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PCR-based DNA markers enable the study of diversity and differentiation of marine organisms at different stages of their life cycle. Our study of European fiat oyster (Ostrea edulis) populations aimed to describe their spatial and temporal dynamics. First, we analyzed adults sampled over the natural range of the species. An Atlantic/Mediterranean differentiation pattern was observed. Similar patterns of variation were found using allozymes, microsatellites and mitochondrial DNA, aIl which support an isolation by distance model. High variability was observed in the range of diversity between populations using a mitochondrial marker, a reflection of small effective population sizes in sorne locations. A 10fold quantitative difference was observed in Fst, between the genomes, which may be due to an unbalanced sex ratio and/or differential reproductive success. Secondly, we initiated the study of reproductive dynamics at a finer scale. Several experiments were performed to document: 1- the variance inallele frequencies during a natural settlement period, 2- the paternal contribution to fertilization by analyzing larvae sampled at the brooding stage within individual females, 3- the individual reproductive success within an experimental population. Our results are relevant in terms of management of the genetic diversity and the potential impact of hatchery propagation., Les marqueurs génétiques basés sur la technique de PCR permettent l'étude de la diversité et de la différenciation des populations d'organismes marins à différents stades de leur cycle de vie, dès la phase larvaire. Notre étude sur l'huître plate européenne avait pour but de décrire la dynamique spatiale et temporelle de la diversité génétique présente chez cette espèce. Dans un premier temps, nous avons analysé des adultes échantillonnés sur l'ensemble de J'aire de répartition. Les résultats sont qualitativement similaires quels que soient les marqueurs utilisés (al1ozymes, microsatellites et ADN mitochondrial), validant l'existence d'un modèle d'isolement par la distance. Une différenciation significative entre populations atlantiques et méditerranéennes est démontrée. Cependant, une différence d'un facteur 10 sur le niveau de différenciation des populations a été mise en évidence entre marqueurs des génomes mitochondrial et nucléaire. Cette différence pourrait s'expliquer par un sex-ratio déséquilibré et/ou une plus grande variance dans le succès reproducteur femelle. De plus, une forte variabilité du niveau de variabilité a été observée entre populations pour l'ADN mitochondrial, indice de tailles efficaces femel1es localement réduites. Dans un second temps, nous nous sommes intéressés à une échelle plus fine, celle de la population, et plus particulièrement à sa dynamique de reproduction. Plusieurs volets ont été abordés dans le cadre de ce travail: 1- la variance des fréquences alléliques au cours d'une saison de recrutement, 2- les contributions paternelles en réalisant des échantillonnages de larves encore non émises de femelles incubantes, 3- le succès reproducteur individuel au sein de populations d'écloserie. Ces résultats seront discutés en terme de gestion de la diversité génétique de cette espèce et de l'impact potentiel du développement de la production de naissain d'écloserie.

Two oyster species are cultivated on the French coasts: the Pacific cupped oyster (Crassostrea gigas), which has been introduced from Japan in the 1970's after the disappearance of the Portuguese oyster Crassostrea angulala, and the European flat oyster (Ostrea edulis). During the second half of the 1960's, the latter suffered successively from two diseases caused by protozoan parasites: marteiliosis (also called "maladie des Abers") and bonamiosis. In France, its production dropped from 20,000 tian in the 1960's to 2,000 today. The impact of these two pathogens as well as the exploitation of the remaining stocks show the importance of the conservation of this species, the only natural European species, whose population is more and more fragmented., Deux espèces d'huîtres sont aujourd'hui présentes sur les côtes françaises : l'huître creuse du Pacifique (Crassostrea gigas), qui a été introduite du Japon dans le début des années 1970 suite à la disparition de l'huître portugaise Crassostrea angulala, et l'huître plate européenne (Ostrea edulis). Cette espèce a subi coup sur coup, durant la seconde moitié des années 1960, deux maladies dues à des protozoaires parasites: la marteiliose ou « maladie des Abers» et la bonamiose. Sa production est ainsi passée de 20 000 tian dans les années 1960 à 2000 tian aujourd'hui en France. L'impact de ces deux pathogènes ainsi que l'exploitation des stocks restants soulignent l'importance de la conservation de cette espèce, seule espèce d'huître naturellement présente en Europe, et dont les populations apparaissent de plus en plus fragmentées.

Natural populations of the European flat oyster, Ostrea edulis, have been harvested since Roman times. This species is distributed along the European Atlantic coasts, from Norway to Morocco, as well as the Mediterranean Sea and Black sea coasts. Previous studies, based on allozymes, have concluded that isolation by distance is taking place in the Mediterranean sea and the Atlantic ocean. Because of the presumed non neutrality of these markers, another approach was made to examine the genetic differentiation of this species using nuclear microsatellite and mitochondrial markers, both considered as neutral, and to compare the results to those observed with allozymes.

As of 1985, Ifremer set up a selection programme for resistance to the protozoan Bonamia ostreae in European flat oysters (Ostrea edulis). Two selection lines, L85 and L89, from the third and second generation, respectively, were produced with 17 and 21 different families. The experimental design also included a mixed line (L85 x L89) of seven families and a control line of five families from Quiberon broodstock maintained without selection pressure., Depuis 1985, un programme de sélection de l'huître plate, Ostrea edulis résistante au protozoaire Bonamia ostreae a été initié par IFREMER. Deux lignées sélectionnées L85 et L89 respectivement de 3ème et de 2èmt: génération ont été produites avec 17 et 21 familles différentes. Une lignée mixage (LBS x L89) de 7 familles et un témoin de 5 familles issu de géniteurs de Quiberon sans pression de sélection complètent le dispositif expérimental.

In France, marine fish and shellfish farming is based mainly on the breeding of wild species, whose natural populations are traditionally exploited. Research programs are conducted by IFREMER, in co-operation with INRA and Universities. French research programs for the genetical improvement of marine fish and shellfish concern primarily seabass (Dicentrarchus labrax) and oysters (Ostrea edulis and Crassostrea gigas). The following programs are developed: •The knowledge of wild and farmed populations. The most recent prodjects involve the highly variable microsatellite markers. Seven markers are presently available for seabass and the work is now initiated on turbot (Scophthalmus maximus). In molluscs, microsatellite markers have been developed for the European flat oyster (O.edulis). • The control of sex-ratios. Research programs in physiology and developmental genetics are initiated in order to control sex-ratios in seabass. These programs include the production of gynogenetic progenies and the study of the sex-ratios of progenies issued from the crossing of hormonally feminised or masculinised breeders with normal ones. •The production of polyploids. Triploid seabass have been produced using high pressure and thermal shocks. The results after 6 months show a lower survival and growth compared with diploid controls. Triploid oysters, produced using chemical treatments (cytochalasin B or 6-DMAP), show better performances compared to diploids. •The establishment of breeding programs for traits of commercial interest. For marine fish, selection programs are initiated by industries and are scientifically supported by research institutes. The transfer to the industries is ensured by SYSAAF, a specialised structure that eases the adaptation of the professional network for t~e development of genetical improvement programs. In the fiat oyster, a breeding program for the resistance against Bonamia osteae has-been initiated in 1985 by IFREMER. Results show a better survival of selected populations compared with wild controls.

At the end of the 1980s, after two decades of aquaculture research in molluscs, Ifremer initiated its first genetic studies. These new studies quickly revealed the limits of knowledge in aquaculture, particularly in terms of controlling oyster reproduction. This presentation attempts to summarize the available information on genetics in aquaculture. In particular, breeding practices in the 1980s involved populations of broodstock, while genetics requires modern techniques where individuals, not just populations, can be identified. Controlling reproduction at the individual level in oysters raises new questions that existing protocols cannot address, especially in terms of controlling sex determination, individual maturity, spawning synchrony, chemical communication between broodstocks, etc., A la fin des années 1980, après deux décennies d'acquis zootechniques sur les mollusques, l'IFREMER engage les premières recherches dans le domaine de la génétique. Très rapidement, ces nouvelles études font ressortir les limites des connaissances zootechniques, notamment en matière de contrôle de la reproduction des huîtres. Cet exposé tente de faire une synthèse dans ce domaine. En particulier, il est souligné que la maîtrise zootechnique des années 80 se situait à l'échelle d'une population de reproducteurs et que la génétique réclame désormais une nouvelle zootechnie, permettant non plus de travailler sur des populations mais sur des individus préalablement identifiés. Cette maîtrise individuelle du contrôle de la reproduction chez les huîtres, pose des questions totalement nouvelles auxquelles ne répondent pas les protocoles existants en matière de contrôle du sexe, de maturation individuelle, de synchronisme des pontes, d'échanges chimiques entre géniteurs...

Nowadays, PCR-based DNA markers allow the study of diversity and differentiation of marine organisms at different stages of their life cycle. Our study of European flat oyster (Ostrea edulis) populations has aimed to describe their spatial and temporal dynamics. First, we analyzed adults sampled over the natural range of the species. An Atlantic/Mediterranean differentiation pattern was observed. Results were congruent between allozymes, microsatellites and mitochondrial DNA and support an isolation by distance model. High variability was observed in the breadth of diversity between populations using a mitochondrial marker, this highlights the small effective population sizes in some locations. A 10-fold quantitative difference was observed in Fst between the genomes, which may be due to an unbalanced sex ratio and/or differential reproductive success. Secondly, we investigated the dynamics of reproduction at a finer scale. Several experiments were performed to study (1) the variance of allele frequencies during a natural settlement period, (2) the paternal contribution to fertilization by analyzing larvae sampled at the brooding stage within individual females, (3) the individual reproductive success within an experimental population. The significance of our results will be discussed in terms of management of the genetic diversity and the potential impact of aquaculture and hatchery propagation.