Abstract
Ostreid herpesvirus 1 (OsHV-1) and Vibrio aestuarianus are the two main pathogens affecting the production of French oyster (Crassostrea gigas). The absence of genetic correlation between the two diseases is promising for the development of stocks with dual resistance. Using unselected and selected oysters concerning enhanced resistance to OsHV-1 infection, we investigated the first generation of mass selection and the response to selection to increase the resistance to V. aestuarianus for two stocks. For each stock, four groups were produced in June 2013 using either parents unchallenged with the bacteria or counterparts that survived experimental infections by the bacteria. Thus, groups were unselected oysters for both pathogens, selected for either the virus or the bacteria, and dually selected for both pathogens. All groups of each stock were evaluated at the spat and juvenile sizes following experimental infection by V. aestuarianus in May 2014. Regardless of their level of selection for OsHV-1, oysters produced from parents that survived V. aestuarianus showed similar mortalities (47% and 53% for stocks A and B, respectively) during the bacterial challenge than those produced from unchallenged parents (43% and 56%, respectively). Thus, no positive response to selection at the first generation to increase the bacterial resistance was found at the spat and juvenile sizes. At the adult stage and with experimental infection with V. aestuarianus, only stock B showed a positive response to selection for increasing the bacterial resistance with a decrease in mortality of 14% in comparison with unselected oysters. Similar results were observed when oysters were tested for 27 months in the field with the absence of response to selection for stock A, while a 13% decrease in mortality was observed for stock B. For stock B, mortality at endpoint after 27 months in the field reached 89% for the control not selected at all, decreased to 84% for oysters selected for their resistance to the bacteria, was 53% for the oysters selected for their resistance to OsHV-1, and was 32% for oysters selected for dual resistance. Realized heritability estimated at the first generation of mass selection for stock B was higher for oysters selected for dual resistance, ranging from 0.47 to 0.80, than those only selected for V. aestuarianus, ranging from 0.05 to 0.30. Selection for dual resistance in C. gigas could limit the impact of both OsHV-1 and V. aestuarianus on oyster production