Great White Shark Populations Have Interesting DNA

0:00 Today I'm gonna be talking about great white shark DNA, and you're probably wondering why. What does it matter? Because it's an enigma, it's a mystery. It's generally just weird. And we're gonna talk about why it's important to talk about how weird great white shark, DNA is and why we need to understand how it happened and what's happening. That's what we're gonna talk about on this episode of the How to Protect the Ocean Podcast. Let's start the show. Hey everybody. Welcome back to another exciting episode of the How to Protect the Ocean Podcast. I'm your host, Andrew Lewin, and this is the podcast where you find out what's happening with the ocean, how you can speak up for the ocean, and what you can do to live for a better ocean by taking action. On today's episode, we're gonna be talking about an article that came out in Science Alert.

0:44 It was a really interesting article. It came out earlier in August, but it was something that I found that needed to be shared. Carly Cassella, I believe is how you pronounce it. This is part of the nature category for Science. Alert wrote this and she titled it, The DNA of Great White Sharks Defies Explanation and Here is Why. So let's talk about great white sharks. Like great white sharks, since the movie Jaws, and by the way, it's Jaw's 50th anniversary this year, back in June, have really captured the minds and the fears of humans all around the world. It captured it so much that after people watching Jaws, including myself, I was a kid when I watched Jaws. I was probably nine or 10 when I first watched it. I had trouble swimming in pools. I had trouble swimming in lakes. I always thought like, there's a great white in my bathtub potentially.

1:27 The imagination of sharks and what they could do and where they could go after watching Jaws was amazing. They became feared quite a bit. It also spurred a lot of funding to find out more about great white sharks, and we have learned a lot because the funding has been there, researchers have been there, and if you listen to Beyond Jaws in any point in time, you know that there is literally a generation of scientists that we coined the jaws generation because that's how they cut their teeth so to speak, no pun intended, within the shark science world. And they learned a lot more, not only about great whites, but a lot of other sharks. but it is different. Like great white sharks are built different and they're a great conservation success story, not only on the west coast, but also on the east coast of North America.

2:14 And we're seeing more and more and we're seeing them, just, I guess, lengthen or increase their distribution every day. Now we're starting to see more and more great white sharks in Nova Scotia, which was a rare sighting, just a few, like five or six years ago. I don't know if it's just 'cause we're searching more, that we have more researchers out there looking at it. But it didn't happen often. Now it's happening this summer seems to be the summer of the great white shark over in Nova Scotia, and we've learned a lot about this, especially with tracking sharks. We've learned about where they go. We've learned about the different populations, and that's where this article comes in, It used to be before the Ice age 10,000 years ago, there was one population of great white sharks, genetically. That's what it was. Now there's a distinct three groups that have emerged.

2:57 So one in the North Pacific, one in the South Pacific in Indian Ocean, and one in the North Atlantic and Mediterranean. So this is intriguing to say the least, right? Because you know, the genetic evidence suggests that these groups all stem from a single population that survived the Ice Age. So 10,000 years ago before spreading into today's three different groups. Yet, despite knowing these genetic divisions exist, mitochondrial DNA and nuclear DNA do not line up, meaning that the common explanations like migration and breeding patterns aren't enough to explain the three different groups. So the mystery remains unsolved and so this is what you kind of need to know.

3:37 Before we get into trying to explain what's been happening. You might be wondering why we care about a genetic mismatch. You know, understanding population structure is essential for conservation. So population resilience. So genetic diversity helps population adapt to change. the more diverse you have populations, so the genetic diversity of each population matters, right? Because if you lose one population, you'll still get another population. 'cause the genetic differences might be the reason why that population survives. So for instance, if you have two shark populations with different genetic makeups and there's one population that just can't find, like say it has a food that it really likes one day and that food disappears, then, you know, you lose that population.

4:20 But the other population that's genetically different has the ability to be able to say, Hey, you know what, I can eat a bunch of different things so I don't have to worry about this population that's gone, but you still have a great white there. So having that genetic differences, whatever reason, a population disappears, whether it's food, whether it's an ice age, whatever that might be. this is really important to stabilize the overall species. It's exactly how a lot of the zoos and aquariums are founded on the basis that they're founded on, and how their breeding programs work to ensure that they preserve the genetic diversity of a species in order to ever, they have to, bring those back to the wild to repopulate and to stabilize the species that may have been just degraded of their species, diversity in the wild for one reason or another.

5:06 So having that resilience is really important. Genetic diversity is such an important aspect of biodiversity and just conservation management and also management planning. If distinct groups exist, protections must be tailored to each of those regions. Not a one size fits all sort of area. So the genetic differences might require unique management approaches, and that's gonna be important in conservation. And then of course, there's the threat assessment. Knowing how populations intermix helps us track human impacts, climate effects as well as fishing pressure and more. So this is something that's really important. So basically, in short, resolving the DNA mysteries gives us the tools to better protect great whites and the ecosystems they anchor.

5:48 You have to remember that. When a great white is in the mix of a population of a food web, it controls that food web a lot of the times, unless there's orcas around. But when you see the disappearance of great whites, whether it be 'cause of orcas or whether it be 'cause of phishing pressure or lack of food, it changes the makeup of a food web, which changes the entire ecosystem, whether it be for good or for bad, depending on what you consider good and bad in an area, but it makes change. So ensuring that these populations stay intact and make sure that we have conservation measures to make sure these great whites are there, then it makes a difference even as great whites move into new ecosystems because of climate change, warmer waters, or the fact that they're trying to get away from competition and extend their range that also matters in terms of how we conserve a specific area.

6:32 'cause a number of great whites showing up within an ecosystem will change that food web of that ecosystem and change the predator prey dynamics. That's really important. So it's a big thing of why it matters, right? It's something that we need to know. But let's get back into the genetic puzzle because like diving into this article, they're wondering, how does this happen? How do we see differences in mitochondrial DNA and or nucleus DNA? that's a big thing. And we're gonna get into a bit of science here, but I think it's important to understand the difference between the different DNA, right? Because that's something that's really important. So nuclear DNA is packaged inside the nucleus of a cell. So that's why it's called nuclear DNA, hence the name, right? But mitochondrial DNA is packaged inside the mitochondria, which turns out energy out for the cell.

7:17 So very different sort of functions of mitochondrial DNA and nuclear DNA. So unlike nuclear DNA, this is quoting from the article, which is inherited from both parents, mitochondrial DNA is thought to be inherited from the mother in most multicellular animals, sharks included. So because mitochondrial DNA can trace a maternal line, conservation biologists have used it for years to identify population boundaries and migration paths. So what that looks like is that, when you start to look at how, especially sharks, some sharks, the mothers will go back to the place where they bred last. So it's called philopatry. I don't know if I'm pronouncing it, this is a new term for me, but essentially what happens is the mother will go back to the same place and will mix DNA with the male and then will put out some offspring.

8:07 That is important to tracing where things are going. But however, when it comes to great whites, the method isn't working. So even after using one of the largest data sets of great white sharks globally, researchers have come up like empty handed. So previously scientists suspected that changes in the mitochondrial DNA were due to female sharks returning to the birthplace to reproduce a concept known as females philopatry, I think I'm pronouncing that properly, but the hypothesis is even supported by the recent observational evidence, which suggests that while both male and female sharks travel vast distances, females return home when it's time to mate. When the researchers and one of the main researchers of Gavin Naylor and colleagues put that into the idea, to the test, however, it failed to explain the groups of mitochondrial DNA.

8:52 So the sequencing of 150 great white sharks from around the world, the sequencing of the genes, Naylor and his team found no evidence of female philopatry. A small signal would be expected in nuclear DNA if females were the only breeding within certain populations. But that wasn't reflected in the nuclear data at all. So even when the team ran an evolutionary simulation showing how sharks might have split off into three groups since their last shared ancestor, it came up that the hypothesis didn't stand. Gavin Naylor's quote is, I came up with the idea that sex ratios might be different that just a few females were contributing to the populations from one generation to the next. So like only a few females were reproducing.

9:34 That also failed to explain the genetic differences so did random genetic changes that accumulate over time called genetic drift. There's a really cool graphic here looking at divergence of great white lineages. So I highly recommend that you click the link in the description to get the article. Because they're looking at, you know, divergence of great white sharks lineages based on genome sequencing, it's really cool 'cause you see how there are some DNA that's mixed into these different populations and sometimes it just doesn't make sense. The team argues that an alternative evolutionary mechanism must necessarily be operating. So there's something else going on here. Right, and so, but the only other known explanation is the natural selection may have honed in each group's mitochondrial DNA, and that seems pretty far fetched.

10:16 So there are only 20,000 great white sharks in the world, which is a very small population. Relatively speaking, when you look at other species, if there's something beneficial in the evolution of some forms of mitochondrial DNA, then it would have to save the sharks from something brutally lethal. This is what Gavin's saying, Gavin Naylor. So Gavin has doubts that this is the case, like some piece of the puzzle is clearly missing. As he says, the mitochondrial DNA observed in natural populations was never reproduced in any of the simulations, even under extreme phenol philopatry, suggesting that other forces have contributed to the discordance. Right. So the same approach would benefit other species of shark where female philopatry has previously been assumed based on genetic data.

11:00 So there's a lot of mystery around here, which we don't even know why this is happening, right? We just don't understand it. But to understand the DNA could be a key to unlocking how to manage great white sharks properly and how to manage other species that might show the same thing. You know, there are other sharks that have similar philopatry, like female philopatry. Some of them include uh, I'm just looking here. I have a list. Lemon sharks, blacktip sharks, sandbar sharks, tiger sharks, nurse sharks, reef associated sharks, like, whitetip reef sharks. That's a huge thing. And you are probably wondering why does it matter? 'cause returning to specific popping grounds means that the protection of small critical nursery habitats can safeguard generations of sharks.

11:42 So just having a marine protected area in a popping zone, that is huge to protecting these sharks. Of course, keeping the genetic structure. So philopatry leads to distinct subpopulations, making localized overfishing more damaging. So being careful of that and management using marine protected areas must consider these natal sites to be effective. So when we actually look and we want to make sure that everything is protected, we can look at these smaller sites and say, Hey, this is a popping zone. We know this is a popping zone because these sharks keep coming back to this area, these female sharks. This is important. We have to protect this. This is a very important area. If the sharks get fished during this time, it's gonna destroy the population eventually. So that's something that's very important as well.

12:24 So that's where we're gonna leave it. It's really just a mystery. It's weird. Great white shark DNA is weird. And we have to understand that we're gonna need more research to find out why it's so weird and how to explain these diversions into these three groups. Very interesting article. Something that I want to share with you. So if you have any questions or comments, please let me know in the comments. If you're watching this on YouTube or you want to get ahold of me, there's a number of different ways. You can go to Instagram at How to Protect the Ocean. Just message me. You can go to speak up for blue.com/contact. Just fill out the form, goes to my email, or you can hit me up on TikTok. Just DM me, at speak Up for Blue. I wanna thank you so much for joining me on today's episode of the How to Protect the Ocean Podcast.

13:06 I'm your host, Andrew Lewin from the True Nordstrom and Free. Have a great day. We'll talk to you next time and happy conservation.