Julien Normand

Responses of diploid and triploid Pacific oysters Crassostrea gigas to Vibrio infection in relation to their reproductive status

Several Vibrio species are known to be pathogenic to the Pacific oyster Crassostrea gigas. Survival varies according to pathogen exposure and high mortality events usually occur in summer during gametogenesis. In order to study the effects of gametogenetic status and ploidy (a factor known to affect reproduction allocation in oysters) on vibriosis survival, we conducted two successive experiments. Our results demonstrate that a common bath challenge with pathogenic Vibrio splendidus and Vibrio aestuarianus on a mixture of mature, spawning and non-mature oysters can lead to significant mortality. Previous bath challenges, which were done using only non-mature oysters, had not produced mortality. Immunohistochemical analyses showed the affinity of Vibrio for gonadic tissues, highlighting the importance of sexual maturity for vibriosis infection processes in oysters. Mortality rate results showed poor repeatability between tanks, however, in this bath challenge. We then tested a standardized and repeatable injection protocol using two different doses of the same combination of two Vibrio species on related diploid and triploid oysters at four different times over a year. Statistical analyses of mortality kinetics over a 6-day period after injection revealed that active gametogenesis periods correspond to higher susceptibility to vibriosis and that there is a significant interaction of this seasonal effect with ploidy. However, no significant advantage of triploidy was observed. Triploid oysters even showed lower survival than diploid counterparts in winter. Results are discussed in relation to differing energy allocation patterns between diploid and triploid Pacific oysters.

Contrasted survival under field or controlled conditions displays associations between mRNA levels of candidate genes and response to OsHV-1 infection in the Pacific oyster Crassostrea gigas

Pacific oyster Crassostrea gigas suffers from chronic or sporadic mortality outbreaks worldwide, resulting from infectious diseases and/or physiological disorders triggered by environmental factors. Since 2008, ostreid herpesvirus OsHV-1 μVar has been identified as the main agent responsible for mass mortality of juvenile oysters in Europe. Previous studies of genome-wide expression profiling have provided candidate genes that potentially contribute to genetically-based resistance to summer mortality. To assess their value in determining resistance to the juvenile mass mortality that has occurred in France since 2008, we analyzed the expression of 17 candidate genes in an experimental infection by OsHV-1 μVar, and in an in vivo field experiment. Individual quantification of mRNA levels of 10 out of the 17 targeted genes revealed significant variation, of which 7 genes were showed differences between conditions that created significant differences in mortality, and 6 depended on the number of OsHV-1 genome copies individually quantified in mantle tissue. Complex SOD metalloenzymes known to be part of the antioxidant defense strategies may at least partly determine susceptibility or resistance to OsHV-1-associated mortality. Furthermore, inhibitor 2 of NF-κB, termed CgIκB2, exhibited highly significant variation of mRNA levels depending on OsHV-1 load in both experiments, suggesting its implication in the antiviral immune response of C. gigas. Our results suggest that CgIκB2 expression would make a good starting point for further functional research and that it could be used in marker-assisted selection.

Rearing practices identified as risk factors for ostreid herpesvirus 1 (OsHV-1) infection in Pacific oyster Crassostrea gigas spat

Early detection of Pacific oyster spat infected with ostreid herpesvirus 1 (OsHV-1) could prevent introduction of OsHV-1-infected individuals into farming areas or onshore rearing facilities, thus reducing the risk of infection of naïve oysters in such production systems. Experiments were conducted on several hundred oyster spat provided by producers in order to examine whether early rearing practices could be considered as potential risk factors for (1) OsHV-1 infection as detected by molecular methods and (2) spat mortality experimentally induced through thermal challenge. Spat groups collected on oyster beds and hatchery spat reared in growout areas during summer exhibited higher viral DNA contamination and mortalities during the trial than spat kept in onshore rearing facilities. Quantification of viral DNA before and during the trial showed that infection prevalence and intensity changed over time and revealed latent infection initially unsuspected in 3 of 10 groups. Thermal challenge induced a clear increase in the probability of detecting infected individuals, particularly for groups exhibiting significant prevalence of OsHV-1-contaminated spat prior to the challenge. The use of detection methods are discussed in relation to early rearing practices and disease control strategies.

Effect of oral exposure to the acaricide pirimicarb, a new varroacide candidate, on Apis mellifera feeding rate: Effect of pirimicarb on Apis mellifera feeding rate

BACKGROUND The ectoparasitic honey bee mite Varroa destructor is a main cause of the gradual decline in honey bees Apis mellifera. Beekeepers currently utilize a wide range of different synthetic acaricides, organic acids and essential oils to keep mite populations under control. Previous work has indicated that pirimicarb may be a new varroacide candidate. The aim of this study was to observe chronic effects on feeding activity in worker honey bees after oral exposure to 1.05 mm pirimicarb. The long-term effects of 24 h exposure to pirimicarb were also tested. RESULTS After three successive trials, no mortality could be detected at the tested concentration, although oral exposure to pirimicarb had a significant effect on honey bees feeding behavior. Pirimicarb added to a sucrose solution led to a rapid decrease in food intake. These tendencies may be reversed when the pesticide is removed. However, recovery seemed to be trial dependent. CONCLUSION This study highlights seasonal variation in honey bee susceptibility, which should be considered in toxicology studies.