Y. Naciri-Graven, A.G. Martin, J.P. Baud, T. Renault, A. Gerard-1998
Journal of Experimental Marine Biology and Ecology, 224 (1): 91-107 (from Current Contents)
Abstract:
In the early 1970s, the appearance of the protozoan parasite Marteilia refringens restricted the production of the European flat oyster Ostrea edulis to subtidal areas. Ten years later a second protozoan parasite, Bonamia ostreae, caused a further serious decline in Bat oyster farming in subtidal areas. Prophylactic measures were taken to sustain oyster farming, but the continuing presence of the two parasites led to the initiation of a programme to select for resistant oysters. Efforts were concentrated on B. ostreae and two selected strains were obtained in 1985 (S85) and 1989 (S89). The programme was based on mass selection and parasite inoculation or natural infections, This paper focuses on the survival and weights of the third generation of S85 (S85-G3), the second generation of S89 (S89-G2) and a cross between S85-G2 and S89-G1. The % survival of S85-G3 oysters was more than 4-times higher than the control group after a 20 month experiment in the wild, and significant differences in parasite prevalence were recorded. No significant differences between S89-G2 and the control group were observed for either survival or parasite prevalence. This was attributed either to a less powerful experiment or to the effect of genetic load clearing as a result of inbreeding. The % survival of the cross between the two strains was more than twice as high as the control group and significant differences were recorded for parasite prevalence. Good evidence for additivity of the resistance was provided by both the intermediate behaviour of a cross between S85-G2 and the controls and the rapid improvement that was obtained from the first generations. The selected strains showed a tendency toward higher weights and higher weight variances when compared to controls. These populations are suspected to have undergone bottlenecks, which would explain the increase in phenotypic variance. No measurable natural resistance has arisen in the wild, This could be due first to the relatively short time that has elapsed since the first infections, secondly to the early sale of resistant oysters as soon as they reach a marketable size, and thirdly to the slow kinetics of B. ostreae infection that allows both susceptible and potentially resistant oysters to participate in recruitment. Improved resistance seems to be related to the delayed mortality of selected oysters. This last result is discussed in the light of a previous study conducted in the laboratory which showed that resistant and susceptible oysters have different haemograms.
(Univ. Lausanne, IZEA, Batiment Biol., CH-1015 Lausanne, Switzerland)