Selecting flat oysters, Ostrea edulis, for survival against the parasite Bonamia ostreae : assessment of the resistance of a first selected generation

In 1989, a selected strain of flat oyster. Ostrea edulis, was produced from breeders that survived Bonamia ostreae and that were over selected by injection of purified parasites. This generation (G 1) was compared to native oysters which had settled during the same year. After a growing period of 21 months, the oysters were tested according to three procedures: exposure to natural infection in an intertidal area, maintenance at the laboratory in tanks near heavily infested oysters, injection of parasites. The test lasted 7 months. In each procedure, oysters of the first generation (G1) showed a better survival rate than control oysters (72 to 94% versus 48 to 66%). On the other hand, the highest mortalities were observed among injected oysters (G1 and controls). In the laboratory tests, however, infestation rates of surviving oysters were equal or even higher in Gl than in the controls. This reduces the gain of resistance, regarding the number of non infested survivors. Neverthe1ess, the difference remains significant between oysters over selected by inoculation and their controls (49% versus 32%). This confirms that the injection technique may provide faster assessment of disease resistance and also may be used for over selection. Survivors of the over selected first generation have been kept to produce a second generation (G2).