Abstract:
Over the last decade, the Pacific oyster, Crassostrea gigas, an important aquaculture species, has been severely affected by the infection of Ostreid herpesvirus 1 (OsHV-1). This virus has caused significant Pacific oyster mortalities in many countries, including New Zealand. Selective breeding has been suggested as an effective method of producing OsHV-1-resistant Pacific oysters. For the determination of OsHV-1 resistance, oysters have needed to be deployed on farms for challenging with OsHV-1. This approach is time-consuming, expensive and affected by annual, seasonal and spatial environmental variability. The aim of this study was to develop laboratory-based methods to quantify the phenotypic trait of OsHV-1 resistance in oysters, and to compare oyster with these traits with their survival on farms. Two proxy challenges were investigated. Survival of Pacific oysters after exposure to heat shock and zinc ion toxicity was a moderately heritable trait (h2 = 0.15 for heat shock and 0.26 for zinc ion toxicity, p < 0.01). Both challenges proved unsuitable as proxies for on-farm OsHV-1challenge, primarily due to the weak correlations with the survival of naïve juvenile siblings on two farms (p > 0.05 for both heat shock and zinc ion toxicity). A laboratory virus challenge for juvenile oysters was established. The heritability for the survival in this challenge was high (h2 = 0.45, p < 0.001) and it correlated strongly with on-farm survival (p < 0.001). As a tool for selective breeding of OsHV-1 resistant Pacific oysters, this method is likely to be lower cost and more time-effective than the on farms assessment and it is independent of factors other than the virus. Meaningful research on the interactions between OsHV-1 and the Pacific oyster either on farms or in the laboratory requires the ability to quantify the virus both in tissue and in water samples. Three commercially available DNA extraction kits and four commercially available qPCR kits were assessed for their future use in OsHV-1 DNA detection and quantification. Significant differences were observed in the performances of these reagents and associated procedures. These differences were in part due to the effect of sample type, as well as the practical difficulties of preparing equal size small solid tissue samples.
