Genetically based resistance to summer mortality in the Pacific oyster (Crassostrea gigas) and its relationship with physiological, immunological characteristics and infection processes

Abstract

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

Hemolymph microbiome of Pacific oysters in response to temperature, temperature stress and infection

“…The outcome in temperate regions can be much less predictable, depending on thermal optima and ranges of hosts and pathogens (Thomas and Blanford, 2003). For example, temperatures over 20 1C are necessary for oyster summer mortalities to occur (Samain et al, 2007;Watermann et al, 2008), but it is low temperatures (o14 1C) that promote development of brown ring disease in clams (Paillard et al, 2004).…”

“…Temperature stress, on the other hand, severely compromised host defenses (Malham et al, 2009), illustrating the need to examine temperature effects in broader context of animal condition and history. Mortalities observed in wild populations of marine poikilotherms are often due to complex interplay of multiple stressors, such as in the well-documented case of oyster summer mortalities, (for example, Samain et al, 2007;Wendling and Wegner, 2013) or Vibrio harveyi infection in abalones (Travers et al, 2008). Controlled experiments are therefore the only way to assess the importance and relative contributions of individual factors.…”

Comparative physiological, biochemical, and molecular thermal stress response profiles for two Unionid freshwater mussel species

“…However, integrative experimental approaches have also successfully yielded results more applicable to conservation and management (Dégremont et al, 2007;Clark et al, 2001;Samain et al, 2007). Furthermore, correlating gene expression data with physiological and traditional life history parameters in response to chronic sub-lethal heat stress has become more prevalent.…”

Ostreid herpesvirus: A pathogen of oysters

“…However, when the oysters are put in the sea, the disease can appear again, probably because the virus can be in the environment. Another way to fight the disease is the production of polyploid oysters: since better survival was recorded using triploid oyster [47], because they commonly have lower gonadal activity, are less fertile than diploids [48] and spare energy to more effectively fight the viral disease. Recently together to polyploid production, studies on genetic selection with the purpose of obtaining a more genetically resistent oysters strain/family have started [49,[50][51][52].…”