Selective breeding of the Pacific oyster (Crassostrea gigas) for enhanced resilience to ostreid herpes virus-1 microvariant (OsHV-1 µ-var): a quantitative genetic analysis of field and lab-based challenges.

Ostreid herpesvirus-1 microvariant (OsHV-1 µ-var) emerged in New Zealand in 2010 causing immediate and severe economic losses to the Pacific oyster industry. In response, we modified the breeding objectives of an ongoing family-based selective breeding program in 2011 to focus on improving survival during OsHV-1outbreaks with minimal loss of previous genetic gain for production and quality traits.

The modified selective breeding strategy currently uses field-based challenges of juvenile oysters during periods OsHV-1 activity as the main selection criterion, but field-based trials are costly and potentially compromised by uncontrolled environmental parameters such food availability, water temperature, water quality, and most importantly virus levels and activity on test farms. To address these potential issues we experimented with lab-based assays to simplify and standardize our assessment protocols for OsHV-1 resilience. As part of this effort, we developed and tested a common tank, lab-based live virus challenge protocol using virus isolated and cryo-preserved using techniques developed by Australian researchers at the Elizabeth Macarthur Agricultural Institute and also tested proxy challenges (survival in temperature and heavy metal stress challenges) as potential indicators of general robustness with potentially high additive genetic correlations with OsHV-1 resilience.

We evaluated the same thirty-seven pedigreed Pacific oyster families from the 2013 breeding program cohort in the field and in lab-based temperature, zinc, and live virus challenges and used a multi-variate animal model analysis of the combined data to estimate the single-trait heritabilities of and pairwise genetic correlations between survival in the four challenges. We found that survival in the heat and zinc challenges had no significant additive genetic correlation with survival in the field, but survival in the lab-based virus challenge had a high additive genetic correlation with survival in the field (0.72).  Furthermore, the heritability of survival in the lab-based virus challenge was higher than the heritability of survival in the field challenge (0.45 vs. 0.38).

We conclude that incorporating lab-based live virus challenges into the selective breeding program could enhance the response to selection for OsHV-1 resilience in the New Zealand Pacific oyster breeding program  and creates new opportunities to study the pathogen/host/environment interaction in greater depth and detail by, for example, systematically varying key environmental parameters or the combining multiple stressors under controlled conditions.