Éditeur(s) : EDP Sciences
Résumé : Bonamiosis is a parasitic disease (causative agent: Bonamia ostreae) affecting the European flat oyster Ostrea edulis, responsible for a drastic decline in its aquaculture production. Selective breeding programs for resistance to bonamiosis have been undertaken to counter this disease. In the present study, a 6-month cohabitation challenge experiment was performed in order to transmit the disease from wild oysters injected with the parasite to two tested families of oysters (20 and 8-month old at the beginning of the experiment, with different genetic backgrounds) originating from a selective breeding program developed by IFREMER in France. Mortalities were checked daily and ventricular heart smears were performed on dying or moribund oysters to detect the level of infection by B. ostreae. Mortality started after 4 months of cohabitation in the tested oysters. The cumulative mortalities after 6 months were 58% for the wild oysters, 9% for Family 1 (20-month old) and 20% for Family 2 (8-month old). In the dying oysters, the parasite could be detected in 67% of the wild oysters, 68% of Family 1 and 89% of Family 2. It was detected in only 11% of the surviving oysters of Family 2. The mortality and the level of infection by the parasite were significantly higher in Family 2 than in Family 1. Our results demonstrate that prespawning oysters as young as 1 year-old can become infected with the parasite and, most importantly, can die from bonamiosis. This result is inconsistent with the commonly accepted critical age of 2 years-old for the disease development. Additionally, no clear relationship between shell length and level of infection was observed. We also review the different methods for infection of the European flat oyster O. edulis with B. ostreae under experimental conditions and their main results.
La bonamiose est une maladie parasitaire (agent causal : Bonamia ostreae) affectant l'huître plate européenne Ostrea edulis, responsable d'un déclin drastique de sa production aquacole. Des programmes de sélection pour la résistance à la bonamiose ont été entrepris pour contrer cette maladie. Dans cette étude, une expérience de 6 mois d'infection par cohabitation a été réalisée de manière à transmettre la maladie à partir d'huîtres sauvages injectées avec le parasite vers deux familles testées d'huîtres (âgées de 20 et 8 mois en début d'expérience, avec des origines génétiques différentes) issues du programme de sélection développé par IFREMER en France. Les mortalités ont été vérifiées quotidiennement et des frottis de coeur ventriculaire réalisés sur les huîtres mortes ou moribondes pour détecter le niveau d'infection par B. ostreae. La mortalité a commencé chez les huîtres testées après 4 mois de cohabitation. Les mortalités cumulées après 6 mois étaient de 58 % chez les huîtres sauvages, 9 % chez la Famille 1 (âgées de 20 mois) et 20 % chez la Famille 2 (âgées de 8 mois). Chez les huîtres mourantes, le parasite a pu être détecté chez 67 % des huîtres sauvages, 68 % de la Famille 1 et 89 % de la Famille 2. Il n'a pu être détecté que chez 11 % des huîtres survivantes de la Famille 2. La mortalité et le niveau d'infection par le parasite étaient significativement plus élevés chez la Famille 2 que chez la Famille 1. Nos résultats démontrent que des huîtres âgées de un an peuvent devenir infectées par le parasite et surtout, peuvent mourir de bonamiose. Ce résultat contraste avec l'âge critique de développement de la maladie communément accepté de 2 ans. De plus, aucune relation claire entre la longueur de la coquille et le niveau d'infection n'a été observée. Nous faisons également la revue des différentes méthodes d'infection de l'huître plate européenne O. edulis avec B. ostreae en conditions expérimentales et leurs principaux résultats.
Aquatic Living Resources (0990-7440) (EDP Sciences), 2008-12 , Vol. 21 , P. 423-439

Droits : EDP Sciences, IFREMER, IRD 2008
http://archimer.ifremer.fr/doc/2008/publication-4783.pdf
DOI:10.1051/alr:2008053
http://archimer.ifremer.fr/doc/00000/4783/

Éditeur(s) : Physiomar 08 Physilogical aspects of reproduction, nutrition and growth "Marine molluscs in a changing environment"
Résumé : Bonamiosis is a parasitic disease (causative agent: Bonamia ostreae) affecting the European flat oyster Ostrea edulis, responsible for a drastic decline in the aquaculture production of this oyster species. Therefore a selective breeding program for resistance to bonamiosis has been undertaken since 1985 bu Ifremer, leading to the production of several selected oyster families. In the present study, a 6-month cohabitation challenge experiment was performed in order to transmit the disease from wild oysters (injected with the parasite) to two tested families of oysters originating from the selective breeding program. Mortalities were checked daily, and ventricular heart smears were performed on dying or moribund oysters to detect the level of infection to B. ostreae. The first infections occurred after 4 months of challenge in the tested oysters (Family 1 and Family 2). The cumulative mortalities after 5 monts were 58% for the wil oysters, 9% for Family 1 (20-month old at the beginning of the experiment) and 20% for Family 2 (8-month, old). The parasite could be detected in 66.8% of the dying wild oysters, 67.5% of the dying oysters of Family 1, 89% of the dying oysters of Family 2 and only 11% of the surviving oysters of Family 2. The mortality was significantly higher in Family 2 thant in Family 1 (x2= 20.87, p<0.001, d.f.) as well as the level of infection by the parasite found in heart smear (x2=24.34, p<0.001, 4 d.f.). This result demonstrates that prespawning oysters as yong as 1 year-old can become infected with the parasite and die from bonamiosis. This result is inconsistent with the commonly accepted critical age of 2 years-old for the disease development. The most probable cause of the dscrepancy in the development of bonamiosis between the 2 tested families is a difference in genetic background.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/2008/acte-4535.pdf
http://archimer.ifremer.fr/doc/00000/4535/

Éditeur(s) : Springer
Résumé : The great fecundity and very high larval mortality of most marine invertebrates and fish make possible substantial variance in the number of offspring contributed by adults to subsequent generations. The reproductive success of such organisms may thus resemble a sweepstakes lottery, in which a minority of progenitors succeeds in replacing an entire population, while the majority fails to procreate. One specific prediction of this hypothesis, that genetic diversity of newly settled cohorts should be less than that of the adult population, is tested in the present study. Microsatellite DNA markers were examined in naturally spawned juvenile European flat oysters Ostrea edulis (L.), collected over a 12-day period in 1993 from the western Mediterranean Sea, near Sete, France (43 degrees 32'N, 3 degrees 56'E) and grown out for a period of up to 10 months. Variation in these juveniles was compared to that in a pooled sample of adults collected in 1994 from two locations (Thau Lagoon and Port St. Louis) that had statistically homogeneous allelic frequencies. Though nearly twice as large as the pooled adult sample, the juvenile sample had only 60% of the adult allelic diversity. Analyses of linkage disequilibrium and kinship, as well as estimation of the effective number of parents, suggested that 10-20 adults produced this juvenile cohort. This observation supports the hypothesis of sweepstakes reproductive success and suggests that partial inbreeding may occur even in species with large populations and dispersing planktonic larvae.
Marine Biology (0025-3162) (Springer), 2007-03 , Vol. 150 , N. 6 , P. 1173-1182

Droits : 2007 Springer
http://archimer.ifremer.fr/doc/2007/publication-2616.pdf
DOI:10.1007/s00227-006-0441-y
http://archimer.ifremer.fr/doc/00000/2616/

Éditeur(s) : Actes Journées Conchylicoles, Ifremer Nantes, 3-4 avril 2001
Résumé : Mangrove oysters include various species that are also present on different coasts. In the southern part of the Atlantic Ocean, Crassostrea rhizophorae has been observed along the southern coast of America and Crassostrea gasar along the western coast of Africa. Fifteen populations of these oysters were sampled from these two geographic areas. Their polymorphism was studied on the 16S mitochondrial fragment both by par sequencing and RFLP analysis. Some samples' caryotype were also analysed. Two haplotypes were identified: C. gasar was found in Africa, but also for the first time in South America, whereas C. rhizophorae was only found on the coasts of South America.
Les huîtres de mangrove englobent plusieurs espèces qui sont présentes sur différentes côtes. Dans la partie sud de l'Océan Atlantique, Crassostrea rhizophorae a été décrite le long des côtes sud américaines et Crassostrea gasar le long des côtes ouest africaines. Quinze populations de ces huîtres ont été échantillonnées dans ces deux zones géographiques. Leur polymorphisme a été étudié sur le fragment mitochondrial 16S par séquençage et analyse en RFLP, le caryotype de certains échantillons a aussi été examiné. Deux haplotypes ont été identifiés: le type C. gasar a été trouvé en Afrique, mais aussi pour la première fois en Amérique du Sud, alors que le type C. rhizophorae a été rencontré seulement sur les côtes d'Amérique du Sud.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/2001/acte-3278.pdf
http://archimer.ifremer.fr/doc/00000/3278/

Éditeur(s) : Inter-Research
Résumé : The marine environment is of special interest for studying hybridization between closely related taxa because of the high dispersal potential of planktonic larvae, such as those of most bivalve species. The oysters Crassostrea angulata and C. gigas are known to be very close genetically and entirely inter-fertile under controlled conditions. However, hybridization in the wild had not been investigated, mainly due to the lack of nuclear diagnostic markers. In the present paper, we first estimated genetic differentiation between these 2 closely related taxa using 8 microsatellite markers. Interestingly, 5 markers displayed significant differences of allele size between taxa. The subsequent sequencing of alleles of one of these microsatellites showed several mutational events, which suggested null alleles and homoplasy. The presence of 1 insertion/deletion event in its 5' flanking sequence enabled us to design a new bi-allelic ('C' and 'NC') nuclear PCR-restriction fragment length polymorphism (-RFLP) marker (CG44R). This, together with a mitochondrial DNA marker, was used to analyze populations of C. angulata and C. gigas. The CG44R allele frequencies were very different between C. angulata (f[C] = 0.91) and C. gigas (f[NC] = 0.92) populations. This analysis also provided evidence for hybridization between C. angulata and C. gigas in a wild Portuguese population where the 2 taxa are in contact due to recent transportation of C. gigas stocks for aquacultural production. Our results represent the first indication of hybridization between these 2 taxa in the natural environment, and contribute to knowledge of the evolutionary history of the Crassostrea genus.
Marine Ecology Progress Series (0171-8630) (Inter-Research), 2004 , Vol. 272 , P. 141-152

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/2004/publication-3355.pdf
http://archimer.ifremer.fr/doc/00000/3355/

Résumé : Les objectifs pour l'année 1997 étaient les suivants : • Poursuivre les premières reproductions des espèces pour lesquelles nous disposons de géniteurs GO. L'objectif est toujours de limiter le stockage d'animaux importés en produisant une descendance simultanément à des témoins C. gigas qui serviront de comparaison. • Tenter des hybridations inter-spécifiques des différents taxons présents au sein du conservatoire en fonction des connaissances disponibles dans la littérature sur ce sujet (voir pour revue Gaffney et Allen, 1993). • Importer de nouvelles souches ou espèces. Élargir la gamme des espèces présentes au sein du conservatoire de souches (les capacités d'accueil de la salle de quarantaine étant limitée).

Droits : 1997 Ifremer
http://archimer.ifremer.fr/doc/00044/15519/12906.pdf
http://archimer.ifremer.fr/doc/00044/15519/

Éditeur(s) : World Aquaculture Society
Résumé : Mangrove oysters encompass several species and their taxomony is unclear. Among then, Crassostrea rhizophorae has been described along the Central and South American Atlantic coasts and Crassostrea gasar along African Atlantic coasts. Most taxonomic studies have so far been based on morphological criteria which are particularly susceptible to plasticity and new insight can be provided by molecular markers. The present work aims to provide information on the phylogeography of these two taxa.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/2000/acte-3496.pdf
http://archimer.ifremer.fr/doc/00000/3496/

Éditeur(s) : Styli 2003. Trente ans de crevetticulture en Nouvelle-Calédonie. Nouméa - Koné, 2-6 juin 2003. 2004. Gorarant C, Harache Y, Herbland A, Mugnier C. Ed. Ifremer, Actes de Colloque, n°38, pp.113-120
Résumé : Two oyster species are currently present along the French coasts : the indigenous European flat oyster (Ostrea edulis), and the Pacific cupped oyster (Crassostrea gigas), that has been introduced from Japan since the beginning of the 70ies. The flat oyster successively suffered from two protozoan diseases during the 60ies and its production decreased from 20 000 tons/year by that time to 1 500 tons/year nowadays. Consequently, the oyster production is principally (99%) based upon the Pacific oyster species with approximately 150 000 tons/year among which 90% are grown from the natural spat. However, the hatchery production of this species is developing and was estimated to 400 to 800 millions spat in 2002. Moreover, strengthened relationships between IFREMER and the 5 commercial hatcheries, that all joined the SYSAAF (Union of the French poultry, shellfish and fish farming selectors), allow to plan for new genetic breeding programs. At the end of the 80ies, IFREMER initiated a genetic breeding program for the resistance of the European flat oyster to the bonamiosis, and obtained strains more tolerant to this disease. After two generations of massal selection, molecular markers had identified a reduced genetic basis in this program. It was then reoriented to an intra-familial selection. However, we were confronted to a zootechnic problem to manage such a scheme and we compromised by an intra-cohorts of families selection scheme managed using molecular markers. The program has now reached the transfer level with experimentation at a professional scale. Concerning the Pacific cupped oyster, and in parallel with the obtaining and the study of polyploids, performance of different Asian cupped oyster strains were compared to the one introduced in France thirty years ago and currently suffering from summer mortalities. The local strain exhibited better performance, certainly based upon a good local adaptation. In other respects, although early growth is a relevant criteria for selection for growth to commercial stage, it is not to be privileged in the context of an oyster producing region with a limited food availability. Contrary, the spat summer mortality became a priority for numerous teams (genetic, physiology, pathology, ecology,...) joined in the MOREST program. The first results showed important survival differences between fullsib and halsib families. They indicate a genetic determinism to this character "survival" and promote for its selection.
Deux espèces d'huîtres sont aujourd'hui présentes sur les côtes françaises : l'huître plate européenne (Ostrea edulis), indigène, et l'huître creuse du Pacifique (Crassostrea gigas), introduite du Japon dans le début des années 1970. L'huître plate a subi coup sur coup, dans les années 1960, deux maladies dues à des protozoaires parasites et sa production est ainsi passée de 20 000 t/an dans les années 1960 à 1500 t/an aujourd'hui en France. Il en résulte que la filière ostréicole française repose quasiment exclusivement (99%) sur la production de l'huître creuse, C. gigas, avec 150 000 tonnes en 2000 dont environ 90% est issu du captage naturel. Cependant, la reproduction de cette espèce en écloserie est actuellement en expansion avec une estimation de 400 à 800 millions de naissains commercialisés en 2002. Des rapports renforcés entre l'IFREMER et les cinq écloseries françaises de la filière ostréicole, qui adhèrent toutes au SYSAAF (SYndicat des Sélectionneurs Avicoles et Aquacoles Français), permettent désormais d'envisager le développement de nouveaux programmes d'amélioration génétique. A la fin des années 1980, un programme de sélection pour la résistance de l'huître plate à la bonamiose a été initié au sein de l'IFREMER, et a permis d'obtenir des souches plus tolérantes à cette maladie. Après deux générations en sélection massale, les marqueurs moléculaires avaient mis en évidence une base génétique réduite au sein de ce programme qui s'était alors orienté vers une sélection intra-familiale. Devant la lourdeur zootechnique d'un tel schéma, un compromis a été trouvé en réalisant une sélection au sein de cohortes de familles biparentales, la gestion de ces cohortes étant permise par les analyses moléculaires. Le programme est maintenant dans sa phase de transfert, avec des expérimentations à plus grande échelle en partenariat avec la profession. En ce qui concerne l'huître creuse, et en parallèle de l'obtention et l'étude de polyploïdes, les performances de différentes souches d'huîtres creuses provenant d'Asie ont été comparées à celles de la souche introduite en France il y a trente ans, sujette à des mortalités estivales. La souche locale présente de meilleures performances, vraisemblablement grâce à une adaptation bien réalisée. Par ailleurs, bien que l'amélioration de la croissance jusqu'à la taille commerciale s'avère possible par une sélection précoce au stade larvaire, le critère de croissance n'est pas nécessairement à privilégier dans le contexte d'un bassin ostréicole disposant de ressources trophiques limitées. En revanche, les importantes mortalités rencontrées par le naissain en périodes estivales constituent le sujet d'étude prioritaire sur lequel se sont penchées de nombreuses équipes (génétique, physiologie, pathologie, écologie, ...) dans le cadre du grand défi MOREST. Les premiers résultats ont mis en évidence de fortes différences de survie entre des familles biparentales d'animaux indiquant un déterminisme génétique du caractère « survie », et encourageant dans la voie d'une sélection pour ce caractère.

Droits : 2004 Ifremer
http://archimer.ifremer.fr/doc/2003/acte-3491.pdf
http://archimer.ifremer.fr/doc/00000/3491/

Éditeur(s) : Blackwell science
Résumé : Patterns of mitochondrial DNA (mtDNA) variation were used to analyse the population genetic structure of southwestern Indian Ocean green turtle (Chelonia mydas) populations. Analysis of sequence variation over 396 bp of the mtDNA control region revealed seven haplotypes among 288 individuals from 10 nesting sites in the Southwest Indian Ocean. This is the first time that Atlantic Ocean haplotypes have been recorded among any Indo-Pacific nesting populations. Previous studies indicated that the Cape of Good Hope was a major biogeographical barrier between the Atlantic and Indian Oceans because evidence for gene flow in the last 1.5 million years has yet to emerge. This study, by sampling localities adjacent to this barrier, demonstrates that recent gene flow has occurred from the Atlantic Ocean into the Indian Ocean via the Cape of Good Hope. We also found compelling genetic evidence that green turtles nesting at the rookeries of the South Mozambique Channel (SMC) and those nesting in the North Mozambique Channel (NMC) belong to separate genetic stocks. Furthermore, the SMC could be subdivided in two different genetic stocks, one in Europa and the other one in Juan de Nova. We suggest that this particular genetic pattern along the Mozambique Channel is attributable to a recent colonization from the Atlantic Ocean and is maintained by oceanic conditions in the northern and southern Mozambique Channel that influence early stages in the green turtle life cycle.
Molecular Ecology (0962-1083) (Blackwell science), 2007 , Vol. 16 , N. 1 , P. 175-186

Droits : 2007 Blackwell
http://archimer.ifremer.fr/doc/2007/publication-2185.pdf
DOI:10.1111/j.1365-294X.2006.03122.x
http://archimer.ifremer.fr/doc/00000/2185/

Éditeur(s) : World Aquaculture Society, Aquaculture 2006
Résumé : Oysters are among the most familiar of all marine invertebrate taxa. However our knowledge of oyster phylogeny and systematics is fragmentary. This is principally due to the plastic growth patterns of these animals, which result in a wide range of overlapping, ecophenotypic variants that greatly reduce the value of analysis based on shell morphology. Besides that, many intentional or accidental anthropogenic transfers have emphasised this situation. In several cases of misclassification or misidentification of oysters, DNA molecular data, have provided valuable new insights on the tracing of introductions or more generally on the phylogeographic relationships between oyster species. We will illustrate this issue with recent case studies from our lab on several flat and cupped oysters: (1) the Crassostrea gigas – Crassostrea angulata couple of Asian species and their successive introductions in Europe, (2) the new species of cupped oysters, Crassostrea honkongensis, described in Hong-Kong and very divergent from the species Crassostrea gigas it was initially supposed to be, (3) the mangrove oyster Crassostrea gasar that was shown not only to be present along the coasts of Western Africa but also along the Atlantic coasts of South America, and (4) the geographic disjunction between Ostreola stentina, described in the Mediterranean Sea, and the genetically closely related species Ostrea aupouria (from New Zealand) and Ostreola equestris (from Mexico Gulf/Atlantic). At the intra specific level, we will focus on one of the main documented case in Europe, the flat oyster, Ostrea edulis. It is a marine bivalve whose natural geographical distribution ranges along the European Atlantic coast from Norway to Morocco, in addition to the Mediterranean and Black Sea. Studies of allozymes, microsatellites and mitochondrial differentiation over the whole range concluded that a significant divergence existed between Mediterranean and Atlantic populations, together with an isolation-by-distance pattern. However, the average mitochondrial haplotypic diversity displayed a high among populations variance, reflecting smaller effective population size in some locations. Additionally, a ten-fold quantitative difference was observed in the same study in Fst between the mitochondrial and the nuclear genomes, which could be due to sex biased differential reproductive success between males and females. In order to further document this hypothesis, two experiments at the population level were conducted. First, brooding females were sampled in a wild population and the number of males fertilizing a given female estimated. Then, parentage analyses were achieved under experimental conditions: successive mass spawnings were collected from a population of potential genitors kept in hatchery, whose genotypes were known, in order to infer a posteriori the relative contribution of each genitor. Moreover, we aimed to better understand the reproduction dynamics of this species. Several patterns of spawning could be distinguished: unique, successive or extended in time. The different parental contributions and reproductive behaviors observed in these experiments are discussed in the context of the hypothesis of a variance in the reproductive success of males and females and consequences in local and temporal reduced effective population sizes.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/2006/acte-3475.pdf
http://archimer.ifremer.fr/doc/00000/3475/

Résumé : Three species of man.-rove oysters, Crassostrea rhizophorae, C. brasiliana, and C. gasar, have been described along the Atlantic shores of South America and Africa. Because the distribution of these molluscs is of great biological and commercial interest, their taxonomy and distribution deserve further clarification. Therefore, 15 populations were sampled from both continents. Their 16S mitochondrial polymorphism was studied by sequencing and PCR-RFLP analysis. Two haplotypes were identified. Haplotype a was the only one observed in Africa, but it was also observed in South America together with haplotype b. Because C. gasar is the only mangrove oyster identified on the west coast of Africa, haplotype a was attributed to this species, which has thus been shown to occur in South America. Haplotype b is attributed to C. rhizophorae. The karyotypes of specimens of C. gasar, from Africa and from South America, were very similar, and both species were observed at the same location in Brazil. The occurrence of C. gasar in South America adds a third species-in addition to C rhizophorae and C. brasiliana-to the list of species present along these coasts. The predominant surface circulation patterns in this part of the Atlantic Ocean favor the hypothesis that C. gasar was transported from Africa to America. Finally, a phylogenetic tree built with seven 16S sequences from Crassostrea and Saccostrea species showed that C. gasar is intermediate between the American Crassostrea species (C virginica and C. rhizophorae) and the Asian species (C. gigas and C ariakensis).
Biological Bulletin (0006-3185), 2002-06 , Vol. 202 , N. 3 , P. 232-242

Droits : Marine Biological Laboratory.
http://archimer.ifremer.fr/doc/2002/publication-2790.pdf
http://archimer.ifremer.fr/doc/00000/2790/

Résumé : Dans le cadre du plan Etat-Région 1994-1998, un programme de conservatoire de souches a été initié au Laboratoire IFREMER de La Tremblade. Ce conservatoire a pour objectif d'acclimater différentes espèces ou souches d'huîtres creuses (genres Crassostrea et Saccostrea) afin d'acquérir des connaissances sur ces espèces et d'évaluer l'intérêt qu'elles pourraient présenter pour l'ostréiculture locale. L'importation d'espèces étrangères sur les côtes françaises est désormais strictement réglementée: la dissémination d'une nouvelle espèce peut se révéler néfaste d'un point de vue écologique et nuire aux productions existantes. De plus, ces huîtres peuvent être porteuses de pathogènes qui, s'il venaient à se répandre dans le milieu, pourraient toucher les stocks locaux. En conséquence, les huîtres importées sont strictement confinées au laboratoire dans une « salle de quarantaine » dont l'eau est traitée à l'ozone avant rejet afin d'éviter à la fois la dissémination des huîtres et d'éventuels pathogènes...

Droits : 1999 Ifremer
http://archimer.ifremer.fr/doc/00044/15514/12901.pdf
http://archimer.ifremer.fr/doc/00044/15514/