Breeding And Domestication Of Eastern Oyster (Crassostrea Virginica) Lines For Culture In The Mid-Atlantic, Usa: Line Development And Mass Selection For Disease Resistance

A selective breeding program for Crassostrea virginica was established in 1997 as part of an initiative in Virginia to address declining oyster harvests caused by the two oyster pathogens Haplosporidium nelsoni (MSX) and Perkinsusmarinus (Dermo). Housed in the Aquaculture Genetics and Breeding Technology Center (ABC), the objective of the program was to develop and disseminate disease-resistant lines that would enable an oyster culture industry. Today, culture of disease-resistant cultivars accounts for more than 90% of oyster production in the state, where 28.1 million half-shell oysters and 2 billion eyed larvae were sold in 2012. Results of our line development program as of 2006 are reported. Eight ABC lines from 3 genetic groups-East Coast (EC), Louisiana (LA), and hybrids between the 2 (HY)-and 1 wild control line, were produced and tested. These 9 groups were deployed in 4 replicates across 4 Virginia sites characterized by low (Kinsale (KIN)), medium (York River (YRK) and Lynnhaven (LYN)), and high (Wachapreague (WAC)) salinity regimes. Groups were sampled routinely for survival, growth, and disease diagnosis between November 2004 and December 2006. At KIN, where salinity was low and below the threshold for MSX and Dermo, survival was 41%-46% greater than survival at the other 3 sites by the end of the trial. Where the diseases were present (LYN, YRK, and WAC), ABC lines in general had greater survival than the control, but this varied by genetic group. The EC groups had 52%-82% greater survival, the HY groups had as much as 40% greater survival, and the LA groups performed worse than the control. Poor performance of the LA groups was a result of their susceptibility to MSX, and the majority of them died before the end of the study. The genetic effects varied with site, and the rank of the lines was inconsistent, such that the best line in one site was, in some cases, one of the worst in another. Genotype-by-environment interaction is clearly driven by disease and salinity. Growth was also influenced by site, genetic group, and an interaction between them. Compared with the wild control, ABC lines were 31%, 20%, 42%, and 24% heavier at the end of the trial in the KIN, YRK, LYN, and WAC sites, respectively. However, unlike survival, the best performers were those from the LA and HY groups. Again, line rankings changed across sites. For this reason, a salinity-specific breeding strategy to develop lines that perform optimally within a salinity range has been adopted

Triploid Oysters In The Chesapeake Bay: Comparison Of Diploid And Triploid Crassostrea Virginica

Diploid and triploid Eastern oysters, Crassostrea virginica, were tested at 3 sites characterized by low on moderate salinity regimes in the Virginia part of the Chesapeake Bay from November 2005 through October 2007. Both diploid and triploid cultures were replicated 3 times by producing separate spawns from different broodstock. Ploidy had a generally consistent effect on the performance of C. virginica at the 3 test sites. At the end of the study, in October 2007, and across all sites, triploid oysters had lower cumulative mortality than diploids (-34%), and greater shell height (+25%), whole weight (+88%), and yield (+152%), as well as a higher proportion of market-size oysters (+114%) than diploids. Both diploids and triploids were similarly infected by Perkinsus marinas and, to a lesser extent, by Haplosporidium nelsoni. In a closer look, growth parameters (shell height growth, whole weight, yield, and percentage of marketable oysters) were always higher in triploids than in diploids regardless of the parental source, strongly supporting the superior advantage of triploids. Similar results were obtained for cumulative mortality, but to a lesser extent as a result of the large variation in mortality for both diploid and triploid cohorts among sites, as well as a significant site-by-cohort interaction. Our report is the first clear illustration of variation for the cumulative mortality exhibited among different spawns in triploids, and comes with the lesson that care must be taken in experiments in which the goal is to test the effect of ploidy on this trait. Our results support the notion that selective breeding programs to reduce mortality, coupled with triploid production to increase growth, can further optimize yield. The best-performing replicate spawn had 80% survival after 2.5 y, and reached an average shell height of 92 mm, weighing 142 g

Etude du polymorphisme des gènes de l'amylase chez Crassostrea gigas (Thunberg, 1793). Relation avec les paramètres physiologiques de l'assimilation

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

Etude des bases génétiques de la mortalité estivale et des relations avec la croissance chez les juvéniles de l'huître creuse Crassostrea gigas

CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

Mortality associated with OsHV-1 in spat Crassostrea gigas: role of wild-caught spat in the horizontal transmission of the disease

The French oyster production of Crassostrea gigas is based on two sources of spat: wild-caught (WC) and hatchery-produced (HP). Massive mortality related to the ostreid herpesvirus type 1 (OsHV-1) has affected both sources in France since 2008. We investigated the mortality in juvenile C. gigas due to the horizontal transmission of OsHV-1 within (separated condition) and between (mixed condition) the two spat sources in three environments from April to June 2010. In the separated condition, no mortality was observed in the HP batches, while the WC batches experienced moderate to high mortality (40–80 %). In contrast, the WC and HP batches experienced high mortality in all tested environments for the mixed condition. At the beginning of the trial, the HP batches were all negative for OsHV-1 DNA detection by real-time PCR, while the WC batches were all positive for OsHV-1 DNA detection by real-time PCR, even though the percentage of virus DNA-positive oysters and viral load were low. During the experiment, all batches that exhibited mortality were positive for OsHV-1 with a high viral load, while OsHV-1 was never detected for the HP batches of the separated condition. Together, our results demonstrated that OsHV-1 was horizontally transmitted from the WC oysters to the HP oysters. Our study is the first to indicate that the mortality related to OsHV-1 in HP oysters can be avoided using ponds or tanks. However, these oysters were always protected from OsHV-1, and HP oysters could also experience mortality and spread the disease similar to the WC oysters if such care is not used. Finally, the persistence of OsHV-1 at a sub-clinical level in certain oysters supports the hypothesis that the virus can be reactivated and cause viral replication. The use of the two spat sources is discussed to better understand the spread of the disease among oyster stocks

Mass selection for survival and resistance to OsHV-1 infection in Crassostrea gigas spat in field conditions: response to selection after four generations

The ostreid herpesvirus 1 (OsHV-1) is one of the major pathogens affecting the Pacific oyster Crassostrea gigas, and numerous mortality outbreaks have been observed worldwide. We report the first results of our experimental breeding program using mass selection focused on survival and resistance to OsHV-1 after four generations of selection for two lines. These lines originated from two stocks of adult wild oysters sampled from the Marennes-Oléron Bay in 2008. Each line was spawned in February 2009 to produce the base populations. Both lines were then either protected from OsHV-1 or tested in the field in 2009 where they were exposed to OsHV-1. For each line during 2010 to 2013, one generation per year was produced using either the survivors of the previous generation for the selected group or the oysters protected from OsHV-1 for the control group. After one generation of selection (G1) for both lines, the mean survival of the selected group was 34.5% compared with 12.3% in the control group. For the fourth generation of selection (G4), the survival of the selected group reached 69.0% and the survival of the control group was 7.3%. The gain in survival of the selected C. gigas spat over the control increased by 22.2%, 43.9%, 50.2% and 61.8% for the G1, G2, G3 and G4 generations, respectively. Our study demonstrates that mass selection for survival and OsHV-1 resistance was successful after four generations of selection, thus indicating a significant genetic improvement for the selected trait. A genotype x size interaction was observed with 55.1% of survival in G4 when selected spat were transferred at 1 g versus 89.9% of survival when they were transferred at 3 g. Our study is the first to provide some estimates of the realized heritability for disease resistance using a mass selection scheme in an oyster species with values ranging from 0.34 to 0.63 depending on the size of the oysters exposed to OsHV-1. Oysters selected for their higher resistance to OsHV-1 infection in G4 showed higher growth (58.4 mm – 19.4 g) than controls (51.4 mm – 15.2 g), and mass selection had significantly improved the yield for the selected oysters (13.3 kg) over the controls (1.2 kg). Mass selection could be easily implemented by a commercial hatchery that cannot afford family-based selection that requires the production of numerous families for the base population

Summer mortality of hatchery-produced Pacific oyster spat (Crassostrea gigas). II. Response to selection for survival and its influence on growth and yield

Response to divergent selection for "high" and "low" survival during the summer period, from July to October, was investigated in juvenile (six-month-old) Pacific oysters, Crassostrea gigas, by producing two sets of progenies in 2002 (Generation 2) and three sets of progenies in 2003 (Generation 3). A strict between-family approach was used and resistance of these selected progenies to summer mortality was assessed in three sites along French coasts, to determine their response to selection and estimate realized heritability of the trait. A significant difference in survival was observed between the "high" and "low" selected groups at all sites for all sets of progenies, indicating a significant genetic component. High realized heritabilities for survival obtained from Generation 2 oysters, ranging from 0.61 ± 0.08 to 0.98 ± 0.15, were in line with previous results from the first generation. Finally, Generation 3 realized heritability, ranging from 0.55 ± 0.18 to 0.81 ± 0.13, supported results from Generation 2. Our results demonstrate that selective breeding to improve survival during the first summer can be successfully implemented and should lead to rapid gains. Overall, selection on survival did not have any impact on growth, although it did have one on yield

Genetic improvement for disease resistance in oysters: a review

Oyster species suffer from numerous disease outbreaks, often causing high mortality. Because the environment cannot be controlled, genetic improvement for disease resistance to pathogens is an attractive option to reduce their impact on oyster production. We review the literature on selective breeding programs for disease resistance in oyster species, and the impact of triploidy on such resistance. Significant response to selection to improve disease resistance was observed in all studies after two to four generations of selection for Haplosporidium nelsoni and Roseovarius crassostrea in Crassostrea virginica, OsHV-1 in Crassostrea gigas, and Martelia sydneyi in Saccostrea glomerata. Clearly, resistance in these cases was heritable, but most of the studies failed to provide estimates for heritability or genetic correlations with other traits, e.g., between resistance to one disease and another. Generally, it seems breeding for higher resistance to one disease does not confer higher resistance or susceptibility to another disease. For disease resistance in triploid oysters, several studies showed that triploidy confers neither advantage nor disadvantage in survival, e.g., OsHV-1 resistance in C. gigas. Other studies showed higher disease resistance of triploids over diploid as observed in C. virginica and S. glomerata. One indirect mechanism for triploids to avoid disease was to grow faster, thus limiting the span of time when oysters might be exposed to disease

Response to selection for increasing resistance to the spring mortality outbreaks in Mytilus edulis occurring in France since 2014

Massive spring mortality outbreaks have been reported in Mytilus spp. in France since 2014. The main areas affected are the Pertuis Charentais Sounds and the coast of the Pays de la Loire, which cultivate mainly M. edulis, and the putative causal agents remain unknown. We report the results of the first generation of mass selection focused on survival and resistance to the spring mortality in M. edulis. Two stocks of mussels were sampled in the Pertuis Charentais Sounds in 2014; one of these went through a spring mortality outbreak and the survivors were used to produce the selected stock, while the second stock was M. edulis considered to be naïve against the putative causal agents of the mortality outbreak and was used to produce the control stock. A first cohort was produced in 2015 and tested at one site in the Pertuis Charentais Sounds. In April and May 2016, a spring mortality outbreak was observed when seawater temperature ranged from 10 to 15 °C. In October 2016, the selected stock had a lower mortality (44%) than the control stock (78%). To confirm this result and investigate genotype by environment interaction, a second cohort was produced in 2016 and tested at six sites. Although no significant mortality was reported at the four sites in the Pertuis Charentais Sounds in 2017, the selected stock had a lower mortality (17–27%) than the control stock (61–74%) at the two sites along the coast of the Pays de la Loire. The survival had increased of 34–48% after one generation of mass selection and the realized heritability was high, ranging from 0.55 to 1.15, but further generations of selection are required to obtain a better estimation of the heritability. Our study showed a positive response to selection in three environments that were different to the site from which mussels were selected, suggesting the absence of strong genotype by environment interaction in sites where spring mortality outbreaks occurred. Selection to enhance M. edulis survival of the spring mortality should be efficient and should be capable of easy implementation through mass selection. Apparently, selection to enhance the survival of M. edulis did not affect the mussel growth in comparison with the controls, but automatically improved the yield of M. edulis in sites affected by the spring mortality outbreaks

Single or dual experimental infections with Vibrio aestuarianus and OsHV-1 in diploid and triploid Crassostrea gigas at the spat, juvenile and adult stages

French production of the Pacific cupped oyster, Crassostrea gigas, is currently threatened by two pathogens, OsHV-1 and V. aestuarianus. While oysters selected for their higher resistance to OsHV-1 are now available for the industry, the impact of V. aestuarianus on such oysters is unknown, especially for triploids. In addition, experimental infection has used the virus or the bacteria alone, but there have been no investigations of dual exposure to these pathogens. This study is the first report of single or dual exposure in spat (Spat1 and Spat2), juvenile and adult naïve oysters. For each of the two stocks evaluated, unselected oysters and oysters selected for their higher resistance to OsHV-1 infection were tested, as well as their triploid siblings of the selected oysters produced using cytochalasin B. We confirmed that resistance to OsHV-1 infection and susceptibility to V. aestuarianus increased with age and size, although selected oysters were not significantly impacted by OsHV-1 whatever their ploidy, size or age. We found different mortality patterns depending on the pathogen tested. The mortality pattern was similar for oysters exposed to OsHV-1 or to both pathogens in the Spat1 trial (4 months old and 1.9 g). The mortality pattern was similar for oysters exposed to V. aestuarianus or to both pathogens in the Adult trial (25 months old and 63.1 g). Surprisingly, mortality was much higher (ranging from 75.9% to 100%), in particular for the selected oysters, for the Spat2 (8 months old/3.9 g) and Juvenile trials (16 months old/18.4 g) given a dual exposure, regardless of the level of selection for OsHV-1 and the ploidy state. Our findings highlight an important threat for oyster farmers: oysters exposed to both pathogens could experience dramatic mortality rates, even in oysters selected for their higher resistance to OsHV-1. Finally, our study demonstrated for the first time that triploid oysters were more susceptible to experimental challenges with V. aestuarianus at the spat stage than their diploid siblings. However, the difference in mortality between the triploids and diploids remained limited and ranged from 22.9% to 6.6% for spat and adults, respectively with a relatively regularly decrease in the difference with increased age