Science Friday - A Strange-Looking Fish, Frozen In Time

Episode Date: March 21, 2024

The term “living fossil” has been applied to any number of animals, from sharks to turtles to the coelacanth. It’s the idea that those animals look very much the same way their species may have looked millions of years ago, with limited evolutionary change over that time.After analyzing the genomes of many different species on that “living fossil” list, researchers report they may have found an animal that evolves more slowly than all the others—a group of fish called gar. The rate of molecular change in gar genomes is the slowest of any jawed vertebrate, the researchers say. In fact, gar genomes change so slowly that two gar species that diverged from each other over 105 million years ago can still interbreed and produce fertile offspring. In evolutionary time, that’s comparable to the distance between humans and elephants. The researchers believe that the slow rate of change in gars may be due to an exceptional ability to repair mutations and other errors in their genes.Dr. Solomon David, assistant professor of aquatic ecology at the University of Minnesota, and Chase Brownstein, a graduate student in Yale’s department of ecology and evolutionary biology, join Ira to discuss the findings, recently reported in the journal Evolution.Transcripts for each segment will be available after the show airs on sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Is a fish called the Gar, the king of the living fossils? It's almost like if it ain't broke, don't fix it situation, and they've found this model that works, and they've stayed consistent with it for 150 million years plus. It's Thursday, March 21st. 25 years ago today, Bertrand Picard, and Brian Jones, became the first to circumnavigate the earth in a hot air balloon. But today's also Science Friday.

I'm CyFRI producer Charles Bernquist. There are lots of members. of the Living Fossil Club, sharks, the Tuatara, the Seleacanth, and more. But a new analysis of hundreds of genomes finds that the gars may have the slowest rate of evolution of any jawed vertebrate. Ira Flato talks with two members of that research team about gar evolution and what it could teach people about our own health. Here's Ira. Joining me now to talk about what they've learned about the gar so far are my guests, Dr. Solomon David, assistant professor of aquatic ecology, University of Minnesota in Minneapolis.

Welcome back to Science Friday. Thank you very much, Ira. Happy to be here. And Chase Brownstein, a graduate student in Yale's Department of Ecology and Evolutionary Biology in New Haven, Connecticut. And he is the lead author on that study I just mentioned. Welcome to Science Friday. Thanks so much.

Thanks again for having me. You're welcome. Solomon, for our listeners, who have never seen one, what is a gar? where are they found? What do they look like? Where do they live? Give us a little summation there, because they're kind of weird looking, aren't they? They definitely are an oddball fish. When people ask what a gar looks like, I tell them to picture a crocodile or an alligator with fins instead of legs. And that's basically what a gar is. They have this prehistoric look to them, which kind of matches this living fossil idea. They've got a long snout, lots of teeth. They've got armored scales. They're

typically found in freshwater, but they're found from Canada all the way south to Costa Rica. And some of them can even live in saltwater for quite a long period of time. So that saltwater one would be a sea gar. That's a classic. You have to say it. I have to say it. So let me get out of the way. Chase, when I think of living fossils, I think of a celicath, right?

And you're saying the gar is fossililer than that? Yeah. Well, so first of all, I want to say, you know, there is a species of gar from Cuba. and my lab likes to joke that that's the Cuban Seaggar. So getting the puns right out of the way. So the sealicant discovered to be a lie in 1936, after being known from the fossil record, is a classic example of a living fossil.

But what we're showing here is that, at least in some lineages, namely gars, and then the sturgeons and paddlefishes, actually have a molecular mechanism behind these intriguing patterns of their evolutionary history and modern diversity. In these lineages, there is actually a very low rate of change across the genome. And as it turns out, gars and sturgeons actually have the slowest rates of change by several orders of magnitude throughout their genomes of all jawed vertebrate. So that's animals with a backbone and a jaw. Wow. So how old a lineage are we talking about you? How much of a fossil is this?

So Garz, at least the common ancestor of all living species, is about 105 million years old. So that's actually a little over 30 million years older than Tyrannosaurus Rex. Wow, you've blown my brain on this one. This same fish basically unchanged has been around since before T.Rex. Yes, I would say virtually unchanged. So, I mean, there are differences, right? I mean, these things have evolved. It's just that they're evolving at such a slow rate.

Dr. David, did this surprise you learning all of this? Yeah, you know, I think as someone who studied Garz for 20 years, we find that Garz have been doing their thing for 157 million years. So, you know, there's a lot of catching up to do. So it was surprising, but, you know, if you look at the fossil record, you look at fossil gars, and then you look at gars that are alive today, they look very much the same. So this was really cool to see that they were slower to change than sealicants and tuataras and crocodilians.

I knew there was something special about them, always thought that, but it took Chase leading the charge here to really kind of bring that science to light, which I think is really cool. That is cool. Part of this work involves hybrid cars, right? Solomon, a mix of two species. What's going on there? Why is that significant? Back in the day, those of us that kept aquarium fish, noticed that there were some weird-looking gars. And the fish that's actually been in a lot of the promotion for this study is a hybrid gar that I founded at a pet shop maybe 20 years ago.

And we thought it was something weird. It looked like a combination of two different species, actually genera of gars. And there was a paper that came out in 2008 that so little information was available about gar hybridization that they, cited the pet trade and some website stuff that I posted as being, you know, sort of of initial evidence. And they ended up showing that gars had hybridized in Shed Aquarium in Chicago. And then reports started coming from the wild of long nose and alligator gars hybridizing in parts of Texas. So this is really kind of a building effect of noticing stuff going on in the wild, stuff going into the aquarium trade, and then bringing that to the science,

the analysis that Chase did, to see that this hybridization is. is actually a big deal. How do you get a hybrid gar? Yeah, we think a lot of it is they're spawning or some individuals happen to be spawning at the same time. And typically this ends up being male long-nosed gars spawning around the same time that female alligator gars might be spawning. It could technically go in both directions.

But they're broadcast spawners. They'll release the gametes at the same time. So if you've got male long-nose gars kind of mixing it in with a group of alligator Garz, you could have a hybrid result there. But what's really interesting is that this is crossing genera that diverged over 100 million years ago. And I'm sure Chase could speak more to that. Yeah, Chase. Fill us in on that. Yeah, absolutely. For the hybrid analysis side of the study, credit where credit is due to my amazing co-lead author, Dan McWegan at the University of Buffalo, who really pushed for the genetic

analyses we feature in the paper. And what we had found, actually, is that the hybrid gars are not only viable. So you have this 105 million years split between the two living gars. That's actually older than the split between you and an elephant, basically. Or, you know, it's like slightly older than that. So if you could imagine that these two lineages,

which last share common ancestry, 105 million years ago, are still able to hybridize. And not only that, but we find in this paper that there was strong evidence, that the hybrids are fertile, the hybrids can reproduce. And then thirdly, that the hybrid gars, although they look distinct relative to their

parents, are actually not pushing out of the range of variation we see in fossil and living gars. So in other lineages, including, you know, the African cichlids, which are very famous, and also often kept in aquarium tanks, you actually see this pattern where hybrids between different species can actually show new patterns of anatomy and color and things like that. Garz actually don't do this. So the hybrids are within that realm of variation, and that's pretty important. And the other thing is that they're fertile. So this is the oldest hybrid cross among eukaryotic organisms. That's organisms that have a nucleus where they keep their DNA, right, ever observed in the wild,

older than any other plant lineage, older than any other animal lineage, older than any other fungi that hybridize. And it's incredible. And we're explaining this pattern as a consequence of this extremely slow molecular rate. Do we know why they have this slower rate of change? Yeah. So that is actually sort of the next frontier of our research. There's a variety of explanations that have been proposed for variation in the rates of change in the genome over millions of years. Population size, how long it takes for new generations to appear as well as age. And in other fishes that actually have faster rates, we see a high degree of variance in these features. And Gars aren't like the most special in any of those parameters.

one thing that we think might be going on based on evidence from sturgeons and paddlefishes is an exceptional DNA repair apparatus. When you have a mutation, right, that occurs in a gene, one of the things that your cells do is they can actually try to repair that. They can try to reverse the change. And this process is called DNA repair. And one possibility is that gars and surgeons might just have really, really good DNA repair abilities. like exceptional DNA repair abilities.

An example I like to use is kind of imagine a perfect game of telephone, where the message just doesn't change from person to person to person or from year to year or millions of years. You know, when we talk about gene damage as you have and gene repair, I'm thinking about cancer, right, where genes go bad. Is there something to learn about people here, Solomon? Yeah, I think so. I think from a conservation angle, there's a bit of irony here where fish that are largely thought of as trash fish might end up turning around and being helpful to humans from a human health and medical standpoint.

With a lot of the genomics work that's being done with gars, establishing gars as a potential model organism, they're doing that to Michigan State colleagues, Ingo Brosh and also at nickel state, we've worked on that. So we're establishing Garas as this sort of model organism to help improve our understanding of human evolution, human disease. And so I think there's a lot of potential there when we think about potential transgenics. Can we identify that DNA repair mechanism? And Chase can speak to that particular gene that we think it is. And can we take that further to maybe look at how this responds in other organisms to different types of environmental stimuli from toxins to other types of other types of impact. Chase, can you take it a little further and tell us?

Yeah, absolutely. So that's, I mean, kind of exactly what we plan to do. I mean, this is what my goal is in looking at seeing if we can experimentally verify whether there are certain repair mechanisms in gars that, when expressed in other species, like model organisms like the zebrafish, for example, whether they actually help these organisms survive, what we call mutagenic. environments or substances. These are things that induce mutation and DNA damage. Solomon, if they are evolving so much more slowly, does that mean they'd be slower to adapt to

some kinds of environmental change? I mean, isn't this actually a liability? How are they able to overcome that? Yeah, I think that's a great question. And you would expect that with this low rate of evolution that Gar's may not be the most adaptive. But in truth, they already are extremely adaptive. They can live in salt water. They can live in freshwater. They range from Canada all the way down to Costa Rica. All of the gar species can hybridize. They can hybridize across general, hybridize across species. They have toxic eggs, armored scales. They grow to relatively large sizes. Some of them can live for over 100 years. So it's almost like if it ain't broke, don't fix it situation and they found this model that works and they've stayed consistent with it for 150 million

years plus. You might be familiar and listeners might be familiar with an evolutionary biologist and paleontologist named Stephen Jay Gould. In the 70s, he published a paper called the Spandrels of San Marcos, which is like one of the papers I love that. It's a great thought paper. And he cautioned people against what he called the evolutionary just so stories. In other words, words, you know, the elephant got its trunk because it needed to reach for water, and the giraffe got its long neck because it needed to reach for the trees and so on and so forth. And I think Garz actually provide a really interesting example of why that's maybe not the best way to think about evolution. So one thing that we do is evolutionary biologists is that we

tend to look at groups that have a lot of species in them. And one reason this is because, well, that's a lot of, you know, kind of replicates. We can sort of. of go through that group and see what's going on and see the diversity and, you know, look and see what stories a lineage has to tell. The problem with that approach is that it denies us a view that's given by these species poor living fossil lineages, if you will. What Garz are saying is that they've, for whatever reason, happen to survive for about 100 million years. And they've been able to do it without drastically changing their morphology and it seems like their genotype, their genes.

So one possibility is that it might just be the case that due to a number of historical accidents and sort of contingency, as we call it, these incredible fishes have survived. So Solomon mentioned a bunch of features of the anatomy and life history of GARs that are really quite interesting and unusual. Again, they have this interesting ability to tolerate saltwater and sort of murky, environments. In 2022, I was working with a paleontologist at the Denver Museum, Tyler Lycin. And we actually described the gar that's like five feet long. It's a relative of the alligator gar. And it lived about 1,300 to 1,500 years after the asteroid impact that killed the dinosaurs.

Actually, Tyler is incredible because he and his team collected a block of sediment from just below the gar that actually has the layer that records the asteroid impact. It's right below the gar and he still has that block in the museum. But even though they're able to sort of tolerate these environments and in sort of an exceptional matter that really is quite interesting and that I almost certainly probably help them survive these kind of major cataclysms of our Earth's history, I would caution a view that gars are like perfectly adapted, whatever that might mean, Because that really doesn't mean anything. Adaptation is variable depending on the context, your developmental context, how you're growing up,

whether different parts of your body plan are jiving together and how much you can change your body plan, how the environment changes, which it certainly has. And what Garz are saying in some sense is that you don't necessarily have to be, you know, perfectly adapted or like drastically changing your anatomy to survive. In some respects, you know, this sort of accident question that they've, you know, managed to survive for how long they have is almost more intriguing because it forces us to reckon with the possibility that we might be just getting part of the story from these really species, rits, you know, adaptive radiations, if you will, of life.

Despite all of that, and despite their kind of incredible evolutionary histories, which people like myself and Solomon can geek out on, they're really not recognized in North America. And I got to say that's really sad. Like, Garz and Bofin, in some respects are like our lungfishes or our kangaroos and stuff. They're not found anywhere else. And they have this incredibly ancient evolutionary heritage. Wow, that is a great way to end.

And what is summation because who knew? I mean, certainly not I, and a lot of people who just look at a gar and say, that's kind of weird-looking creature. We have run out of time. I want to thank you both, Dr. Solomon David, Assistant Professor of Aquatic Ecology at the University of Minnesota in Minneapolis, and Chase Brownstein, graduate student in Yale's Department of Ecology and Evolutionary Biology. Thank you for enlightening us today. Gargantuan, thank you for having us. Yes, thank you so much. It's been a pleasure. That's it for today. Tomorrow, we'll check in on some of this week's top stories in science with the Science Friday News Roundup.

I'm SciFri producer Charles Bergquist. Thanks for listening. We'll see you soon.