Progress in selective breeding for resilience to Ostreid herpes virus in Crassostrea gigasin New Zealand

Zoë HILTON¹, Norman RAGG¹, Hannah MAE¹, Aditya KESARCODI-WATSON¹, Samantha GALE¹, Mark CAMARA¹, Nick KING¹, David BURRITT², Brendon DUNPHY³, Ben MORTENSEN³, Andrea ALFARO⁴, Tim YOUNG⁴.

¹Cawthron Institute, Private Bag 2, Nelson, New Zealand. 

²Department of Botany, University of Otago, Dunedin, New Zealand. 

³School of Biological Sciences, University of Auckland, Auckland, New Zealand.  

⁴Institute for Applied Ecology New Zealand, School of Applied Sciences, Auckland University of Technology, Auckland, New Zealand.

e-mail: zoe.hilton@cawthron.org.nz

Key words: Selective breeding; Ostreid herpes virus; Crassostrea gigas; physiology; growth; oxidative stress

New Zealand’s Pacific oyster industry was devastated by mass mortalities upon the arrival of OsHV-1 µ-Var in 2010. Traditionally, the industry relied predominantly on wild spat capture, but the crisis caused by the arrival of OsHV-1 µ-Var led to major innovation. Collaboration between researchers and industry led to the re-focusing of a selective breeding programme established by the Cawthron Institute for commercially important traits, towards selection for OsHV-1 resilience. Lab and field challenges were developed, and successful selective breeding combined with changes in husbandry practices and stock movements have now allowed almost complete recovery of the industry, with export values now exceeding pre-virus values. After 3 generations, survival of selectively bred spat is now over 80% in 83% of families, compared to < 25% survival in down-selected families (under standardised lab challenge).

Using the third generation of families, an integrative biology approach was used to investigate the mechanisms underlying resilience, as well as to examine potential trade-offs from selection for virus resilience. In conjunction with the larger selective breeding programme, 10 novel pedigreed, selectively bred families were created that exhibited a range of resilience phenotypes, and a wide range of growth potential. These families were then exposed as spat to different food conditions to induce differential rates of growth. We also exposed groups to hypoxia and re-oxygenation stress (7d at 1.44 mg/L O₂ [~ 20% ambient] at 22ºC), and undertook lab challenges to confirm levels of viral resilience. Clearance and assimilation parameters, scope for growth, anaerobic and antioxidant enzyme levels, and ROS damage were measured, and samples were taken for metabolomic analyses.

Results indicated that in these 10 families, growth rate did not correlate with viral resilience or susceptibility, and that resilient fast-growing families had much higher baseline levels of some anaerobic and antioxidant enzymes and a greater capacity to increase these after stress than susceptible families. Resilient families suffered lower effects of ROS damage. These results are helping to inform husbandry as well as new avenues for markers of resilience. Metabolomics results will allow us to examine in greater depth the mechanisms underlying viral resilience in these families.