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The French project "Morest", coordinated by IFREMER, aims to a better understanding of the causes of summer mortalities in juvenile oysters by a multidisciplinary approach. Within this program, a first generation (G1) was bred in 2001 to assess to what extent genetic variability exists for this trait in French populations. 17 half-sib families (HSF) including 44 full-sib families (FSF) were obtained in the G1, and each FSF was reared in three sites along the French coast during the summer period. Significant differences in survival were observed among HSF, clearly indicating a genetic basis for survival. Estimation of heritability for survival was very high: 0.81  0.29. In 2002, 3 "good" HSF and 3 "bad" HSF were selected on their survival performances to constitute a second generation (G2) by divergent selection. Survival of the G2 was recorded in the same 3 sites according to the same protocol as in 2001, and additionally, the G2 was also monitored in several laboratory experiments. Significant differences in survival were found between the offspring of the "high" and "low" selected groups for all experiments, and the realized heritabilities of the survival in the field were 0.75 for the high group and 0.71 for the low group. These results confirm those observed for the G1, i.e. a high heritability for survival in juvenile cupped oysters, indicating that selective breeding programs could efficiently improve survival of juvenile oysters.

In the frame of MOREST project, a common biological material, resulting of a mixture of different families produced in experimental hatchery, was reared in two different environmental fields : Normandy and Charente. Concomitantly, a pool was conditioned at the Ifremer Argenton hatchery with three different algae levels : 4%, 8% and 12% of algal dry weight per oyster dry weight. During the experiments, five immune parameters were studied in parallel with survival rate and reproductive status (stages and intensity). Site location, seasonal variations and experimental diet level clearly influenced oyster immune parameters. Haemocyte counts were higher for oysters reared in Normandy than those reared in Charente and in hatchery. Granulocyte percentage was drastically reduced in hatchery conditions compared to in situ conditions. Moreover, hemocyte activities were also affected by the in situ conditions and dietary treatments in relation to reproductive cycle and mortality events. In example, haemocyte adhesion capacities were more affected by the presence of bacteria during high mortality period.

Bi-parental families were produced in hatchery and tested in the field in 2001. Two sets of 5 families were constituted, selected on their high (R) and low (S) survival. These two sets were reared in Brittany from March to November 2002. Samplings were performed twice a month to obtain data on biometry, survival, reproductive cycle, biochemical composition, adenylate energy charge, hemolymph parameters (ions and defense system) and muscle strength. "R" and "S" oysters exhibited different reproductive effort and spawning strategy. "R" oysters allocated less energy in gonad than "S" ones and presented a complete spawning at the end of August contrary to the "S" which spawns partially. Mortality started in July when the seawater temperature reached 19 degree C and affected mainly "S" oysters. Concentrations of Na and Cl ions in hemolymph were different for "S" and "R" from May to June. A bacterial increase in hemolymph (R and S) was observed during the same period. The adenylate energy charge was more lowered for "R" than for "S" oysters, just before the spawning event However, other physiological and immunological parameters were similar between the two sets during the phases of maturation and mortality but discriminated groups after the spawning peak.

The French project "Morest", coordinated by IFREMER, aims to understand the causes of the summer mortalities in Crassostrea gigas. In 2001, three sets of families were bred following a nested half-sib mating design. 17 half-sib families (HSF) were obtained in this first generation (G1) and reared in 3 sites. Significant differences in survival were observed among HSF, and some HSF showed high levels of mortality in all sites, clearly indicating a genetic basis for survival. In 2002, a second generation (G2), including divergent selection and inbred lines, was constituted. Monitoring of survival and growth of G2s were the same as in 2001. Significant differences in survival were found between the offspring of the "high" and "low" selected groups and between inbred lines. The high realized heritability for survival indicates that selective breeding programs could efficiently improve survival of juvenile oysters.

Field characterization of summer mortality was performed in France in the frame of the Morest project. Natural and hatchery spat were compared between three oyster production areas in France. Regardless of the natural or hatchery origin, oysters died during the reproduction period after temperature reaches 19 degree C. Thus, in southern areas, temperature accelerated gametogenesis of small spat (10mm) as well as adults, and mortality appeared for the two age classes. In contrast, sexual maturation proceeded more slowly in northern where spat mortality was lower compared to 18 months old oysters. However, critical gametogenesis and temperature were not sufficient to induce mortalities, as observed in examples with stable environment. Alternatively sediment proximity in addition to oyster manipulations increased mortality during spring and summer, suggesting that some additional environmental stresses were necessary to reproduce the phenomena. These interaction processes will be detailed in the other Morest contributions.

Phenotypic and genetic correlations between fitness-related life-history traits - such as survival, growth and reproductive allocation - have to be considered to better understand selective processes, both in the wild and in breeding programs. Quantitative genetic experiments, based on nested half-sib mating designs, were perfomed in the Pacific oyster C. gigas (1) to estimate genetic variance and the response to selection and (2) to document phenotypic and genetic trade-offs between life history traits. Physiological trade-offs, i.e. plastcity of resource allocation, were shown to vary among genotypes. Genetic polymorphism was primarily observed for the the plasticity of reproductive effort. In addition, the degree of plasticity in reproductive effort appeared to be genetically correlated with mean survival. The signs of some signficant genetic correlations were found to reverse accross environments. These results could explain the maintenance of genetic polymorphism for the studied traits. These may help us to better understand the causes of summer mortalities in C. gigas juveniles, on which a national program recently started in France.

L'analyse du polymorphisme des gènes de J'amylase chez Crassostrea gigas (Thunberg, 1793) a été établi sur deux générations dans le cadre du programme européen GENEPHYS (génétique et physiologie chez C.gigas). Après une amplification de l'ADN par PCR à partir de deux couples d'amorces qui sont spécifiques à chacun des deux gènes de l'amylase, une digestion par EcoRI de l'ADN amplifié suivie d'une électrophorése en gel d'agarose sont effectuées. Cette technique (RFLP) permet de déterminer le polymorphisme génétique des huîtres creuses. Une troisième génération a été réalisée afin d'obtenir quatre familles et d'en analyser le génotype. Ces familles ont été tout d'abord placées pendant cinq mois dans un milieu riche en nourriture, puis ensuite, chaque famille est séparée en deux lots, un lot restant dans le milieu riche tandis que l'autre est placé dans un milieu pauvre pour une durée de six semaines. Les génotypes obtenus pour les individus des quatre familles ont permis de trouver un nouvel allèle pour le gène B qui n'était pas amplifié par le couple d'amorce spécifique lors de la PCR à cause de la non fixation de l'amorce sens. De même, il a été démontré pour la première fois que les deux gènes de l'amylase sont portés par le même chromosome. D'un point de vue performances physiologiques et croissance, il a été démontré que deux génotypes particuliers présentent des résultats antagonistes selon les conditions trophiques. Ainsi le génotype 22 66' est le plus performant en milieu riche avec les meilleurs taux de croissance, d'ingestion et efficacité d'assimilation alors que le génotype 11 66 est le moins performant. Par contre en milieu pauvre, c'est ce dernier le plus performant alors que le génotype 22 66' n'est pas du tout adapté à ce milieu avec des performances médiocre.