Fruit flies’ ability to sense magnetic fields thrown into doubt

Shamini Bundell

Welcome back to the Nature Podcast, this time: the search for animals’ magnetic sense suffers a potential setback...

Nick Petrić Howe

...and what the closing of the Arecibo Observatory means for science. I'm Nick Petrić Howe...

Shamini Bundell

...and I'm Shamini Bundell.

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Shamini Bundell

First up, a recent Nature paper has raised questions about the ability of fruit flies to sense the Earth's magnetic field, a finding that could have repercussions for scientists efforts to answer one of the biggest questions in sensory biology. Here's Benjamin Thompson with more.

Benjamin Thompson

Lots of animals can detect the Earth's magnetic field with some, like certain species of bird or turtle, using it to help them navigate as they migrate vast distances around the globe. But exactly how animals do this still elude scientists. A leading theory is that photoreceptor molecules, called cryptochromes, are central to this ability. But studying this isn't always easy in animals like birds. So many researchers have turned to animals that are easier to work with, namely, Drosophila, or fruit flies. And that includes Peter Hore, from the University of Oxford, in the UK. He and his colleagues primarily focused on migratory songbirds, but wondered if Drosophila could help inform their research.

Peter Hore

I mean we acknowledge that flies aren’t birds, but flies have cryptochromes, the proteins that we think of the sensors in the birds. We had previously studied the Drosophila cryptochrome in the lab. And it shows magnetic field effects, very similar to the ones that we've subsequently saw from the bird cryptochromes. So, even if the Drosophila didn't actually need to sense the Earth's magnetic field, if they did it just by accident because they happen to have the same protein, we could still potentially learn something about the mechanism by studying the flies.

Benjamin Thompson

The team planned to use fruit flies to learn more about what might be going on in the birds. So, they turned to the literature and selected two types of tests they could use to put the flies through their paces. One, first published in Nature in 2008, and another in Nature Communications in 2014. Both showed that fruit fly behaviour could be affected by weak magnetic fields.

Peter Hore

One study was of flies in a maze where they were given a binary choice to move towards or away from a magnetic field. And the other assay was a natural behavior known as ‘negative geotaxis’ in which the flies move up a vertical surface. We've all seen flies climbing our kitchen. windows. And they will do this. If you put them inside a transparent tube. And the claim was that the speed at which they would climb could be affected by a weak magnetic field.

Benjamin Thompson

But Peter and the team ran into a problem.

Peter Hore

Well we tried to replicate these two studies and we could find no evidence with either of those assays that for Drosophila, in our hands, responded to weak, magnetic fields.

Benjamin Thompson

The team say they went to great lengths to reproduce the experimental setups as faithfully as possible, reaching out to the original authors and getting the blueprints of the equipment used in the 2008 T-maze paper, and even getting hold of the actual equipment itself used in the 2014 climbing experiment. They also took great care to reduce the chance that outside effects were influencing the results.

Peter Hore

We did all the measurements in magnetically shielded chambers in a purpose-built non-magnetic laboratory, which we had built to study the birds a few years before, made of entirely non-magnetic materials. And the walls and ceiling and floor of these chambers were very carefully electromagnetically shielded, to reduce the intensity of any electromagnetic interference from outside was reduced by a factor of 10^5 or 10^6.

Benjamin Thompson

The team tested almost 110,000 flies spread over six years.

Peter Hore

We couldn’t find anything, so a very large numbers of flies tested under very carefully controlled conditions, and we could find no evidence of magnetic field effects on their behaviour.

Benjamin Thompson

Now, the two studies that Peter tried to replicate are not the only ones that suggest fruit flies can sense magnetic fields. And lab experiments have also shown that Drosophila cryptochrome molecules can be affected by magnetic fields in vitro. Here's Eric warrant from Lund University in Sweden, who wasn't part of Peters team, but has written an analysis about it in Nature.

Eric Warrant

I mean, after all, they've only replicated two of at least 15 studies that have been done in recent years, showing or claiming at least a magnetic sense in Drosophila. It's obviously impractical and more or less impossible to replicate to that level that they've done every single study that's ever been conducted. So the authors haven't categorically said that the magnetic sense of Drosophila is now completely ruled out, but they have nonetheless raised quite serious flag over its existence I would say.

Benjamin Thompson

But Eric, who works on magnetic sensing in migrating moths, doesn't foresee the finding completely upending the field.

Eric Warrant

I don't think it will have any impact on the hypothesis of cryptochromes being used to sense the magnetic field. I don't think you'll have any impact on that, that I think is fairly clear that that's a very promising hypothesis and there's good evidence now mounting up in different labs around the world to suggest that this mechanism does truly exist. But the implication it does have is that this unbelievably useful genetic model organism Drosophila may no longer be as useful as we previously had hoped. That is, to some extent, very disappointing because it means that we're further away now from probably dissecting the mechanism of how magnetic information is sensed then we were before this study was conducted.

Benjamin Thompson

It's important to say here that not everybody agrees with this assessment. Nature reached out to Steven Reppert and Bambos Kyriacou, the corresponding authors of the two studies Peter and his colleagues failed to replicate. Steven was unable to speak to us on tape, but stood by his lab’s work and defended their conclusions, citing various concerns and pointing out the nuanced nature of these kinds of behavioral experiments in flies, and potential environmental impacts that may not have been considered. Bambos, from the University of Leicester here in the UK, also does not believe this study invalidates his work. Here he is.

Bambos Kyriacou

There is a context to our work. I mean, we published a paper in Nature a couple of months ago, with our model of how we think this molecule cryptochrome mediates magnetosensitivity. And it's very different to the model that Mouritsen and Peter Hore advocate. And our model has been around since 2014. And was supported in very interesting way, both with a whole organism behavior assay, and with a single cell physiological assay.

Benjamin Thompson

He also points to a group in Korea that, he says, has already fully replicated his results and offers a potential explanation as to why fruit flies might need such abilities.

Bambos Kyriacou

The Korean paper was a beautiful paper. They do have a reason why flies have a geomagnetic sense and that's because the magnetic field keeps flies flying low to the ground because this particular species lives on rotting fruit, which is found on the floor of orchards and vineyards. So, they need to be close to the substrate and the geomagnetic field helps them do that. It’s a very compelling argument.

Benjamin Thompson

It's clear that the debate is unlikely to be resolved anytime soon. But nonetheless, Eric thinks there are lessons to be learned.

Eric Warrant

Well, one thing it does do, and I think it should do this for all researchers working on the magnetic sense. It should help us sharpen our experimental methods and be very, very careful about what influences may exist in our laboratories that may influence the magnetic sense, and make sure that those influences are eliminated.

Benjamin Thompson

In his News and Views article, Eric writes that, quote "any sensory biologist who has dared to work on the magnetic sense of animals knows it is notoriously difficult" unquote. And this new paper highlights this. Quite what effect it will have on scientists search for the elusive mechanism, or mechanisms, involved in magnetic field sensing remains to be seen. But it has had a direct result on the direction of research for Peter and his colleagues.

Peter Hore

Well, it means for us but we won't be doing any more experiments with Drosophila. We’re back for the birds, which of course we have been studying in all the intermediate years since we started this. Other people, I guess we'll have to make their own choice. I mean, we certainly don't claim that Drosophila have no magnetically affected behaviors. All we're saying is that we have found no evidence of it, in this study, trying to replicate very carefully the assays of two different set of authors. So, it's up to other people to judge whether they think it's worth continuing or getting into this area and using Drosophila as a model organism for magnetoreception. I mean, maybe people should be trying to replicate our study, form their own opinion about whether we've done it right or whether there are other conditions that might show effects.

Shamini Bundell

That was Peter Hore from the University of Oxford, here in the UK. You also heard from Eric warrant from Lund University in Sweden, and Bambos Kyriacou from the University of Leicester also in the UK. Head over to the shownotes for links to Peter's paper, and Eric's News and Views.

Nick Petrić Howe

Coming up, what the shutting down of an iconic observatory means for science. Right now though, it's the research highlights, with Dan Fox.

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Dan Fox

A labrador-sized swimming reptile, with a big snout, that lived nearly 250 million-years-ago, might have been the world's first filter feeder. Researchers reconstructed the skull of the reptile using two early Triassic fossils, found in China. The team then compared the reconstruction to 130 skulls of modern animals. They found that the skull structure was most similar to that of a modern baleen whale. The jaw had no teeth, but could have contained baleen plates and connected to the skull in a way that suggests that the long mouth could expand to accommodate water exactly what you need to be a filter feeder. The researchers think that this reptile probably swam slowly along the ocean surface, gulping plankton rich water as it went. You can feed on that research over in BMC Ecology and Evolution.

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Dan Fox

New research suggests that deforestation has led to climate change, in Bronze Age Europe. Between seven and two-and-a-half thousand-years-ago, many Europeans switched from hunting and gathering to agriculture, and this resulted in a 10% decline in forest cover across the continent. To quantify the effects that this had on the climate at the time, a team of researchers used global and regional climate models along with two models to describe land cover, that's the physical material on the surface of the Earth, one based on dynamic vegetation predictions and one reconstructed by analyzing pollen records. Their results indicate that part of northern Europe was up to one degree Celsius warmer in winter and southern Europe up to 1.5 degrees Celsius warmer in summer than the regions would have been had they never been subject to this Bronze Age tree cutting drive. You can uncover that research in Climate of the Past.

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Nick Petrić Howe

On the 14th of August, Arecibo, the famed Observatory in Puerto Rico shut down. For decades, it has been used for everything from discovering exoplanets to detecting gravitational waves. But now after the collapse of its main instrument, with no fixed plans for its future, and with its funding exhausted, many astronomers have been left scrambling to retrieve their data, equipment and belongings before it closes its doors. Last week, just before the doors were shut, I spoke to him Anil Oza, who's been writing about this for nature.

Nick Petrić Howe

Anil, hi.

Anil Oza

Hi, Nick, how are you?

Nick Petrić Howe

I'm good. Thank you. Well, thank you for joining me. And so Arecibo is closing. Now, I'm sure this is going to be sad news for many scientists, as this is a pretty iconic observatory, right?

Anil Oza

Yeah, it's pretty hard to understate how gutting this is for researchers in the community. I mean, my family's from Puerto Rico, and I have this memory of going to the site and seeing this huge sort of larger-than-life telescope, it feels like something out of a sci-fi movie. And a lot of the researchers I spoke to that now use and even run the site have a similar experience. And I think that's sort of the biggest thing that people are worried about, especially with the NSF, which is the National Science Foundation here in the US, which has funded Arecibo and a lot of research here in the US for the past couple of years, they've not been the most excited about funding research at the site, which has a lot of researchers really worried. But it was also for a very long time, the most powerful instrument that we had to study radio astronomy, planetary science and atmospheric science. And so that research that's going on at the site is sort of up in the air right now.

Nick Petrić Howe

I mean, I'm sure a lot of things are up in the air for many people. And I think we'll get to that. But I was wondering if you could give us a bit of background. What's led up to this? Why is the telescope closing down?

Anil Oza

Yeah. So it's been a rough couple of years for Arecibo, and it's been a combination of cycles of neglect and unfortunate circumstances. And so the NSF, which helps run and manage the site for the past decade or so has been trying to decrease the amount of money it spends on the site and divert that money to other sites, which has left Arecibo without the necessary maintenance that it needs. But there's also been earthquakes and hurricanes that sort of have battered the site too, which led to the collapse of it's sort of like marquee largest instrument in 2020, which was this huge 305 meter telescope.

Nick Petrić Howe

Yeah, as you say, it's been taking a bit of a battering over the past few years. But as I understood it, there was a plan to kind of save the site, it was going to become an educational center. Where is this plan now?

Anil Oza

So, that big telescope collapsed in 2020. And last year, in 2022, the NSF announced that rather than reopening the site as a research facility, they want to open an education center. And a couple of the researchers sort of said that wasn't really their first choice. Arecibo was doing a ton of education and outreach before it was even called an education center. And so they said that it could do the same thing if it was rebuilt and reinvested as some other sort of research facility. And they didn't say that it has to be in astronomy, but a lot of them would love to see another sort of huge world class telescope at the site again.

Nick Petrić Howe

No, I'm sure. And speaking of the sort of reactions of the researchers, we have this sort of imminent news that the telescope is going to close down, what the researchers doing now?

Anil Oza

Yeah, so I talked to a bunch of researchers both in Puerto Rico and outside and a lot of them have viewed this last year as sort of like a rush to get as much as they can out of the site. And so even though they don't have this huge 300 meter telescope, they've upgraded one of the smaller instruments on the site, which is a 12 meter radio telescope. And these researchers that are at Arecibo right now told me that it's been basically us 24/7, for the past year, that researchers are clamoring to use it to get as much data as they can and get a couple of like, academic publications out of it. And so a lot of them are also having to say goodbye to the site that sort of means so much to them.

Nick Petrić Howe

I mean, it almost sounds as if researchers are kind of scrambling at this point, would you say that's sort of the vibe you get from them?

Anil Oza

Yeah, I think it's really hard because a lot of these researchers don't want to leave Arecibo, but they're sort of being forced out, because there just isn't opportunities for them anymore. And part of the reason that is, is that that proposal we mentioned earlier from the NSF, for an education center hasn't been chosen yet. So the NSF asked researchers sort of from around Puerto Rico and from the US to submit proposals to take over this education center, but they haven't yet chosen who that person will be. And until that person is chosen, we don't really know what the future the site will be. We don't know what type of education, what type of research will be going on there. And so in the sort of uncertainty, a lot of these researchers and even the director of the site are having to leave Arecibo to find opportunities elsewhere.

Nick Petrić Howe

And it's not just the actual science that's been done there. As you sort of mentioned at the start, this telescope means quite a lot to people.

Anil Oza

Yeah, absolutely. I think almost everyone that I spoke to that comes from Puerto Rico mentioned that, like, as a school child, they would visit the site. And that was sort of what inspired them to go into some form of astronomy or planetary science. And I think, repeatedly, that's what people said was the biggest loss that it wasn't necessarily just the data and the research that people were getting out of there. But the fact that this has inspired a generation of Puerto Rican, not just astronomers, but scientists in general.

Nick Petrić Howe

So with all this in mind, what does the future hold for this observatory?

Anil Oza

So, like I said, everything is really really up in the air until the NSF chooses the next sort of manager of the site, but researchers have a lot of pretty lofty goals right now. And I think it was best put by Jennifer González Colón, who is the resident commissioner of the island, meaning that she represents Puerto Rico in Congress. She sort of has three goals, some for the short term, the medium term and long term. Right away, she hopes that whoever takes over the site takes us to the instruments that are already at Arecibo, and like don't let them go to waste. And she hopes over time that researchers can start applying for funding elsewhere to build some more research instruments before they sort of ramp up to this sort of like world class instrument that they hope can replace that 305 meter telescope. And right now, the NSF proposal doesn't have anything to do with research. So, researchers are hoping that whoever takes over the site will find money elsewhere from either the government or private companies to build up more instruments. And one of those goals is there's this big telescope array that the NSF is currently funding called The Next Generation Very Large Array. And this would be a bunch of really small telescopes placed across the country to sort of like work together and help researchers study the stars. And the hope is that a couple of these telescopes will be placed at Arecibo as sort of a stepping stone, and that it'll bring researchers to the area it'll keep the area free of radio pollution, which will be important if they want to build this bigger telescope. But the real big goal for a couple of years researchers is something called The Next Generation Arecibo Telescope. And it's in it's really really early phases of development, but it would sort of help out the three groups of scientists that use the old telescope. So it would be really important for radio astronomy, planetary science and atmospheric sciences. But right now, we sort of 1. don't really know how much it will cost or if 2. the plan as it is right now will even work. And so it would require a couple of years of testing and research to even know if that's a feasible design.

Nick Petrić Howe

That was Anil Oza. For more on this story, check out the show notes for a link to Anil's news article.

Shamini Bundell

Finally, on the Nature Podcast, it is time for the Briefing Chat, where we discuss a couple of stories that we have read this week in the Nature Briefing. So, Nick, tell us about your pick.

Nick Petrić Howe

So this week, I've been reading a news article in Nature about how the Standard Model continues to be correct.

Shamini Bundell

The physicists love the Standard Model, don't they? But they do. Do they want it to be correct? Or do they want it to be wrong? What? What's going on with the standard model?

Nick Petrić Howe

So yeah, I was being a little bit tongue in cheek with my headline, physicists' Standard Model is basically their description of three of the four known fundamental forces, the electromagnetic, the strong, and the weak forces, but not gravity, because gravity is weird. And basically, they kind of want it to be wrong. Because if it's wrong, it implies there's sort of new physics out there. And you know, that'd be very exciting for physicists.

Shamini Bundell

Would be quite good—

Nick Petrić Howe

—Yeah—

Shamini Bundell

—I'd quite like that. But I suppose ultimately, what they want is to find out... find out the truth in an unbiased way, as good scientists, as do we all.

Nick Petrić Howe

Yes, they're cursed by being good scientists. And so they do want to find out the truth of what's going on. And so this story requires a little bit of background, because essentially, this news article is about latest in a set of results that have been trying to explore one aspect of the Standard Model. So back in 2001, there was a result about a fundamental particle known as a muon, which you can think of as kind of like a fat electron — it's similar to an electron, but 200 times more massive. And what they did is they measured something called its magnetic moment. And what that means is its tendency to align with a magnetic field. And in 2001, this measurement seemed to go against what the Standard Model decreed. So physicists obviously got very excited about this.

Shamini Bundell

Okay, so that's great. So the Standard Model covers the electromagnetic force, this muon magnetic moment, didn't match... Excitement, fireworks! But I'm guessing that didn't last very long?

Nick Petrić Howe

Well, the trouble was the result was not precise enough for them to definitively say, "okay, this definitely deviates from the Standard Model". So physicists have since been trying to make more and more precise measurements. So they can say for sure that it does deviate from the Standard Model. And so there's been a few of these such experiments, especially from Fermilab, and they've found sort of agreement with that original 2001 result. However, since then, how we determine what the standard model says about the muon has also changed. So maybe the fact that these measurements are confirming this deviation doesn't actually mean, the Standard Model is wrong.

Shamini Bundell

Oh, no. Okay, so since they've measured the muon's magnetic moment and found it to be different, or potentially different, from the prediction, that prediction itself has shifted, and is no longer necessarily completely different from the latest muon measurements.

Nick Petrić Howe

Exactly. And so a couple years ago, we covered this on the podcast, there was a new way of determining how the muon behaves. Because basically, in order to determine things like magnetic moment, you can either do a bunch of experiments, or this new way was doing some computer simulations for how they behave. And this computer simulation way seems to agree with this different muon magnetic moment that's been seen since 2001, which therefore implies that the Standard Model is in fact right. And therefore there aren't all sorts of new exciting physics on the horizon.

Shamini Bundell

And what's this this very latest paper that you said, and in this whole story, what does that actually say?

Nick Petrić Howe

So this latest one, which is a preprint at the moment, is basically the most accurate measurement of the muon so far, and it has agreed with all the previous ones and the 2001 result. But as I say, that may not necessarily imply that it actually deviates from the Standard Model. So, it's exciting to be able to be super accurate, but it also doesn't imply that lots of super exciting new physics exists either.

Shamini Bundell

So does this suggest that if physicists are out looking for places where the standard model might be wrong, a muons magnetic moment is no longer the hot new place to look?

Nick Petrić Howe

Perhaps. There is a lot more still to be done. There are more measurements on the horizon. and also, those predictions that I talked about, there are further ways to refine them as well. There are also some other experiments that have come out that seem to also show different ways that this discrepancy from the Standard Model could be explained. So it could be the some of the things that physicists plug into their calculations weren't quite right when they made the predictions before. But all in all, it seems like the Standard Model, at least with regards to the muons magnetic moment, seems to still hold strong.

Shamini Bundell

So we'll be hearing lots more about the Standard Model still being correct, at least until maybe one day it isn't.

Nick Petrić Howe

Indeed.

Shamini Bundell

Well, I'm gonna take us now from the small scale of a muon's magnetic moment, to the large scale of the biomes and ecosystems of the Earth. This was an article in The Guardian, based on a PNAS paper. And I got to say, I picked this story, because it's got a whole load of really nice sort of headline facts that you can share with people who are interested in such things. And you can even make it a quiz, right? Go to your family and friends. And ask them what kind of habitat on Earth is the most species rich? Where has the highest diversity of species? I'd ask you, Nick, but I know you've you saw the headline for this one. So you can do you already know.

Nick Petrić Howe

I do already know. I probably would have said the oceans. But from glancing at the headline, I guess it's soil.

Shamini Bundell

Soil! Points to anyone who guessed soil without knowing this story. So, people have hypothesized that soil is extremely biodiverse habitat, but it's really under studied. So there wasn't really enough information to pin it down. So a previous paper in 2006 tried to, as this one has, estimate how much of life on Earth lives in, or on, or sort of based closely in association with soil. The 2006 estimate was 25%.

Nick Petrić Howe

Pretty good.

Shamini Bundell

Which is quite a lot. This new estimate does have some quite wide error bars on it. But the headline number is 59% of life calls soil it's home, at least at some point in its lifecycle. So over half of species on Earth.

Nick Petrić Howe

Whoa, so like a full blown majority of species live in the soil. But when you say like different species, is this lots of different species of bacteria? Or is this like worms and stuff? What sort of species are we talking about here?

Shamini Bundell

So, what they've done is they've gone through different groups, and tried to estimate it. And and the key thing here is that they can't just sort of count them, if you were only to count the species that we do know about, you know, soil would be really under counted underrepresented there. So, they have divided life up into certain key groups. And they're actually looked at all groups, but they've picked a whole bunch of them, including bacteria, insects, arthropods, mollusks, mammals too. They even looked at viruses at one point, which is even harder and debatable whether that counts is life or not. And for each of those different groups, they use these various data analyses and statistical methods to try and figure out, given that it's largely unknown, given what we know about other species elsewhere, or given what we know about the sort of proportion of different species that we do know, how many do they think could there be including all the ones that we don't know about. And they came up with sort of different numbers, therefore, for each of the four different groups. So for example, fungi, they estimate 90% of fungi are soil based, that was one of the highest. Plants, obviously, a lot of plants need to grow and sort of... pretty key, but not as many as fungi, so it's 85%. Because of course, you've got a lot of plants living in water, or even sort of with aerial roots or up trees and stuff. So 85% of plants, more than half of bacteria. And they also looked at mammals and mammals was the smallest soil based group, only 3% of mammals living in or directly on the soil.

Nick Petrić Howe

Wow. I mean, it's still a staggering number of species then. So does this, the authors think, imply that we need to do more to protect this very species rich environment?

Shamini Bundell

Well, yeah, they definitely note that you know, soil is under studied in general. This kind of information is not known. And soil is, unsurprisingly, pretty important. Most of our food we grow in soil is important for climate change impacts even sort of directly on human health. It's a pretty important habitat overall, and also one that we are having big impacts on so obviously through agriculture, but also pollution, deforestation, climate change, again, having impacts on the soil that maybe we don't fully understand what these impacts are.

Nick Petrić Howe

And so you said the error bars here are quite big. Did they say what they need to do to sort of refine the estimates going forward?

Shamini Bundell

Yeah, it's a really difficult thing to estimate, basically. And so yeah, they were quite high that, you know, they, they were really specific about the sort of lowest and highest possible numbers, again, for each of their groups. So, like for bacteria, that was a really large range, like, the lowest estimate was maybe 22% of bacteria living in the soil, whereas the highest was 89%. The first author sort of really emphasized the sheer challenge of this undertaking, and how much variation there is it within their estimates, they said, you know, "it's a first attempt to organise existing global richness" were their words. But also, you know, they say the first realistic estimate of global diversity and soil, which is needed to be able to sort of advocate for soil life.

Nick Petrić Howe

Well, thanks Shamini, I'll be sure to look out for the species under my feet in the future. But I think that's all we've got time for on the Briefing this week. Listeners, if either of those stories sounded like a good read to you, you can check out the links in the show notes, and we'll also put a link of where you can sign up to the Briefing to get more like them straight to your inbox.

Shamini Bundell

That's all for the show this week. Don't forget, you can get in touch with us if you fancy. You can find us on Twitter or X, @naturepodcast, or you can send us an email to podcast@nature.com I'm Shamini Bundell.

Nick Petrić Howe

And I'm Nick Petrić Howe. Thanks for listening.