Podcast: Mapping fruit flies’ neural circuitry, and perfecting the properties of metallic glass

Host: Benjamin Thompson

Welcome back to the Nature Podcast. This week, the neural circuits of fruit flies…

Host: Nick Howe

And improving the properties of metallic glass. I’m Nick Howe.

Host: Benjamin Thompson

And I’m Benjamin Thompson.

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Interviewer: Benjamin Thompson

Insect eggs come in a dizzying array of shapes and sizes, and insects lay them in an equally diverse range of environments – buried underground, glued underneath leaves, laid inside of other creatures, you name it. Egg laying is a tightly controlled process in insects, and it’s something that Barry Dickson from the Howard Hughes Medical Institute in the US is investigating.

Interviewee: Barry Dickson

There are two really fascinating things about it. One is how they decide where to lay eggs. They clearly choose out certain sites. And the other is how their egg-laying behaviour is regulated by mating. It obviously doesn’t make much sense for a female to start laying eggs until she’s mated to get the sperm to fertilise the eggs.

Interviewer: Benjamin Thompson

Barry is trying to map the neural basis of behaviour – seeing which neurons have to fire to trigger an output – and he says that fruit fly egg laying is a great system to use.

Interviewee: Barry Dickson

Many people now are trying to understand the fly brain and any brain at the level of circuits. How does a whole behaviour come together? Egg-laying behaviour is a pretty good model for this, where there’s a clear decision – when and where to lay an egg. And so, we can begin to explain that now in terms of the activities of individual neurons all the way through the female’s brain.

Interviewer: Benjamin Thompson

As circuits go, this one has a fairly clear input – mating – and output – egg laying – but the full picture of the wiring in between wasn’t clear. However, one thing that was understood was the role of a particular molecule.

Interviewee: Barry Dickson

We had known for many, many years that there’s a small peptide that the male provides to the female in the ejaculate that goes by the unfortunate name of the sex peptide. And that peptide then changes the female behaviour, and one of the things it does is initiate egg laying.

Interviewer: Benjamin Thompson

The sex peptide was discovered in the 1980s, but it wasn’t until the 2000s that Barry and his colleagues got an idea of what it was doing. They showed that this molecule binds to sensory neurons in the uterus of female fruit flies, sending a signal to their brains.

Interviewee: Barry Dickson

That part was relatively easy, but then you get into the brain and it was really difficult. It’s a mess, in terms of the neuronal wiring, so it was difficult to figure out where that signal went from there.

Interviewer: Benjamin Thompson

Despite having a brain the size of a poppy seed, fruit flies’ brains are still really complex, containing over 100,000 neurons. In recent years, researchers have been working to build up a 3D map to show the wiring between them in fine detail. Using this map, Barry was able to follow the signal’s route through the fruit flies’ brains and work out how it affected egg laying, also known as oviposition.

Interviewee: Barry Dickson

We identified these descending neurons, these neurons descending from the brain, and these neurons turn out to be command-type neurons for oviposition. So, that means if you activate them in a female, whether she’s mated or not, this will trigger all the motor sequences of oviposition.

Interviewer: Benjamin Thompson

Conversely, when these neurons were disrupted, fruit flies were unable to lay eggs, even after mating. Barry concluded that these descending neurons must be key to the egg-laying process. So, that wraps everything up then, right? Well, not quite. It turns out that female fruit flies are actually very picky about where they lay their eggs, and Barry also identified a set of neurons that feeds into this system. These neurons seem to be involved in selecting the best place for a fruit fly to lay an egg. He suggests these neurons work together to coordinate egg laying, making sure it only happens when a female has mated and found a suitable place to lay. But given the complexity of connections in the fruit fly brain, Barry doubts this is the whole story.

Interviewee: Barry Dickson

When you do the tracing, it’s kind of scary how many different uncategorised inputs and outputs there are from all these different neurons. Our functional analysis gives us confidence that we’ve found the key ones, at least that we were looking for. But all these other things – time, day, nutrition, whatever – sure they matter, and maybe those other neurons, those other inputs, provide some of those additional controls.

Interviewer: Benjamin Thompson

Barry has demonstrated the tight coordination between mating and egg laying, filling in some of the gaps in the neural circuit diagram linking the two. There is a way to go yet before all the intricacies of the system are uncovered, but Barry is hopeful.

Interviewee: Barry Dickson

The fly has a relatively complex brain. Sure, it’s a simple brain by comparison to vertebrates, but it’s at that sort of sweet spot, I think, of complexity but tractability, and in this system, we can put together these circuit-level explanations of behaviour.

Interviewer: Benjamin Thompson

That was Barry Dickson. You can read his paper over at nature.com.

Host: Nick Howe

Later on, we’ll hear about a giant survey of bird populations in India and about the genomes of rats in New York – that’s coming up in the News Chat. Before that, it’s time for the Research Highlights, read this week by Noah Baker.

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Noah Baker

Two newly discovered cockroach species are the earliest known animals adapted for life in caves. Researchers found the 99-million-year-old roaches exquisitely preserved in amber in a mine in Myanmar. One of the Cretaceous critters has many of the features typical of their modern cave-dwelling relatives, such as body size and small eyes. This suggests that many modern cave-living cockroaches could have origins dating back 100 million years or more, which would make these the only cave animals from the Cretaceous known to have living relatives. The authors think the roaches might have fed on the dung of pterosaurs and dinosaurs, much like some modern cockroaches that feed on the waste of birds and bats. Scuttle over to that study in Gondwana Research.

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Noah Baker

Many moths sport luxurious pelts of scales, and aside from being stylish as, it turns out there’s function in their fuzz. It absorbs the echolocation signals from bats hunting for a mothy meal. Researchers simulated bat echolocation and beamed it at two particularly hirsute species of moth. Then using tweezers and a stiff paintbrush, they de-fuzzed the moths and repeated the test. On average, the moth fuzz absorbs two-thirds of incoming sound energy, which is much better than commercially available sound insulation of similar structure and thickness. The researchers say that this acoustic camouflage means bats have to be significantly closer to moths to detect them, which helps the moths better evade capture. You can read more on that research over at the Journal of the Royal Society Interface.

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Host: Nick Howe

You’ve probably heard of obsidian – a type of glass-like structure that forms as lava rapidly cools. Well, it turns out that metals make similar glassy structures when cooled very quickly, which have different properties to conventional metals. These metallic glasses have a structure which makes them much stronger and more flexible. But under stress, they tend to fracture and break. Now, scientists may have found a way to solve this problem. Anand Jagatia tells us more.

Interviewer: Anand Jagatia

Metallic glasses are metals that capture the atomic structure of a liquid while still being a solid. This might sound strange, but it’s something that’s true of all types of glass. The atoms in a normal metal are extremely ordered in a regular 3D lattice. They’re crystalline. Metallic glasses, on the other hand, have a disordered structure, with the atoms all jumbled up. This disorganised structure of metallic glasses gives them some interesting properties compared to crystalline metals. They’re much better at storing elastic energy. They’re springy, and they’re also much stronger. These properties make them incredibly attractive materials for a whole range of applications, from computer components to surgical instruments. But there’s a catch. Despite being strong and springy, metallic glasses have a tendency to break if they are bent, unlike crystalline metals which can resist fracture. Here’s Lindsay Greer, a materials scientist from the University of Cambridge, to explain.

Interviewee: Lindsay Greer

For a crystalline metal, as you start to deform it, you shape and you bend it, the natural expectation is that when you do something like that to a material, it would get weaker. You would try to keep bending it and it would get weaker and weaker and it would break. But actually, crystalline metals are not like that. They get harder, they get stronger as you do that, and that’s the phenomenon called strain-hardening, and it’s a really important property really underpinning why metals are useful materials, you could say.

Interviewer: Anand Jagatia

But rather than showing strain-hardening like crystalline metals, metallic glasses undergo strain-softening, meaning they become weaker as they are bent. When you bend a metallic glass, you can get something called shearing, where layers of atoms end up lining up and then sliding past each other. This weakens the material and loosens the atoms, making further shearing even more likely, and so the metallic glass gets weaker and weaker and eventually breaks. But this week in Nature, Lindsay and his colleagues have discovered a process that could solve this problem.

Interviewee: Lindsay Greer

What we did was taking a cylinder of metallic glass, machining a notch around the circumference, and then compressing that cylinder along its axis.

Interviewer: Anand Jagatia

The team then cut out a cylindrical core about 3 millimetres long and 1.5 millimetres across out of the middle of this compressed notched cylinder and tested its properties.

Interviewee: Lindsay Greer

Well, when we deform it, it behaves more like a crystalline metal, so it actually shows strain-hardening, not strain-softening. It does not have the same yield stress at the original glass. That is lowered, so it is softer and that’s part of the trade-off. We have essentially traded off a bit of softness for the ability to strain-harden.

Interviewer: Anand Jagatia

Normally, bending a metallic glass introduces a bit of order into the atoms, and they can line up in a way that causes shearing. But this process of compressing a notched cylinder causes the atoms in the metallic glass to become even more disordered than usual, so bending it doesn’t cause shearing. Instead, the atoms pack more closely together, strengthening the material. In other words, strain-hardening. So, how much of an advance is this research? I reached out to Jan Schroers, a material scientist who wasn’t associated with this new study.

Interviewee: Jan Schroers

So, to me, the most interesting thing or why I really liked this paper is kind of like in crystalline metals, this was done almost exactly 100 years ago, that people understood these strain-hardening processes, and this has really revolutionised crystalline metals, structural metals. I would say this is a very big step.

Interviewer: Anand Jagatia

So, does this research mean that we’ll soon be seeing an explosion in the use of metallic glasses?

Interviewee: Jan Schroers

There are some challenges, in my eyes, because the process – they put a notch in the sample, they loaded and took that material out, and then characterised it – so that, of course, is not a scaleable process. So, the next steps, of course, for the community now has to be how can we create these glasses in the first place. That of course is the major work that has to follow now to make this a reality in applications.

Interviewer: Anand Jagatia

There is still a way to go then before this process can be adapted to metallic glasses on a larger scale but, in principle, it can be done, and if it can be scaled up, then these remarkable materials could be used in some pretty exciting ways.

Interviewee: Jan Schroers

So, what this work here has shown is you can create very strong materials that are also very, very tough. I would say for advanced structure applications – technologies like satellite technology, like maybe robotics, aerospace – that’s where you need these properties and this is where you will find this application. Really, then, it’s an unprecedented material.

Host: Nick Howe

That was Jan Schroers from Yale University in the US. You also heard from Lindsay Greer from the University of Cambridge, right here in the UK. You can find Lindsay’s paper over at nature.com.

Interviewer: Benjamin Thompson

Finally on the show, it is time for the News Chat, and I’m joined in the studio by Nicky Phillips, Nature’s Asia-Pacific Bureau Chief, who’s over here in London. Nicky, hi.

Interviewee: Nicky Phillips

Hello, thank you for having me.

Interviewer: Benjamin Thompson

Not at all, Nicky. Well, three stories today, and for the first one, I think it’s important we check in again on the COVID-19 coronavirus outbreak, and listeners, usual caveat applies – we’re recording this on Tuesday morning, but go to nature.com/news for all the latest updates on this. Nicky, what’s been going on since the show went live last week?

Interviewee: Nicky Phillips

Yes, well, obviously, listeners will know that it’s been a very serious situation in China, but things seem to have changed in the last week or so in that the number of new cases in China are actually slowing down but cases in other countries outside of China have actually surged. So, over the weekend, there was a jump in cases in places like South Korea, in Italy and in Iran, and then a bunch of countries in the Middle East actually reported their first infections, so that’s places like Kuwait, Bahrain, Afghanistan and Iraq. So, it seems to be shifting from a situation that’s just mostly contained in China to something that’s spreading to more countries around the world.

Interviewer: Benjamin Thompson

So, cases continue to increase, but in some cases, there has been some controversy about how these are counted.

Interviewee: Nicky Phillips

Yeah, in China there was a bit of controversy last week because it became apparent that China wasn’t counting cases where people tested positive for the virus but they didn’t have any symptoms, so these are called asymptomatic cases. And China says that they haven’t been counting these all along because they don’t definitely know that these people are infected with the virus. They could just have some traces of the virus in a nasal swab, but they might not be infected and so might not be able to pass the virus on to other people. Now, infectious disease experts that Nature spoke to disagree with this. They think that if a person tests positive for the virus that means the virus is replicating in enough numbers to be detected. They think also that not counting these cases can sort of skew the picture of the outbreak. If we’re not counting asymptomatic cases, it came make it harder to model, so that’s one thing they’re worried about. The other thing they’re worried about is that other countries who are preparing for the virus in their own countries might get the sense that the virus is only very severe, when actually there are cases of people that don’t have symptoms.

Interviewer: Benjamin Thompson

Well, it seems maybe regardless of counting methods, levels do continue to go up and around the world, as you say, Nicky. What does this mean about what this outbreak is?

Interviewee: Nicky Phillips

So, I guess the p-word, and by p-word I mean ‘pandemic’, is being discussed at the moment because of the increase in cases outside China. The World Health Organization has decided that despite the spread of the disease, it doesn’t yet amount to a pandemic. So, I guess what they’re worried about is that calling it a pandemic could create a lot of panic and they don’t think it fits the facts of the outbreak at the moment and it would just cause fear if they called it that. So, for the moment, they’re maintaining that it’s not a pandemic, but as we’ve seen, it’s a moving situation so we could see it called that yet.

Interviewer: Benjamin Thompson

Certainly one to keep an eye on there, and listeners, as I say, head over to nature.com/news for all the latest updates on the coronavirus outbreak. But for the time being, Nicky, let’s move on to our next story in today’s News Chat, and it’s about the levels of bird species in India.

Interviewee: Nicky Phillips

Yeah, so it’s a bit of a gloomy story, really. One of the first ever major reports on the state of bird populations in India has revealed that, actually, a lot are suffering pretty dramatic declines.

Interviewer: Benjamin Thompson

Well, what can you tell me about this report then? Where has this data on the levels of species come from?

Interviewee: Nicky Phillips

Most of the data from this report has come from amateur birdwatchers. Actually, more than 10 million observations over decades have been entered into a database called eBird, and the researchers used this data to kind of track either long-term trends in species, so over the last 25 years, or short-term trends, so in the last five years.

Interviewer: Benjamin Thompson

And what are some of the headline findings from the report?

Interviewee: Nicky Phillips

So, for long-term trends they looked at just over 260 bird species, and they found that more than half had declined in that time. And then for another group of birds, another 146 species, they looked at what has happened in the last five years, and they found that quite strikingly, 80% of those birds have been in decline. So, one of the most at-risk species is actually raptors, so species of eagle and harrier have decreased quite severely and, actually, vultures were another one that have really suffered, and that’s because they seem to be getting poisoned by a drug that is typically used as an anti-inflammatory for livestock and these birds are inadvertently eating it and dying.

Interviewer: Benjamin Thompson

Fairly bleak news then. Are there any bright spots at all in this survey?

Interviewee: Nicky Phillips

There are, so India’s national bird is called the Indian peafowl, and it has actually increased in population and expanded its range. The other one that’s doing quite well is the common house sparrow. It’s declined in some cities, but overall, throughout the whole country, it seems to be doing okay.

Interviewer: Benjamin Thompson

Well, it seems like quite a sizeable piece of work, Nicky. What are researchers saying about it?

Interviewee: Nicky Phillips

Researchers have been really impressed by this body of work, partly because it’s the first effort to look at so many species across such a huge country as India. They’re also really impressed by the fact that it was using citizen science data.

Interviewer: Benjamin Thompson

Well, let’s move on to our final story in today’s News Chat, Nicky, and let’s continue our tour of the globe. I think we’ll stop off in New York this time, and we’ve got a story about the rats that live there.

Interviewee: Nicky Phillips

Yeah, if you’ve ever been to New York, you were probably lucky enough to see a rat because they do inhabit much of the city and, actually, now some researchers in the US are looking at the genomes of rats specifically from New York because they want to see whether or not these rats have adapted to city living.

Interviewer: Benjamin Thompson

Right, and how on Earth do you go about seeing whether a rat has adapted to city living?

Interviewee: Nicky Phillips

Well, first, you have to catch them. So, the researchers spent quite some time catching the rats of New York – I can imagine they’re quite wily figures – but they managed to lure them with a mix of bacon, peanut butter and oats, and then they sequenced the genomes of 29 of them.

Interviewer: Benjamin Thompson

Right, and what did the researchers find?

Interviewee: Nicky Phillips

Well, they were comparing the New York rats’ genomes to the genomes of rats from rural China, which is where we think brown rats originated from, and they were looking for variations in the New York rats’ genomes.

Interviewer: Benjamin Thompson

And presumably, any useful genetic variations would then quickly sweep through the New York population. Did anything stand out?

Interviewee: Nicky Phillips

So, they found dozens of genes that were associated with diet and behaviour and mobility, and these might be linked to their behaviour but it’s too early to say that yet. This is just a starting point. They’ll have to do more studies to see whether these changes have actually influenced the rats’ behaviour.

Interviewer: Benjamin Thompson

Well, we’ll have to wait and see how that one pans out, and listeners, for more on these stories, head over to nature.com/news, and all that remains is to say, Nicky, thank you so much for joining me.

Interviewee: Nicky Phillips

Thank you for having me.

Host: Nick Howe

That’s it for this week. But before we go, there’s just time to tell you about some of our new videos. One explores facial expressions. How well do we really understand them? We’ve also got an animation all about a rare blood disorder. Find both of those over at youtube.com/NatureVideoChannel. I’m Nick Howe.

Host: Benjamin Thompson

And I’m Benjamin Thompson. Thanks for listening.