Résumé
The flat oyster Ostrea edulis is an endemic European oyster species, found on both Atlantic and Mediterranean coasts. Its aquacultural production has dramatically decreased due to two successive diseases caused by the intracellular parasites Marteilia refringens and Bonamia ostreae. Since 1985, Ifremer has undertaken a breeding program to produce oyster families which are tolerant to Bonamia. In this context, a genetic map was therefore established as a basis for Quantitative Trait Loci (QTLs) analysis. The reference family was an F2 family coming from a first bi-parental cross between a wild oyster and an individual from the fifth generation inbred line of the Bonamia tolerance selection program. This inbred line had shown no mortality when reared in sites where the parasite was present. Twenty microsatellites and 60 AFLP (Amplified Fragment Length Polymorphism) primer pairs were scored. Linkage analysis was carried out using CRIMAP v.2.4. Among the polymorphic markers, 296 AFLPs segregated in the mapping family: 235 were used in the linkage analysis as well as 16 microsatellites. The linkage map of the first parent consisted of 137 markers grouped into 8 linkage groups (2n=20). The linkage map of the second parent consisted of 149 markers grouped into 9 linkage groups with more than 5 markers and 3 doublets. Moreover, three sex-average linkage groups were built with more than 3 microsatellites as anchor loci. Average marker spacing was 6 cM and length spanned from 69 to 98 cM. Then a QTL search for resistance to bonamiosis was performed after an experimental challenge to Bonamia ostreae. A total of 550 F2 individuals cohabited with over infected wild oysters with the parasite. Mortality was checked daily during six months and smear achieved on dead oysters to detect the presence of the parasite. Among the 105 dead oysters, 46 very infected oysters were randomly chosen, as well as 46 oysters among the 445 live individuals, where no parasite was detected. After a multistage testing strategy (marker by marker analysis), 15 AFLP markers showed a significant difference of allele frequencies between the "dead" group and the "alive "group". The classical QTL analysis detected 6 QTLs. Some of the 15 markers identified in the marker by marker analysis were mapped in the same locations as the QTLs, strengthening our results. Those QTLs will be further investigated and more precisely located by adding codominant markers such as microsatellites.