimated family variance on the South Australian farms was approximately five to seven times greater than on Tasmanian farms. Apart from one environment at T4, the estimated genetic correlations between environments averaged 0.91, 0.91, 0.90 and 0.91 at times T1, T2, T3 and T4 respectively, indicating that G × E interactions in the form of re-ranking of families across environments were relatively unimportant. The exception was Tas1 at T4, where the estimated correlations ranged from 0.11 to 0.46. The data were then combined and genetic correlations between family means at different time points were estimated. For T1 onwards, the estimated correlations were 0.96 or greater, indicating that weight is effectively the same trait at all time points between T1 and T4. Correlations of average animal weight at T0 with measurements at later time points were slightly lower, but still strong, ranging from 0.79 to 0.97. This indicates that measurement regimes based on early measurements could increase the efficiency of the breeding program. It was concluded that G × E interactions for weight will not be detrimental to the breeding program for this population, provided appropriate models for genetic evaluation of families are used.