International audienceSummer mortality of Pacific oysters is known in several countries. However no specific pathogen has been systematically associated with this phenomenon. A complex combination of environmental and biological parameters has been suggested as the cause and is now starting to be identified. A high genetic basis was found for survival in oysters when a first generation (G1) was tested in three sites during summer. This paper presents a synthesis on physiological characteristics of two selected groups (‘R' and ‘S', from families selected for resistance and susceptibility to summer mortality respectively), of the second and third generations. R and S showed improvement or reduction of survival compared with the control in both field and laboratory trials confirming the high heritability of survival of juveniles <1 year old. Interestingly, no correlation was observed between growth and survival. Comparison between the two selected groups showed that S oysters invested more energy in reproduction and stayed a longer time without spawning than R oysters which had high synchronous spawning. This was mainly shown with high rather than low dietary rations (respectively 12% and 4% DW algae/DW oyster) in a controlled experiment. Moreover, early partial spawning was detected in S oysters and not R ones in the high dietary ration. S showed a higher respiration rate and an earlier decrease in absorption efficiency than R during gametogenesis, but they were not significantly different in glycogen or ATP utilisation. Two months before a mortality episode, hemocytes from S oysters had a higher adhesive capacity than R hemocytes and significantly higher reactive oxygen species production capacity. One month before mortality, S oysters had the highest hyalinocyte concentration and their expression of genes coding for glucose metabolism enzymes (Hexokinase, GS, PGM, PEPCK) was significantly lower in the labial palps. After a thermal increase from 13 °C to 19 °C, during 8 days in normoxia, S oysters showed a large HSP70 increase under hypoxia contrary to R oysters, suggesting their high susceptibility to stress. Their catalase activity was lower than in R oysters and showed no further change to subsequent hypoxia and pesticide stresses, in contrast to R oysters. These observations suggest possible links between higher reproductive effort in S oysters, their specific stress response to temperature and hypoxia, ROS production, partial spawning, hyalinocyte increase and the infection process. To compare R and S oysters in a more integrated way, a suppression subtractive hybridisation (SSH) library and a micro-array strategy are being undertaken

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Summer mortality has been reported in the Pacific oyster, Crassostrea gigas, for many years in different parts of the world. The causes of this phenomenon are complex. The multidisciplinary program "MOREST", coordinated by IFREMER, was initiated to understand the causes of summer mortality of Crassostrea gigas juveniles in France and to reduce its impact on oyster production. Within this program, three successive groups of bi-parental families were bred in a hatchery in 2001 and placed in the field during summer in three sites (Ronce, Rivière d'Auray, and Baie des Veys). This paper reports the relative importance of family, site and field placement timing for three characters of major importance for oyster production: survival, growth, and yield. At the end of the summer period, significant differences for the three characters were observed among sites and families for each group. Family effect was the largest variance component for survival, representing 46% of the total. Variance component analysis revealed that variation in yield among families depended either on survival or on growth according to the site. Significant family × environment interactions were observed for yield and survival but not for growth. No difference in survival was found among groups in the three sites at the end of the experiment, but a critical period of mortality was identified from late July until early September. The influence of environmental conditions, notably on reproductive allocation and its relationship with the studied traits, is discussed.

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.

Correlation between DNA microsatellite heterozygosity and growth rate was investigated in two hatchery-propagated stocks of the shrimp Penaeus stylirostris, which had been genetically isolated from wild founders for 17 generations, and from each other for five more generations. Presumed demographic history of these populations suggested that they were maintained at small effective population sizes, and this was confirmed by significant changes in allelic frequencies between the two stocks. Despite a small sample size (n=60) and a small number of loci screened, a significant positive correlation was detected between microsatellite tri-locus heterozygosity and growth rate in one of the two stocks (r=0.3, P=0.02). In the other stock (n=48), the positive correlation was not significant, but the combined test of single locus heterozygote advantage over the three loci was significant (P=0.005). Use of DNA microsatellite markers, whose selectively neutral status is generally accepted, allowed us to reject the hypothesis of direct overdominance at marker loci as an explanation for an association of growth rate with heterozygosity. The average depression associated with one locus in our case is above or equivalent to the amount observed for a 10% increase of inbreeding in other species. These results suggest that heterozygosity at neutral marker loci is sufficiently well correlated with individual inbreeding coefficients to reveal a significant residual inbreeding load for growth rate in Tahitian P. stylirostris stocks. Inbreeding during hatchery propagation was, thus, insufficient to purge the ancestral load.