Introduction
Cupped oyster (Crassostrea [Magallana] gigas Thunberg, 1793) production is regularly threatened by episodes of mass mortality from spat to adult, representing a major economic issue for the oyster farming industry (Jourdan et al., 2026). Among the pathogens involved in mortality events in adult oysters, the bacterium Vibrio aestuarianus has been identified in various environments (D��gremont et al., 2019). Consequently, the onset and intensity of mortality events often result from interactions between the bacterium, the environment, and the genetic and farming-related factors of the populations (Travers et al., 2014). The genetic resistance to V. aestuarianus was estimated as moderatly heritable (h2=0.2) in a genetic design based on separated families and a controlled challenge in laboratory (Az��ma et al., 2017). The estimation of heritability in commercial conditions according to different rearing conditions and/or environments is not documented. The aim of this study was to estimate the selection potential for adult survival in the cupped oyster in different production environments to optimize breeding programs.
Material and Methods
300 families were produced by artificial fertilization and the size of maternal families was balanced before pooling all larvae into one cohort. At 18 months, 2 060 progeny were electronically tagged and biopsied (mantle) for later genotyping (as their parents). Three batches were created at 24 months and reared in subtidal environments: one in Normandy (Asnelles; 687 oysters) and one on the Atlantic coast in Vend��e (Bay of Bourgneuf; 579 oysters). The third batch was reared suspended in the Mediterranean Sea (Thau Lagoon; 794 oysters). Parents and progeny were genotyped on the 40k Axiom Oyster Genotyping Array. Parentage assignment was performed using the R package APIS (Griot et al., 2020) to reconstruct the pedigree. Genetic parameters for survival were estimated within and between sites using a single-step threshold model with the THRGIBBSF90 software (Tsuruta & Misztal, 2006).
Results
In the Thau Lagoon, mortality was intermediate (33%), as in Normandy (24%), and was limited in Vend��e (12%). Heritability estimates for survival in the three different sites were about 0.26-0.27, while overall survival, regardless of sites, was estimated at 0.21±0.04. Genetic correlations for survival were low to moderate (0.09±0.2 to 0.58±0.1) across sites (Table 1). The highest correlation was observed between Normandy and Thau, and the lowest was observed between Thau and Vend��e.
Discussion
This study provides the first estimates of genetic parameters and GxE interactions of adult survival of cupped oysters across different commercial rearing conditions. The moderate heritabilities estimated for survival within and between sites confirm that survival, and probably also resistance to V. aesturianus, in adult oysters can be genetically improved by genetic selection.
The intermediate to low genetic correlations between across environments suggest differences in the selection potential in each condition. The correlation between Normandy and Thau indicates that genetic gain can be expected across these sites, while the correlation between Thau and Vend��e highlights the need to consider specific site effects. However, estimates involving the Vend��e site should be interpreted with caution due to the low mortality and high standard errors (up to 0.26), suggesting limited accuracy.
Additional genetic analyses considering inter-annual variation within and between sites and across generations are necessary to better evaluate the importance of these and their impact on the expected genetic gain. Genetic correlation across rearing conditions should be taken into account to organize selection by site and optimize oyster breeding programs within the limit of economic barrier.
Table 1 : Heritability (diagonal) and genetic correlation (above diagonal) for survival between site and ages estimated in GBLUP
30 months-Normandy
30 months -Thau
30 months -Vend��e
30 months -Normandy
0.26 [0.06]
0.58 [0.13]
0.23 [0.23]
30 months -Thau
0.27 [0.06]
0.09 [0.20]
30 months -Vend��e
0.27 [0.07]
References
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