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Breeding a Pacific Oyster Virus Resilience

Monday 11 August 2014

With the results of the second field trial just in, Cawthron scientists are optimistic that virus resistance is genetically controlled and so they will be able to breed a resistant stock for commercial production.

The feared OsHV-1?-variant devastated North Island oyster farms when it appeared in 2010. Some British oyster beds were wiped out the same year.

The virus may have been present in oyster beds for ages with no discernible impact.

Environmental changes may have made the virus more aggressive or the oysters more vulnerable to infection. Alternatively, the virus may have mutated or arrived from overseas. Nobody knows.

What was clear, was the industry was taking a huge hit from a virus that could spread astonishingly rapidly throughout a bay or harbour within a few days of the first signs of trouble. The virus kills all ages of Pacific oyster, with juvenile stocks especially susceptible. Something had to be done.

Based on an educated guess, and hope that there was enough natural genetic variation in the introduced oyster population to make selecting for virus resistance successful, Cawthron and the oyster industry initiated virus research at the Cawthron Aquaculture Park just out of Nelson.

The industry selected and collected a wide range of brood stock from the wild and set up experimental sites at Waiheke Island off Auckland and in Whangaroa Harbour in the far north. The sites were carefully set out so the oysters would get broadly equal exposure to the virus.

Don Collier of Aotearoa Fisheries says they decided on two North Island locations, but sufficiently far apart from each other to be able to validly show survivorship differences between oyster family lines.

According to Cawthron shellfish genetics and breeding scientist Mark Camara, who has worked on the numbers, the strong differences between families which showed up are a good indication the population could be selectively bred.

“There are groups of related individuals who have very high survival in the face of this virus and others that don’t. So the trick is to identify the good ones and use these in our next breeding round to make more of them and eventually move the whole population far enough in that direction so they can be farmed profitably,” he said.

Cawthron Cultured Shellfish Programme leader Nick King says that because there are numerous oyster families with high resistance, there is enough genetic variation to also select for other traits such as growth rate and shell shape, or even for resistance to another virus turning up in the future.

“With this much genetic variation, we’re able to focus on the top end of the distribution but still maintain a quite broad population, which gives us the ability to choose from a wide range of families with different characteristics.”

Don Collier says breeding for virus resistance is the absolute priority for the breeding programme. He says the initial field trial results provided some promising results.

However the proof of true success will be when oyster progeny from the programme are grown in commercial volumes right through to maturity, in a now virus laden environment.

Starting out, though, the scientists weren’t sure there would be a genetic basis for susceptibility, or if there was, whether it could be detected through the environmental “noise” of where individual oysters were located or how they were managed.

The current programme has benefited from 12 years of breeding work on Pacific oysters which was also a collaborative project between industry (Aotearoa Fisheries Ltd) and Cawthron.

The selective breeding work is financially supported by the MPI Sustainable Farming Fund project “Oyster Industry Modernisation”, which was awarded to the New Zealand Oyster Industry Association. That original programme was for quality traits, such as growth rate and shell appearance, and was more predictable and straightforward.

“Breeding for disease resistance is harder and more unpredictable,“ Nick King admits.

“In comparison, it’s easier to put oysters out and measure how fast they grow or how heavy they are at harvest, and you can look at the shell and decide whether you like its shape or not.”

“But if you put oysters out in the field for disease testing, you can’t know that they are going to get a consistent exposure to the virus. You don’t know if their susceptibility varies depending on the farm or the other stresses there. You can’t even be sure at the outset that there’s a genetic basis to the resistance. They are all unknowns until you get a generation or two down the track,” Nick said.

The virus outbreak and the resulting breeding project made the scientists take a wider view in their research, to anticipate another mutation or more environment change.

“We tend to focus a lot on the oyster itself. But this made us really concentrate on the epidemiological triangle; you get the host, you get the pathogen and you get the environment. And they are all interlinked. You can have a resistant oyster but if you stress it enough it’s probably going to succumb to the virus anyway, so you need to understand these complex interactions.”

Taking such a wider perspective means he is reluctant to rule out factors such as ocean acidification for the virus outbreak, not directly, but potentially as another stressor that drive the oysters over the edge.

Overseas research on the virus shows it can’t function in water temperatures lower than 16 degrees. Unfortunately nothing much can be done totypo3/# control water temperature on an oyster farm.

Another potential way to control the virus is to have the oysters located sufficiently above the low tide mark. The persuasive downside of this solution is that this also slows their growth right down.

Trying to isolate oysters from the virus is unlikely to work in open oyster farms. Scientists don’t know how it spreads, nor why it spreads so rapidly. Two theories are that the virus may attach itself to sediment particles, or the oysters may be infected by ingesting infected oysters’ sperm and eggs.

Long term, Mark Camara acknowledges that the virus may mutate, as viruses do, and become active again.

“There’s nothing you can do about that. You just have to live with the possibility and try to be prepared”.

He says that it’s taken centuries to successfully control most livestock diseases on land. Similar work is just starting in aquaculture. One advantage in shellfish such as oysters, is their huge reproductive output, many millions of eggs per female, which makes it possible to apply very high selection pressure.

"There is enough genetic variation to also select for other traits such as growth rate and shell shape"

Nick King thinks sequencing genomes, as was done with oysters a couple of years ago, has huge prospects for breeding, though identifying the specific genes that control desirable traits and understanding their significance and potential will take time and money.
He says since shellfish have not been domesticated there is a lot of natural genetic variation which still offers a lot of low hanging fruit which traditional selective breeding can utilise.

Gazing into his crystal ball across species of shellfish, Nick King says it is probably not too much to ask if we parallel the kiwifruit industry, which started out with a green variety, and then added a gold one. Perhaps a golden shell mussel, to go with the green one?

These sorts of developments sometimes can be achieved quite rapidly because compared to growth rate and disease resistance they often have a very simple genetic basis and aren’t strongly influenced by the environment.

For the OsHV-1? resistance Nick King initially estimated the oyster breeding would take 10 years. He’s now hopeful about offering a resilient oyster stock to the industry and that may be within another year.

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