Sea squirts teach new lessons in evolution

Host: Shamini Bundell

Welcome back to the Nature Podcast. This week, the mystery of some missing genes.

Host: Benjamin Thompson

And an iodine-powered satellite engine. I’m Benjamin Thompson.

Host: Shamini Bundell

And I’m Shamini Bundell.

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Interviewer: Shamini Bundell

First up today, I’ve been finding out how studying a tiny, squidgy, sea squirt-like creature could provide some exciting insights into the evolutionary process. Now, the oceans of Earth are filled with some quite peculiar animals, many of which seem quite unlike our bony human selves.

Interviewee: Cristian Cañestro

Tunicates is not a group that we are very familiar with. They are called sea squirts and they are marine animals. These filter feeders normally are at the bottom, attached to the rocks. And they eat what they filter. Normally, that’s small microalgae that live in the oceans.

Interviewer: Shamini Bundell

Tunicates certainly seem nothing like us at first glance. Early biologists classified them as some kind of mollusc. But there was a twist. Some of these sessile, unmoving filter feeders have a larval stage, and that larval stage looks surprisingly similar to backboned animals known as vertebrates. That includes humans.

Interviewee: Cristian Cañestro

They have a trunk, a tail, a notochord, muscle. They have a brain and a nerve cord like we do dorsally. We have reclassified these weird animals that are in the bottom of the ocean to be the sister group of vertebrates – our own group.

Interviewer: Shamini Bundell

Genetic evidence suggests that humans and tunicates both evolved from tiny free-swimming creatures, much like the larval tunicates. But what did this free-swimming ancestor look like? And how did most tunicates end up with a sessile way of life? These are some of the questions that Cristian Cañestro and his colleagues set out to answer, and the results of their investigation are published this week in Nature. Their research focused on one particular kind of tunicate – an unusual group called the appendicularians.

Interviewee: Cristian Cañestro

There is a group of tunicates – the appendicularians – which all of them are free-living throughout their life. They have complete freedom. They have no metamorphosis.

Interviewer: Shamini Bundell

The team wanted to know how these appendicularians had evolved and why they were different from the other tunicates – known as ascidians – with their sessile adult forms. It could be that appendicularians simply lost the adult stage present in their sister group and stayed living like free-swimming larvae. Or it could be that all the tunicates started off as free-swimming creatures and the appendicularians retained that primitive state, never having evolved a sessile way of life. And that last option presents a tantalising possibility.

Interviewee: Cristian Cañestro

If appendicularians would represent the last common ancestor of all tunicates, then that would help us understand how was the last common ancestor between tunicates and vertebrates, or at least give us an insight into how looked the last common ancestor from which vertebrates originated.

Interviewer: Shamini Bundell

So, we could have all evolved from something that looks like a modern day appendicularian.

Interviewee: Cristian CañestroHowever, if appendicularians are not the ancestral condition, then appendicularians will help us to understand that, so it’s key to understand how appendicularians look like or how they have evolved within tunicates, to help us understand the last common ancestor for all tunicates and vertebrates.

Interviewer: Shamini Bundell

The research on appendicularians started with Alfonso Ferrández-Roldán, the PhD student whose project this paper was based on. He went looking for some of the genes that you’d usually see in both tunicates and vertebrates.

Interviewee: Cristian Cañestro

He was frustrated because he was not finding many of his favourite genes. And then at some point we had the genome, and then we realised that we were not finding them because the genes were gone. The genes had been lost during the evolution of those appendicularians.

Interviewer: Shamini Bundell

Digging into the developmental pathways of appendicularians allowed Alfonso to figure out what impacts those missing genes had on the growing animal. Some of the missing genes were associated with a filter-feeding structure that appendicularians don’t have but ascidians do. In another case, the loss of certain genes changed the appendicularian heart to a structure better suited to a free-swimming organism. The fact that these genetic losses seem to be driven by a transformation to a free-swimming life suggests that the ancestors of the appendicularians were sessile. In fact, appendicularians are likely to have evolved from a creature more similar to their unmoving sister group – the ascidians.

Interviewee: Cristian Cañestro

Our data points that the last common ancestor for tunicates was sessile. Then we cannot use appendicularians as a proxy to get closer to that common ancestor between tunicates and vertebrates.

Interviewer: Shamini Bundell

So the appendicularians can’t represent the primitive state from which we all evolved. But for Cristian and his colleagues, the way that they returned to a free-swimming state – through major gene losses – had fascinating implications, in particular because of the specific genes that had disappeared.

Interviewee: Cristian Cañestro

Genes don’t work separately. They make webs, they make networks of genes that they interact with each other. So, then now, if we imagine in the space that network, all of the losses were not randomly scattered. They were concentrated in particular places – the genes that interact with each other.

Interviewer: Shamini Bundell

Looking at the groups of genes that had been lost revealed the modular structure of the gene networks they were part of.

Interviewee: Cristian Cañestro

And by locating, by identifying in which part of the network, that helps us to identify modules that maybe we didn’t realise were there. And then we can relate those modules to specific functions that have evolved in that particular animal. So, then, by studying gene losses, now we can see it as a tool to help us understand how gene networks work.

Interviewer: Shamini Bundell

Cristian thinks these insights into gene loss could be widely applicable in the study of evolution.

Interviewee: Cristian Cañestro

How we look at gene losses as an evolutionary force is changing. We always think, in evolution, in innovation of new features, in the invention of new genes, what conditional new properties by the genes that were there. And we always think of gene losses as a secondary output with no consequences. There is no impact. And that is true in some cases. For instance, fish that go to live in a cave which is dark, then all of the genes responsible for vision are useless so then they can be lost with no consequences. However, there are many examples now of gene losses that are adaptive. For instance, there have been cases with humans in which the loss of some particular genes, grow resistance to malaria or to HIV. And losing one gene is a very easy event mechanistically because it only needs some few mutations or even one single mutation to make the gene not to be functional. So, mechanistically, it’s a very rapid response.

Interviewer: Shamini Bundell

This research set out to better understand the evolution of sea squirts and their relatives. And indeed, the final paper gives insights into the ancestors of all tunicates, the development of appendicularians and the genetic changes that allowed some members of this group to return to a fully free-swimming life. But for Cristian, it’s the deeper insights into the role of gene loss in evolution that has some of the most intriguing implications.

Interviewee: Cristian Cañestro

And now, for the first time, we have a map of what we call the lossosome. We can compare any two species and see not only which genes are present but which ones are absent. By also understanding which genes are not there, we are trying to understand how those losses contributed to the differences of different animals or the acquisitions of new innovations.

Interviewer: Shamini Bundell

That was Cristian Cañestro from the University of Barcelona in Spain. You can find his team’s paper linked to in the show notes.

Host: Benjamin Thompson

Coming up, we’ll be hearing about a miniature iodine-powered engine, propelling a satellite up in space. Right now, though, it’s time for the Research Highlights, read by Dan Fox.

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Dan Fox

The glittering green forewings of an African beetle have been found to contain a unique type of light-reflecting crystal. Photonic crystals are microscopic structures that block certain wavelengths of light while allowing others to pass through. In the lab, they’re difficult to make, but many organisms grow them with ease. The structures can be found in butterfly wings and also give an emerald sheen to the Sternotomis callais beetle of Central Africa. Using electron microscopy, researchers were able to reconstruct the internal 3D structure of one of the beetle’s scales. They found that it looks like a complicated mixture of spheres and wide tunnels, resembling an ideal mathematical surfaced called an I-WP. But while the structure of the beetle’s photonic crystals reflect green light as expected, it’s still unclear how they form. Reflect on that research in Journal of the Royal Society Interface.

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Dan Fox

The cold-loving Pacific Ocean rockfishes have wildly varying lifespans, with some species averaging a mere 11 years while others can live for more than 2 centuries. Now, researchers have pinpointed genes linked to this remarkable range. To understand this longevity, scientists examined the genomes of 88 rockfish species. They discovered that the longest lived have more genes linked to DNA maintenance than their cousins that live for less than 20 years. Some of those genes seemed to indirectly affect longevity by influencing a fish’s size and ability to adapt to different environments. The long-lived fish also have more copies of genes involved in suppressing the immune system, suggesting that these fish are more protected against the effects of an increase in immune-related inflammation throughout the body that occurs with age in many vertebrates, including humans. It won’t take a rockfish lifespan – long or short – to read that paper in full in Science.

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Interviewer: Benjamin Thompson

Up in space, propulsion systems are a key part of a satellite’s apparatus, helping them, for example, to manoeuvre into the correct orbit and keep them there when they arrive. Many of these are electric propulsion systems, and broadly they work by ionising gas inside the engine, then use an electric field to accelerate these ions out the back of the engine, providing thrust and pushing the satellite forwards. The most common propellant currently being used is xenon gas. But there is a potential alternative: iodine. This week, a paper in Nature demonstrates for the first time an iodine-based electric propulsion system that works in space, and the team behind it hope it will one day power shoebox-sized satellites. Now, xenon is the propellant currently used, in part because its atoms are heavy and easy to ionise, and the heavier the ion, the more thrust you get for your buck. But xenon is not without its issues, as one of the authors of the new work, Dmytro Rafalskyi from the French space company ThrustMe, explains.

Interviewee: Dmytro Rafalskyi

Xenon is used a lot indeed on Earth for medical applications for different industries. The little problem is that it’s really rare, so it’s very expensive and now, currently, demand is higher than the production rate.

Interviewer: Benjamin Thompson

And xenon has some disadvantages as a satellite fuel as well.

Interviewee: Dmytro Rafalskyi

It requires very high pressure to store it and this brings a lot of risks.

Interviewer: Benjamin Thompson

For bigger satellites, this isn’t so much of an issue, as there’s room for pressurised storage and safety features. But for very small satellites, which are increasingly being used in the space industry, that’s not going to fly. And so, researchers have been looking for a long time for an alternative propellant.

Interviewee: Dmytro Rafalskyi

Well, the winner is iodine, definitely. The first works on iodine appear in 1970. Then there was pretty much nothing until the early 2000s when the research started to reappear in many centres actually.

Interviewer: Benjamin Thompson

As well as being cheaper and more abundant than xenon, there were other reasons that iodine was a great candidate for a satellite propellant.

Interviewee: Dmytro Rafalskyi

Iodine, it has similar or even better performance than xenon in some range. It has much higher storage density. It’s stored as a solid. It doesn’t require any high pressures. And it’s really safe to handle a system filled with iodine.

Interviewer: Benjamin Thompson

Iodine sublimes, meaning it readily turns from a solid into a gas upon heating, and that gas can be easily ionised, making it a good fit to fuel this type of electric propulsion system. And because solid iodine doesn’t require high-pressure storage equipment, engines can be shrunk down. And the one that Dmytro and his colleagues have built is small.

Interviewee: Dmytro Rafalskyi

The entire system, which combines all of the subsystems necessary to run it, it’s 10cm by 10cm by 10cm cubed. We store iodine in the container. There is a matrix which holds this solid iodine. We warm up the iodine. It’s pretty much simple. It gets sublimated, and then the vapour is directed towards the chamber where we ionise iodine using the radiofrequency of electric fields, and then we have a set of grids which will accelerate ions to very high velocities.

Interviewer: Benjamin Thompson

Developing this new system was no mean feat. While iodine has a lot of positives, it’s also really corrosive to metal, so the team needed to design the insides of their engine carefully, using things like ceramics and polymer coatings to protect the delicate machinery. Dmytro and his colleagues are by no means the only group developing iodine-fuelled electric propulsion systems, and others have been tested here on Earth. But in this work, they’ve gone one further. The team attached their system to a small satellite called a cube-sat, and sent it up into space to put it through its paces.

Interviewee: Dmytro Rafalskyi

So, the cube-sat is a 12U satellite. It means 12 units of 10cm by 10cm by 10cm. So, one of those 12 units is our engine, and it was built by our partners, and it has been launched from China in November 2020.

Interviewer: Benjamin Thompson

By firing the engine at precise times, the team could change the cube-sat’s orbit, moving it closer to or further from Earth, showing for the first time that this sort of system does work in space.

Interviewee: Dmytro Rafalskyi

This mission is purely a demonstration mission to demonstrate the basic capability. Right now, we have performed 40 manoeuvres with almost 4 kilometres of orbit altitude change, so we were raising and increasing the altitude. We are very happy about the fact that we made these manoeuvres because there was a lot of fear in the community about how this will happen, how iodine will behave, will the platform be destroyed just because of the corrosive nature of iodine, and things like that. We hope that it will be a really good, simple flight for others to follow and I really hope that in ten years from now, most of the electric propulsion systems will use iodine in space.

Interviewer: Benjamin Thompson

Around the world, there is a lot of interest in iodine-fuelled electric propulsion systems, and this work has shown their potential. There’s a long way to go, but Dmytro thinks that systems like his could help reduce demand for rare xenon gas, help reduce the costs of satellite deployment, and maybe even help clear up the clutter of small satellites by bringing them out of orbit once their missions are completed.

Interviewee: Dmytro Rafalskyi

Everyone is speaking about space junk and so many satellites around, and many of them actually don’t have any propulsion, so they eventually create junk, they create collisions and so on and so forth. There’s now so many objects around Earth, just there, and it’s getting more and more. It’s extremely important to put now the solutions.

Interviewer: Benjamin Thompson

That was Dmytro Rafalskyi from the space company ThrustMe. To read his paper, look for a link in the show notes.

Host: Shamini Bundell

Finally on the show, it’s time for the Briefing chat, where we discuss a couple of recent articles that have been highlighted in the Nature Briefing. So, Ben, what have you found for us to discuss this week?

Host: Benjamin Thompson

Well, Shamini, we heard just now a story about satellites orbiting the Earth, and this story, I guess, is kind of related, and it’s something that I read about in Wired and in this case it’s about the iconic Hubble Telescope.

Host: Shamini Bundell

Ah, how is the Hubble Telescope doing? That’s probably one of the most famous telescopes there is.

Host: Benjamin Thompson

Well, absolutely true, Shamini, and sadly, the Hubble hasn’t been doing too well recently. It’s been a little bit sick, and engineers and technicians and the whole crew behind it are trying to get it back online.

Host: Shamini Bundell

Oh, no. What’s gone wrong?

Host: Benjamin Thompson

So, problems began at the back end of October. And from what I understand, some of the instruments on board the telescope weren’t receiving a standard kind of synchronisation message that comes from the control unit on the telescope. And so, as a result of that, Hubble put its science operations into this induced coma known as a ‘safe mode’.

Host: Shamini Bundell

Oh, so it hasn’t actually been collecting any data recently until they’re able to go and try and fix it?

Host: Benjamin Thompson

Well, as soon as this happened, Shamini, the team back on Earth kind of sprang into action. They’ve been trying to figure out what’s been going on and bring it back online. And this is something you have to be very, very careful doing. It’s not just like turning your computer on and off again loads of times, which maybe I’ve done here on Earth, I don’t know, because of course, these are very delicate instruments and changes in temperature can mess things up if you keep warming them up and cooling them down again by turning the power on and off. But the good news is that the Advanced Camera for Surveys, now that’s back on, and this is one of the more straightforward things to recover, and this was installed back in 2002, and it images large areas of the sky at once and in huge amounts of detail.

Host: Shamini Bundell

It must be pretty tricky to fix a telescope that is in orbit. Are they able to go there or do they have do this entirely remotely?

Host: Benjamin Thompson

Well, that’s an entirely reasonable question, Shamini. So, previously, people have gone up to manually work on the Hubble Telescope – five service missions in all – to repair it or upgrade it. And one of them that I certainly remember happened back in 1993. So, soon after the telescope was put into orbit, researchers realised that it was a bit fuzzy. Things were a bit out of focus. So, what they had to do was send up a pair of glasses, essentially, to put over the top, to fix this kind of aberration in this big mirror and to help get things into focus. So, things have been done in the past but, to answer your question, that really kind of is in the past now. The last mission up there was in 2009, and now it’s very much a case of trying to fix things from back on Earth.

Host: Shamini Bundell

Well, I was going to say, I mean, Hubble maybe needed glasses from a young age, but it’s certainly been up there for a while. Is this Hubble sort of breaking apart? Is this the end of days?

Host: Benjamin Thompson

Well, I mean, this is what people have started to talk about, but I think there’s life in the old telescope yet, Shamini. So, ten years was the lifespan of the Hubble mission and it’s now in its 30s, so it’s doing well. And what I will say is, the team behind it, are still trying to work out the ways to power on the rest of the systems. But despite its age, it is still a super important thing, right? It’s massively oversubscribed still. Astronomers want to get on it to use it to look at things, and it’s been involved in so many discoveries over the years. We know so much more about space because of its existence, so ‘iconic’, I think, as we said, is the word to use to describe it.

Host: Shamini Bundell

You said one of the systems was now fixed. Is there a time by which hopefully it will all be good to go again and collecting all that delicious space imagery?

Host: Benjamin Thompson

Well, I think watch this space, Shamini, no pun intended. I guess people much smarter than me and NASA are working on the problems right now and, say, fingers crossed that Hubble will be back up in the very near future.

Host: Shamini Bundell

Get well soon, Hubble. If you can hear us here on Earth, we’re sending you our best wishes.

Host: Benjamin Thompson

Very much so. Alright, Shamini, well, what have you got for me this week?

Host: Shamini Bundell

Right, so, I have got, for you, a paper in Science that was reported on in Nature. It’s about the discovery of a new mineral that is a key component found deep in the Earth that basically can only really exist at these like intense pressures and temperatures found in deep, deep in the Earth’s mantle.

Host: Benjamin Thompson

Right, I mean, my first question is how did someone get hold of it then? I guess it wasn’t digging a very, very, very big hole?

Host: Shamini Bundell

Well, that’s my favourite part of this story, is the fact that this mineral was theorised, but, yes, how do you get some to actually look at it? And the answer to that is they found some inside a diamond.

Host: Benjamin Thompson

Wow. So, pretty fancy then?

Host: Shamini Bundell

Yes, well, diamonds aren’t just for looking sparkly in jewellery. They’re often pretty important for science, and this one looks like a shiny chunk of diamond material, but it’s got these little black specks in, and these black specks are the new material, the new mineral, which they have named davemaoite.

Host: Benjamin Thompson

Why have they named it that then? What can you tell me about it? How far down in the ground is it found?

Host: Shamini Bundell

So, it’s named after Ho-kwang ‘Dave’ Mao, who is a researcher. This is the second mineral he’s had named after him. He is, I understand, something of a big deal in high-pressure geochemistry and geophysics. And davemaoite, the mineral, aside from being sort of trapped in this diamond and thus sort of preserved at high pressure, it can only exist in the lower mantle, so that’s 660–2,700 kilometres below the surface of the Earth. It’s basically the bit of the Earth between the core and the crust and where a lot of the movement that contributes to things like plate tectonics happens. And the reason people were actually interested in this particular kind of mineral is because they think it’s actually really important for sort of heat transfer and thus the movement of the mantle and potentially the crust above it.

Host: Benjamin Thompson

What’s it doing then in these big geological processes?

Host: Shamini Bundell

Most of this mineral is calcium silicate. Now, calcium silicate is a pretty common compound found it quite a lot of minerals. But this particular form of it, and this is the form that can only exist at these high temperatures and pressures, is a perovskite. Now, you might remember us talking about perovskites on the podcast. There’s quite a lot of different research that involves perovskite structures, which can be made out of different elements. And one of the exciting things about them that’s sort of particularly relevant in this case is the way they can incorporate other atoms into their structure. So, in this case, this calcium silicate structure can scavenge radioactive isotopes, so things like uranium, thorium, potassium, things that are contributing to the heat that keeps the mantle and the core molten in the Earth, and therefore knowing how this davemaoite works and sort of where it is in different sort of parts in the mantle is going to be really key for understanding how heat is moving around down there.

Host: Benjamin Thompson

Well, maybe let’s leave it there then, Shamini. We have covered some ground today, from really, really far underneath our feet to really, really, really high above our heads, so a heck of a Briefing chat today. And listeners, if you’d like stories like these delivered directly to your inbox, then why not check out the Nature Briefing, and we’ll of course put a link on where to sign up in the show notes.

Host: Shamini Bundell

Well, that’s it for the show, but you may remember we’ve been talking a lot about the UN’s Climate Conference (COP26) on the podcast recently. The conference has wrapped up now but, as we mentioned last week, several of our colleagues were on the ground at the event in Glasgow, putting your questions to researchers and reflecting on the key discussions. You can watch how they get on in our YouTube series and links to that, as always, are in the show notes.

Host: Benjamin Thompson

Yeah, I would thoroughly recommend giving those a watch. Some excellent stuff in there. Don’t forget, if you want to get in touch with us, you can. We’re on Twitter - @NaturePodcast – or on email – podcast@nature.com. I’m Benjamin Thompson.

Host: Shamini Bundell

And I’m Shamini Bundell. Thanks so much for listening.