Abstract
Since 2008, the emergent virus OsHV-1µvar has provoked massive mortality events in Crassostrea gigas spat and juveniles in France. Since 2012, mortality driven by the pathogenic bacteria Vibrio aestuarianus has stricken market-sized adults. A hypothesis to explain the sudden increase in mortality observed in France since 2012 is that selective pressure due to recurrent viral infections could have led to a higher susceptibility of adults to Vibrio infection. In our study, two OsHV-1-resistant lines (AS and BS) and their respective controls (AC and BC) were experimentally challenged in the laboratory to determine their level of susceptibility to V. aestuarianus infection. At the juvenile stage, the selected lines exhibited lower mortality (14 and 33%) than the control lines (71 and 80%), suggesting dual-resistance to OsHV-1 and V. aestuarianus in C. gigas. Interestingly, this pattern was not observed at the adult stage, where higher mortality was detected for AS (68%) and BC (62%) than AC (39%) and BS (49%). These results were confirmed by the analysis of the expression of 31 immune-related genes in unchallenged oysters. Differential gene expression discriminated oysters according to their susceptibility to infection at both the juvenile and adult stages, suggesting that resistance to V. aestuarianus infection resulted in complex interactions between the genotype, stage of development and immunity status. Finally, survivors of the V. aestuarianus challenge at the juvenile stage still exhibited significant mortality at the adult stage during a second and third V. aestuarianus challenge, indicating that these survivors were not genetically resistant.
Highlights
The French oyster industry has regularly suffered from massive mortality episodes (Figure 1)
A particular OsHV-1 genotype that was first described during a period of C. gigas mortality in 2004–2005 in Normandy [5] has been ascribed to the mortality [6]
While mortality related to OsHV-1 and V. aestuarianus was reported in C. gigas in France prior to 2008 [27, 29], their impact on French oyster production became predominant due to recurrent and intense mortality in spat and adult oysters
Summary
The French oyster industry has regularly suffered from massive mortality episodes (Figure 1). Once a disease affecting an oyster species has been introduced. 1970s with the massive introduction of Crassostrea gigas from Japan and British Columbia to replace C. angulata during the RESOR operation (Figure 1) [3]. French oyster production of C. gigas has ranged from 100 000 to 150 000 tons for several decades, but has begun to decrease due to two diseases (Figure 1). Significant mortality has been observed in market-sized adults since 2012 [7–9], and C. gigas production is expected to decrease again (Figure 1). Because it would not be reasonable to introduce another oyster species to replace C. gigas, the possibility of genetic selection for disease resistance might limit the impact of diseases on wild and cultivated oysters
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