The ostreid herpesvirus 1 (OsHV-1) is one of the major pathogens affecting the Pacific oyster Crassostrea gigas, and numerous mortality outbreaks have been observed worldwide. We report the first results of our experimental breeding program using mass selection focused on survival and resistance to OsHV-1 after four generations of selection for two lines. These lines originated from two stocks of adult wild oysters sampled from the Marennes-Oléron Bay in 2008. Each line was spawned in February 2009 to produce the base populations. Both lines were then either protected from OsHV-1 or tested in the field in 2009 where they were exposed to OsHV-1. For each line during 2010 to 2013, one generation per year was produced using either the survivors of the previous generation for the selected group or the oysters protected from OsHV-1 for the control group. After one generation of selection (G1) for both lines, the mean survival of the selected group was 34.5% compared with 12.3% in the control group. For the fourth generation of selection (G4), the survival of the selected group reached 69.0% and the survival of the control group was 7.3%. The gain in survival of the selected C. gigas spat over the control increased by 22.2%, 43.9%, 50.2% and 61.8% for the G1, G2, G3 and G4 generations, respectively. Our study demonstrates that mass selection for survival and OsHV-1 resistance was successful after four generations of selection, thus indicating a significant genetic improvement for the selected trait. A genotype x size interaction was observed with 55.1% of survival in G4 when selected spat were transferred at 1 g versus 89.9% of survival when they were transferred at 3 g. Our study is the first to provide some estimates of the realized heritability for disease resistance using a mass selection scheme in an oyster species with values ranging from 0.34 to 0.63 depending on the size of the oysters exposed to OsHV-1. Oysters selected for their higher resistance to OsHV-1 infection in G4 showed higher growth (58.4 mm-19.4 g) than controls (51.4 mm-15.2 g), and mass selection had significantly improved the yield for the selected oysters (13.3 kg) over the controls (1.2 kg). Mass selection could be easily implemented by a commercial hatchery that cannot afford family-based selection that requires the production of numerous families for the base population. Highlights ► Mass selection to enhance survival and OsHV-1 resistance was successful in C. gigas spat. ► This is the first study to report realized heritability for disease resistance in an oyster species. ► The gain of survival per generation was 12.1% to 20.2% for 1 g and > 3 g oysters, respectively. ► The selected oysters grew faster and had a much higher yield than did the unselected oysters. Statement of relevance Strong impact on breeding program working on OsHV-1 resistance in C. gigas.

Pesticides represent a major proportion of the chemical pollutants detected in French coastal waters and hence a significant environmental risk with regards to marine organisms. Commercially-raised bivalves are particularly exposed to pollutants, among them pesticides, as shellfish farming zones are subject to considerable pressure from agricultural activities on the mainland. The aims of this study were to determine (1) the genotoxic effects of diuron exposure on oyster genitors and (2) the possible transmission of damaged DNA to offspring and its repercussions on oyster fitness. To investigate these points, oysters were exposed to concentrations of diuron close to those detected in the Marennes-Oleron Basin (two 7-day exposure pulses at 0.4 and 0.6 μg L(-1)) during the gametogenesis period. Genomic abnormalities were characterized using two complementary approaches. The Comet assay was applied for the measurement of early and reversible primary DNA damage, whereas flow cytometry was used to assess the clastogenic and aneugenic effect of diuron exposure. Polar Organic Chemical Integrative Samplers (POCIS) were used in exposed and assay tanks to confirm the waterborne concentration of diuron reached during the experiment. The results obtained by the Comet assay clearly showed a higher level of DNA strand breaks in both the hemocytes and spermatozoa of diuron-exposed genitors. The transmission of damaged genetic material to gamete cells could be responsible for the genetic damage measured in offspring. Indeed, flow cytometry analyses showed the presence of DNA breakage and a significant decrease in DNA content in spat from diuron-exposed genitors. The transmission of DNA damage to the offspring could be involved in the negative effects observed on offspring development (decrease in hatching rate, higher level of larval abnormalities, delay in metamorphosis) and growth. In this study, the vertical transmission of DNA damage was so highlighted by subjecting oyster genitors to short exposures to diuron at medium environmental concentrations. The analysis of POCIS showed that oysters were exposed to integrated concentrations as low as 0.2 and 0.3 μg L(-1), emphasizing the relevance of the results obtained and the risk associated to chemical contamination for oyster recruitment and fitness.

Highlights► M. edulis that survived a mortality outbreak continued to show mortality in the following year, ► Naïve M. edulis adults had significant mortality only when they were in contact with M. edulis that survived a mortality outbreak suggesting a horizontal transmission of a putative causal agent, ► No putative causal agent identified yet, although the bacteria belonging to the Splendidus clade were detected at high prevalence, ► M. edulis spat seems less susceptible than M. edulis adults to mortality, ► M. edulis spat seems more susceptible than M. galloprovincialis spat to mortality,

The effect of ploidy on the mortality of Crassostrea gigas spat caused by the ostreid herpesvirus (OsHV‐1) genotype μVar was investigated at five sites along the Atlantic coast in France in 2011. Sibling diploids and triploids were produced using either unselected or selected OsHV‐1‐resistant oysters. No significant interactions were found between the factors of environment, genotype and ploidy at the endpoint dates. The mean mortality rates at the sites were 62% and 59% for diploids and triploids, respectively, and the two rates were not significantly different. The mean mortality rates were 33% and 32% for sibling diploids and triploids, respectively, when OsHV‐1‐resistant parents were used and 91% and 85%, respectively, when unselected parents were used. The results were confirmed through other broodstocks tested in 2013. Our study is the first to clearly show that mortality related to OsHV‐1 is similar between diploids and triploids in C. gigas when the same germplasm is used for both ploidy. Furthermore, OsHV‐1 resistance was not substantially altered by triploidization, indicating that the achieved selective breeding of diploid oysters for OsHV‐1 resistance can be translated into improved survival in triploids.

Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. Statement of relevance Husbandry practices can decrease OsHV-1 mortality in C gigas, but it depends on the genetic resistance of the oyster stocks used. Highlights ► Larger oysters had lower mortality than smaller ones when exposed to OsHV-1 for most of the 40 families tested. ► Lower mortality for high growing heights but only for families which are not highly resistant or highly susceptible to OsHV-1. ► Higher growth is associated to higher susceptibility to OsHV-1. ► Moderate heritability for OsHV-1-associated mortality ranging from 0.44 to 0.49