Probing the die-off of Pacific oysters

[Music] Welcome to Science Sessions, the podcast of the proceedings of the National Academy of Sciences, where we connect you with Academy members, researchers, and policymakers. Join us as we explore the stories behind the science. I'm Paul Gaprelson. Mass mortality events in specific oysters can be devastating to small oyster farming operations. In many cases, the causes of oyster diodes aren't known. In a recent PNAS study, Curtis Suttle and Kevin Xu Zhong of the University of British Columbia and colleagues, found a previously unknown Nido virus in farm oysters. There's not enough evidence yet to conclusively link the virus to mortality events, and there's no sign at all that the virus would be harmful to people who eat the oysters. There are, however, some characteristics of the virus that are remarkable, and different enough from other Nido viruses to merit classification of a distinct virus family. Curtis, how important are Pacific oysters to aquaculture? They are, by far, the most important shellfish globally. It's a multi-billion-dollar industry. It's cultured in Europe, in France, in UK, in Australia, in South America, in British Columbia, where we're located. It's also really, really important because it supports individual growers. Many of them are family-operated, many of them are indigenous First Nations communities. So when you go out and you talk to people, they'll come up to you and tell you about how a mortality event is really, really devastated their ability to generate income for their families. Tell us about the mass mortality events that have been observed. It's a worldwide phenomenon, and it's been going on for a long period of time. So these mortality events usually happen incredibly quickly, and they're devastating because they affect adult oysters, primarily, usually at near the time of harvest. We had a project called Sustainable Oyster Aquaculture that we've been working on since about 2017, and we were actually trying to understand what the causative agents of some of these mortality events are. In some cases, they're known, things like curfews viruses and bacteria like fibriosis, even proteins, which are devastating to oyster aquaculture. But in many cases, it's not known, and it doesn't seem to be any of those particular organisms. And so we were actually on the coast traveling around looking at oysters, and we came into an area where there's a lot of aquaculture going on, and we actually came in to get water for the boat, and we came in and they were pulling up racks of oysters, and it just couldn't believe it because literally every oyster was dead or dying. Kevin, how did you go about identifying the night of virus associated with oyster mortality events? So we didn't identify the cause of mortality events, we just showed these viruses strongly associated with mortality events. In this farmed oyster, we sampled them, and then we extra nucleosid, genetic material, we could RNA and DNA inside the oyster. And we sequenced all those genetic information, and we see who is replicating the oyster. We use cutting-edge bioinformatical tools to assemble the genome. The way we identified the possible aging, associated with mortality events, we see which kind of virus is replicating in the disease oyster, but not in the healthy oyster in the beach surrounding this farm area. So we think about 10 more locations around these two farms, impacted by this virus. And we just saw this specific ion virus, P-On-V1, phosphoryloreous and nitrovirus, which is occurring in this disease oyster, but not in the healthy oyster. What's notable about the night of virus you identified? First, there is the Hesla1 of the largest genome, known for ion virus, which is 64 KB. That one's the kilo-base pair of the nucleosid. Most ion viruses are very usually very small, the genome is often around KB, or even like coronaviruses, about 30 KB. And this big size really pushing the boundary, or what we know for the ion virus, and has a bigger implication for understanding the virus evolution. And the second thing says this virus has a unique genome architecture, so have double segments, one segment which is 41 KB, and the other segment which is 22 KB. And double segment genome for nitrovirus, which is an unusual. And also this virus has a specific protein domain that cannot be found in the other night of virus, which suggests that there was a specific virus hosting the interaction mechanism that cannot be found in the other night of virus. The third thing about what's notable about this virus is that we found this virus is widespread in the Pacific oyster. We found this virus in China, in France. You can chase back to the data in 2012. That means this virus has been there for a long time, is until now we reveal them to the public. Tell us about the nitrovirus family you identified. We found this nitrovirus, which is evolutionally divergent. So we proposed one new family called Megah I-V-E days. So Megah means giant, bigger genome size. I-8 that means bigger I-V-E days. So this is a family. So this P-O-V-U-L virus can be found in France in China, is widely spread not only here. And also this family, the virus we found, they only mostly found in the Pacific oyster, not in the other oyster species. Certainly, that means this virus has a fiery species of host-a-age. So if it turns out to be true, a family that's associated with only a single tax on, for example, a single species, that would be pretty unusual. So we can't say that that's the case right now, but we don't have evidence that the family is broader than that. And so that would, to me at least, would indicate a tight, co-evolutionary relationship between Pacific oysters and this group of viruses. What are the takeaways of your study for oyster farmers and regulators? That's the question that people always ask, but this is what the oyster growers want to know. Okay, so you've told us what might be killing our oysters. So how do you stop it killing our oysters? Now that we know that we have a pathogen, we can now start to interrogate for that particular virus. So now we can ask the question if we go back to other oysters in which we've seen mass mortality events. Is this virus associated with it? One of the things that's a bit unusual about the oyster industry is that historically it's really relied on translocation of the tiny little oysters which are smaller than the size of your pinky fingernail. Oysters are actually reared initially in hatcheries, and then the seed or the spat are sent to growers. So the people that do the growing don't produce the oyster seed typically, right? So there's been mass mortality events in hatcheries that people don't understand, but now we can start asking the questions, did the virus potentially come in the seed? Historically, in British Columbia, seed has been purchased from the United States, from Chile, from Europe, and brought into the country, and we see these mass mortality events on the farm, in this case, but it was no evidence of the virus at all in the adjacent oysters on the beach. So it makes us wonder, did the viruses come in with the seed, for example? Is that why they saw a mass mortality event? If we can start understanding where the virus is, how it comes to dominate the system or how it spreads, then we can start thinking about things like best practices, for example. Now that we have something which is likely associated with disease, the next thing is to find out, does it really cause mass mortality? And so that's one thing we didn't look at in this paper, it's a complex thing because you have to start doing challenge studies and things like that. What are the caveats or limitations of the study? We don't know that this virus actually causes mass mortality, so that's a major caveat. We found a virus which belongs to a group of viruses which are known to cause disease in other kinds of organisms, and so they very likely could cause disease in oysters. And they're clearly replicating in oysters, but that doesn't mean that they're necessary, the causative agent of a mass mortality event. So you can imagine we've got climate change going on, we've got oysters that are being exposed to different environmental conditions and the growers have ever had to try and deal with before. A lot of those things put stress on oysters, right? And so they have fairly complex immune system as well, so you can have stressors on that immune system which could allow something else to kind of propagate and take over. But ultimately, these things are probably going to be not a single factor. The immune system could be suppressed. They could be infighting another kind of infection, and then a neuro virus comes in and just finds this a great environment to replicate in because the oysters are fighting something else off. So that's why these stories are really, really complicated and how even in the human scenario, when the new virus is discovered, trying to figure out whether it's actually a causative agent or disease or not can be an extremely difficult proposition. Thanks for tuning into science sessions. You can subscribe to science sessions on iTunes, Spotify, or wherever you get your podcasts. If you like this episode, please consider leaving your review and helping us spread the word. (gentle music)