Exploring the origins of a glacial relict seal

[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 Gabrielson. The legacy of the last Ice Age persists today in the landforms carved by glaciers, like Lake Saima in Finland. It also persists in the histories of species who were isolated by the advance and retreat of glaciers. Scientists have long thought that Saima Ring Seals diverge from other Ring Seals when they were landlocked in Lake Saima around 10,000 years ago. In a recent PNAS study, Yaku Pahersmiki of the University of Eastern Finland, Yuko Yurnwal, an Aare Littinoia of the University of Helsinki and colleagues, looked at the genetics and even tooth shapes that make the Saima Ring Seal unique. They found that Saima Ring Seals have a longer history than previously thought, and can teach us about how new species emerge. Yaku introduces to Lake Saima and the Saima Ring Seals. Finland is known as the land of thousand lakes, and Saima is the biggest one. It's 4,300 square kilometers in area, that's about 1,700 square miles, so there's several basins of the lake and straits and almost 14,000 islands, and the shoreline is almost 15,000 kilometers, so like 9,000 miles. 11,000 years ago, all of this finoscundian area was under thick continental ice, which then was retreating and actually generating all these network of big lakes, which were opening up to the northwestern part of what was to become a Baltic Sea. And somewhere in that process, there were ringed seals that got entrapped into the lakes, and so when the land was repounding after the continental ice had melted, then the seals got locked into the lakes. The ones that remained in Lake Saima until the present day. There's now about 500 of them, so it's heavily endangered, and they were really close to extinct and back in the 1950s. One of the key threats currently is climate change, when there's no snow, they need to keep birds to the pups on a barren ground or a barren ice, and then they're really exposed to predators. But overall it's a remarkably resilient landlock seal to have survived so far. Yuka, what do we typically see in populations of animals isolated by glaciation? There are many species that are clasio relics. In the past, they had much broader distributions, and they were adapted into cold environments. And as the clasias they retreated in the end of the last clasiation. They were left stuck in places like mountain tops, like in Europe. We have a marmord in Alps living there because the only way they could go is up in the mountains to follow the cold air environments, or then they got stuck into lakes like we have these seal forms. And what kind of characterizes them all? They are typically small populations, they are open and endangered. But then also because of the geological history is pretty well known, we actually know when they got isolated, or we think we know, like in the case of the Saima ring seals 10,000 years ago. Ari, what did you look at to determine the evolutionary history of the Saima ring seals? In previous analysis, Saima rings is happy and compared with geographical closest populations from the Baltic Sea and Lake Latica. But this species actually has a really, really white distribution. And in this study, we expanded the sampling of the whole gene of data. To represent this global distribution of ring seals, we could show that this species is globally unique. We performed demographic analysis using isolation with migration model. And this gave us some estimates of the split times between Saima rings and other populations and also estimates of migration between populations across time. Why were the results surprising? Actually, the genetic analysis went hugely surprising. They rather confirmed what people had been seeing using less sophisticated methods over tens of years. And now we replicated these analysis with full genome data. And we could still see that Saima rings are genetically more diverged that we would expect given the age of the lake less than 10,000 years. Now we had full genome and we couldn't make the pieces to match. It still didn't fit into picture. We need something like higher than normal mutation rates to explain the diversions of Saima rings. And we just simply had to accept that they are genetically different and they must have had longer history than has been previously thought. So actually, the surprising result is that the species is older than the happy that it lives in. So this result made us to turn into the phenotypes because many analysis from the past have found differences in the scale more forward, see between the ring seals in Saima and the other populations. But this time we turned into dentition because dentition is informative about the diet. We used the extensive collections throughout different museums. And actually, the surprising result was how distinct the Saima rings seal dentition was from the others. And this is something that we have simply overlooked in the past. The ring seal dentition is characteristic of this kind of mixed feeder type where the seals eat on small fish, but then they also suction and filter feed on small marine invertebrate crustaceans. The teeth are quite complex, but this was not the case for the Saima ring seals. The dentition actually has kind of lost or reduced the filtering capacity and increased the piercing capacity. Really something interesting that the Saima ring seals has been adapted to eat only on fish for a long long time, which is well reflected also in the Saima leg where there's really not high abundance of invertebrate prey. But it's also subjusts with the genetic information that this adaptation goes much deeper in time. What are the implications for biodiversity and post-glacial habitats? Implications for the conservation biology are clear. This study underscores the fact that Saima ring seal is a really unique ring seal, possibly even the species of its own. And that means that there are no related seals around which could be used to restock it or otherwise replace it if it's lost. So, other spines have really great responsibility in trying to safeguard this species also for the future generations. We typically see in these models of ice age where the ice glacial is where and what was dry land. But they don't really show that there has been ice lakes around these glaciers. And what we suggest here is that these ice lakes actually have been crucial for aquatic organisms to survive during the ice age that they were pushed south and east during ice age and to survive in these waters surrounding the glaciers or waters connected to them. And once the glaciers again melted these animals and waters, they moved with the glaciers boundaries back towards north and then they formed the starting point of the newly forming Baltic Sea. And if that is true then this aquatic marine organisms in the area, they actually have also a richer history. They are not simply colonized from the Atlantic but they can actually have this kind of ice age refugee year. What are the caveats or the limitations of the study? In science usually we need more data because the deeper origins of the cymarine seals, some just that it has existed in the past, somewhere in lake systems, it would be wonderful if we would be able to obtain data where they would be ancient environmental DNA or even fossil finds. They would in a way help us to really map where these seals existed in the past. From genetics point of view, we would have liked to resolve the origins of cymarine seals. We have now a hypothesis that it comes from somewhere east or southeast but we cannot really pinpoint where it is. During this study we tried another approach we tried to model cymarine seals as mixtures of different other lineages. However the results depended very much on the outroop that we used in this study and we used gray seals and harbours seals and so on. The results changed depending on the outroop and that was first confusing before we actually realized that we cannot resolve the history of ring seals without resolving the history of all these other species. This species group consists of six other species. There's been competing models of the history of these species and no good agreement what the history is. I think that this is typical to species evolution. It's increasingly accepted that the history is not very much tree-like but it's more like a bush where different lineages separate and mix and there is no clear phallogeny but lineages coming and going and mixing up. With the advent of genomic and genetic data in general it's become more tricky almost to define the species limits because initially people were just thinking some kind of genetic distance and then we can split it but we wanted to also look not only genomic data but phenotypic data especially qualitatively important features like the feeding adaptations and then of course the isolation put together is suggestive really that this is a bona fide even though it's probably very young species, but still we are kind of really seeing how that can happen almost in real time at least in paleontological sense within last 60,000 years or so. 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 a review and helping us spread the word. [Music]