In this episode:

00:46 Solid-state flight

Powering a plane with ions. ResearchArticle: Xu et al.; News & Views:Flying with ionic wind; Nature video: Ion drive: thefirst flight

06:46 Research Highlights

Ant assassins and termites’ boringdebris. Research Highlight: Skull-collecting ants slay with acid; ResearchHighlight: Termite mounds dating back millennia can be seen from space

08:35 The brain's mosaic DNA

Researchers have shown how neurons'genes can change over time. Research Article: Lee et al.; News and Views: A mosaic mutation mechanism in the brain

16:03 News Chat

An overhaul of SI units, and predicitingvolcanic eruptions. News:Largest overhaul of scientific units since 1875wins approval; News:World’s first automated volcano forecastpredicts Mount Etna’s eruptions

TRANSCRIPT

This week, a solid-state plane engine, and ‘mosaicism’ in brain cells.

Host: Shamini BundellWelcome back to the Nature Podcast

Host: Benjamin Thompson

And hearing about the DNAdifferences in individual brain cells. I’m Benjamin Thompson.

Host: Shamini Bundell

And I’m Shamini Bundell.

Interviewer: Shamini Bundell

If you watch early-1900s footageof the Wright brothers demonstrating one of the first powered, controlledaeroplanes and you listen very carefully… you probably won’t hear anythingbecause the footage doesn’t have any sound. But the invention’sgasoline-powered engine turning two propellers presumably made pleasing rattleas it flew along just above ground level. This week, you can see footage of anew type of flying machine, one that even with the sound turned up isremarkably silent as it gently glides across a sports hall a couple of metresabove the floor. And silent it just the way that one of its creators, StevenBarrett, wanted it.

Interviewee: Steven Barrett

Well, the idea is, in a way, achildhood fantasy. I used to be a big fan of Star Trek and sort of thought thatthe future of flight shouldn’t be things with propellers and turbines andshould be more like with a kind of blue glow and something that silently glidesthrough the air.

Interviewer: Shamini Bundell

Steven works in the Department ofAeronautics and Astronautics at the Massachusetts Institute of Technology, sohe’s pretty well placed to create a glowy, futuristic-looking flying machine.But the method he’s used to power his new plane has actually been around sincenot long after the first commercial flights.

Interviewee: Steven Barrett

It dates back until at least the1920s where an eccentric inventor at the time started experimenting withhigh-voltage electrodes and thought he had discovered antigravity, which ofcourse was not the case, but that set some of the initial groundwork, some ofthe very old patents on mechanisms for creating what’s called an ionic wind.

Interviewer: Shamini Bundell

And those basic mechanisms arewhat lie behind the development of Steven’s new plane – a lightweight,fixed-wing aircraft with a 5-metre wingspan, powered by electricaerodynamicpropulsion, or what’s sometimes known as an ion drive.

Interviewee: Steven Barrett

So, what we did for this designis to try and stick to something that looks somewhat like a conventionalaircraft but under the wing, rather than conventional engines, it has a seriesof electrodes and those consist of an array of very thin wires at the front andthen an array aerofoils at the back. Now those thin wires at the front are setat a very high voltage – +20,000 volts – and where that high field strengthoccurs it creates a source of ions.

Interviewer: Shamini Bundell

The ions are created when electronsare knocked off nitrogen molecules by the wires of the positive electrode atthe front. The ions are therefore positive nitrogen ions, meanwhile theaerofoils at the back of the plane are negative electrodes. Opposites attractso the positive ions move towards the back of the plane.

Interviewee: Steven Barrett

And so, on that path frompositive to negative, the ions collide with air molecules many, many times,transferring momentum to the air, creating a breeze or an ionic wind that’sleft behind.

Interviewer: Shamini Bundell

And so, as nitrogen ions pushagainst the air molecules, thrust is created, silently and invisibly propellingthe plane forward. Well, that was the theory anyway.

Interviewee: Steven Barrett

Many attempts failed because ofvarious things going wrong like structural failures, the power electronicsfrying itself. The first day that it actually worked it was about 50% power soit was a power glide but there was quite a lot of excitement and a lot ofcheering when that happened.

Interviewer: Shamini Bundell

From that first glide, the teamwere soon able to make the first fully powered flight, and it’s no surprisethey were so excited about it – it’s taken decades to put this technology intopractice in this way. For example, spacecraft have been using ion thrusters fordecades but with a design that only works in a vacuum. Here on Earth, it’srelatively simple to create a little ion-driven lifter that jumps off a table,but that requires the craft to be attached by wires to a large power sourcenearby. The new plane has on-board batteries and is remote-controlled.

Interviewee: Steven Barrett

So, what we achieved was thefirst ever sustained flight of an aeroplane that is propelled byelectroaerodynamic propulsion, and that’s also by many definitions the firstever solid-state flight, meaning no moving parts.

Interviewer: Shamini Bundell

This achievement has been madepossible with modern technology such as lightweight batteries, and it’s animpressive feat of engineering to get it to work. Here’s Kris Pister of theUniversity of California, Berkeley.

Interviewee: Kris PisterNo one has ever been able to do this before and plenty of peoplewould have said, ‘No, that’s not possible, that will never work.’

Interviewer: Shamini Bundell

Kris works in this area himselfand is optimistic about some of the applications, though perhaps not the onesinvolving futuristic, glowing flying machines or ion-powered passenger planes.

Interviewee: Kris Pister

I’m sceptical of whether it willhave practical application at large scale in the atmosphere. I think that it’sa technology that scales well, so for me as a micro-robot person, propellersdon’t work well at a millimetre scale whereas this technology has the sameperformance kind of independent of scale. So, at a small scale, this may end upbeing the best game in town.

Interviewer: Shamini Bundell

Potential applications includecreating silent drones which could be used to observe wildlife or monitor trafficin urban areas without creating noise pollution. Hopefully this is just thefirst step in developing useful flying ionocraft, and the sight of thissilently gliding machine with no visible power source or propulsion may wellinspire future researchers to explore new uses for this strange technology.

Interviewee: Kris Pister

It’s cool because the physics isso different from the physics of flight that we’re used to and you don’t haveto be a physicist to appreciate that.

Interviewer: Shamini Bundell

That was Kris Pister and we alsoheard from Steven Barrett, whose paper is published in Nature today.Find more coverage of the work at nature.com/news, where you’ll also find thevideo of the plane in action.

Host: Benjamin Thompson

Later in the show, Lizzie Gibneywill be joining us to talk about a momentous day for scientific measurement –that’s in the News Chat. Up next though, it’s time for the Research Highlights,read this week by Anna Nagle.

Interviewer: Anna Nagle

The nests of Formicaarchboldi ants are filled with a rather macabre collection: thescattered body parts of their prey, ants from the Odontomachus genus.Until now, the way the Formica ants were able to take downtheir targets had been something of a mystery, as their Odontomachus preyare much larger and fiercer with spring-loaded jaws. Researchers discoveredthat the Formica ants are coated in chemical waxes that mimicthat of their prey. These waxes disguise their odour, allowing the ants to getup close to their victims before delivering a precise stream of paralysingformic acid. Head over to Insectes Sociaux to find out more.

Interviewer: Anna Nagle

In the semi-arid forests ofnortheastern Brazil, there stand tens of millions of huge earthen mounds.Equally spaced and up to four metres tall, the dirt hillocks cover an area thesize of Great Britain and are thousands of years old. To find out more aboutthem, a team of researchers cut into hundreds of the mounds. Most were soliddirt, but many contained entrances to an underground network of tunnels createdby the subterranean termite Syntermes dirus. The team suggest thatthe discarded detritus is the result of the termites tunnelling travails. It’sthought that by dumping dirt at regularly spaced intervals, the termites minimisethe time taken to reach a disposal zone from anywhere in their undergroundnetwork. To dig that research out, go to Current Biology.

Host: Shamini Bundell

Next up in the podcast, AliJennings has been learning about how neurons change their DNA.

Interviewer: Ali Jennings

Most cells in your body carry acopy of your genome – information written in DNA. And for the most part, theDNA sequence doesn’t change. However, things are a little different when youlook at the brain. It’s been known for a while that cells in the brain can havedifferent DNA sequences from each other, and not just small mutations – largechunks of genome can be copied and moved around. Having different DNA sequencesin different cells is called genomic mosaicism, and it can have profoundeffects on gene function and cell survival, both positive and negative. Butdespite its importance, researchers have struggled to explain how it happens.Now, this might be about to change because a group of researchers haveuncovered a possible mechanism for mosaicism. They wanted to understand how asingle gene could vary between cells, so they chose a gene that is known toshow many mosaic changes – the amyloid precursor protein gene, or APP forshort. The researchers examined APP in neurons taken from human brains. Butinstead of reading the DNA sequence directly, they looked first at RNA which istranscribed based on the DNA. This let them spot possible changes in the APPgene in different cells. Here’s Jerold Chun, from the Sanford Burnham PrebysMedical Discovery Institute in California.

Interviewee: Jerold Chun

The first thing we did is welooked at RNA and what that revealed was a whole menagerie of previously unseenvariants of APP. There were truncated forms that had lost the central portionsof the gene, and they had been stuck back together to form new molecules.

Interviewer: Ali Jennings

This suggested that the neuron’sDNA contained different versions of the APP gene along with the original. Andindeed, looking through the genome of the neurons revealed a whole host of APPgene variants. To visualise what they’d found, the team ran the samples througha gel to separate the differing DNA segments into different bands, depending ontheir size.

Interviewee: Jerold Chun

Sure enough, we pulled up a wholebunch of bands – not just a few, just a ton of bands. In fact, so many bandsthat they were smearing, they formed a smear within our gel.

Interviewer: Ali Jennings

So, how did each cell end up withmultiple differing versions of the APP gene? And why did different neurons havedifferent gene variants? Jerold and his team think that something unusual ishappening to the RNA from the APP gene.

Interviewee: Jerold Chun

We have APP. It’s transcribed toproduce an RNA, and then it can be copied in its entirety back into the genomeas a genomic cDNA.

Interviewer: Ali Jennings

cDNA, or complimentary DNA, isformed when you copy an RNA sequence back into DNA, reversing the usual routeof DNA being transcribed into RNA, which then produces protein. This reversetranscription could explain all of the APP gene variation.

Interviewee: Jerold Chun

It appears that they are reversetranscribed through an enzyme called reverse transcriptase. Reversetranscriptase is kind of a sloppy copier, and so it starts to produce errors asit copies.

Interviewer: Ali Jennings

So, neurons gain multiple,badly-copied versions of the APP gene thanks to the enzyme reversetranscriptase. This leads to the mosaicism of APP – the changes in its DNAsequence. Mosaic changes in the genome seem to occur as a part of normalneuronal activity, but there’s another side to the story. One of the reasonsthat Jerold’s team chose to look at APP is that amyloid precursor protein islinked to Alzheimer’s disease. One element of this is that people withAlzheimer’s show more mosaicism in their APP gene. The team wanted to see iftheir newly discovered mechanism – the reverse transcription of APP – was alsoresponsible for the mosaicism in Alzheimer’s brains. So, they compared neuronsfrom people with Alzheimer’s to neurons from healthy people.

Interviewee: Jerold Chun

And so, in a normal case, wedon’t see nearly as many of these variants. In Alzheimer’s disease, we havemore variants, variants that have more mutations or variations within them, andjust plain old more variants inserting within the neuronal genome that maycause them to die. So, you might think of this in the case of Alzheimer’sdisease as something gone awry with the normal process.

Interviewer: Ali Jennings

This work offers new insight intoa potential mechanism underlying some forms of Alzheimer’s disease. But whatdoes it mean for future treatments?

Interviewee: Jerold Chun

What was very, very interestingabout that conclusion was that it suggested that maybe if you were able toinhibit reverse transcriptase, you might have an effect on the course ofAlzheimer’s disease.

Interviewer: Ali Jennings

So, by stopping the sloppyreverse transcriptase from inserting lots of incorrect versions of APP backinto the neurons’ genomes, you might be able to tackle the disease. We askedPierluigi Nicotera, the Scientific Director of the German Centre forNeurodegenerative Diseases, for his thoughts on the new paper.

Interviewee: Pierluigi Nicotera

So, I must say when I startedreading the paper I was initially a bit sceptical because it seemed a bitfar-fetched. As I read over it I got more and more enthusiastic about it. Tosee the extent to which they went into making sure that there was absolutely avery thorough validation of their own observations and results I think isbeyond many other papers that I’ve seen in my entire career. I think this isreally a breakthrough.

Interviewer: Ali Jennings

But could targeting reversetranscriptase really offer a new root to treating Alzheimer's disease?

Interviewee: Pierluigi Nicotera

In principle, it’s absolutelyplausible to me. If I had the possibility to run a clinical trial like this ina group of patients with an early stage of Alzheimer’s disease and without toomuch side effects, I think I would go for it.

Interviewer: Ali Jennings

This study has revealed a new wayfor neurons, although born with identical genomes, to become unique throughmodification of their own DNA. The paper shows how this mechanism is importantfor APP and Alzheimer’s disease, but both Pierluigi and Jerold wonder if thiskind of DNA rewriting is also happening with other genes, perhaps contributingto the huge variety of neurons that we see across the brain.

Host: Shamini Bundell

That was Ali Jennings talking toPierluigi Nicotera. You also heard from Jerold Chun, whose paper you can readover at nature.com/nature.

Interviewer: Benjamin Thompson

Finally this week, it’s time forthe News Chat and joining me in the studio is Lizzie Gibney, Senior Reporterhere at Nature. Lizzie, thanks for stopping by.

Interviewee: Lizzie Gibney

Hello Ben.

Interviewer: Benjamin Thompson

Well, for our first story todaythen Lizzie, we’re going to talk a bit about science history and perhapshistory in the making, and you were there in the room. What’s been going on?

Interviewee: Lizzie Gibney

That’s right. So, I went toVersailles which is just outside Paris, for the official event that defined theredefinition of some of the units of measurement that we use, both in scienceand in the rest of the world.

Interviewer: Benjamin Thompson

So, what sort of measurements arewe talking about then?

Interviewee: Lizzie Gibney

So, the unit of mass – thekilogram, the unit of current – the ampere, the unit of temperature which isthe kelvin, and the unit of substance – the mole.

Interviewer: Benjamin Thompson

Well, before we talk about maybewhat they’ve been redefined as, maybe we should talk about what they weredefined from.

Interviewee: Lizzie Gibney

Well, the most famous one is thekilogram so that is the very last unit that’s defined according to a physicalthing. So, this is like about a palm-sized block of metal, platinum iridium,and since 1889 that has been the kilogram. So, every time any kilogrammeasurement has been made in the world, that effectively has been calibrated atsome point traced back to this one kilogram that sits in Paris. Now, that isclearly a bit of a ludicrous situation to be in because although it’s keptunder lock and key in very careful conditions, someone could lose it, it mightgain or lose a few atoms here or there, and that would be a big problem for theworld.

Interviewer: Benjamin Thompson

So that’s where we were then, sohow much does a kilogram weigh now then and how do you define it?

Interviewee: Lizzie Gibney

A kilogram still weighs akilogram, although the former kilogram may not weigh exactly a kilogram anymorebecause the definition is changing. So, the definition is now going to be basedon fundamental constants of nature so for mass that’s Planck’s constant. Now,it might not seem easy to see the relationship between mass at Planck’sconstant. This constant relates frequency to the energy of a particle. It’s aquantum number. But the way that physicists have done this is a very, veryclever system. They essentially put a mass on one side of a balance, and on theother side of the balance produced an electromagnetic force, and if you plug inthe current and the magnetic field that you’re using to create that, you canrelate these quantum constants to the mass that you have on the other side.Now, for years they’ve been doing experiments like these to come up with very,very, very precise measurements for Planck’s constant, measuring it against thekilogram. Now, in the future, what they’re going to do is they’re going to flipthat experiment and they put in Planck’s constant and that they can use tomeasure any mass that they want on the other side of the balance.

Interviewer: Benjamin Thompson

Right, well that seems quiteclever, but it also seems kind of complicated as well though. Why would we wantto do that compared to this block that we have, and have been using?

Interviewee: Lizzie Gibney

Well for one reason as I said,that block is vulnerable because by definition it always has to weigh akilogram. There was something very nice said at the conference which was if youleft a fingerprint on that metal block, it would still weigh a kilogram, butthe whole rest of the world would weigh less. That is clearly a problem. Otherthan that, the idea of defining mass by these experiments and fundamentalconstants means that you can actually do that anywhere – it democratises thesystem. So, if you have the right setup – which is very precise but they’retrying to make it easier and cheaper – then you can create an exact a kilogramas possible anywhere in the world.

Interviewer: Benjamin Thompson

Brilliant, well if that’s thekilogram then, you mentioned some others too, how have they changed then?

Interviewee: Lizzie Gibney

So that’s a little bit moresubtle. Some of them involved the kilogram in their definition so they’veshifted as a result of the change in the kilogram. One of the other moresignificant ones is the ampere, which used to be to do with two infinite wires andthe force between them. Now, two infinite wires clearly can’t exist, so thiswas a hypothetical, abstract concept which wasn’t very satisfying formetrologists – the scientists who study measurement – so instead, it’s going tobe defined in terms of the flow of individual electrons, so the actual chargeon a single electron, which is just a wonderfully precise way to measurecurrent.

Interviewer: Benjamin Thompson

Alright then, well let’s thinkabout this then, so we’ve had these hundreds of years, these old standards, andnow we’ve got these new standards. Is it a flip of a switch from old to new?When are they going to get brought in?

Interviewee: Lizzie Gibney

So, this was the official greenlight and people have been working on it for decades now, but it will come intoforce on the 20th May next year, so mark your diaries.

Interviewer: Benjamin Thompson

Well, you talked about kelvinthere, what was the temperature in the room? How excited were people to bethere and do this vote?

Interviewee: Lizzie Gibney

It was an absolutely wonderfulatmosphere. As I say, there’s been a huge amount of work that’s gone into thisand they were certain the vote would go through. They knew that it would begiven this green light, but there was a standing ovation, there was champagneafterwards. They were all just wonderful quotes from people saying this is adream come true, this is a thrill ride. Yeah, this is the biggest day inmetrology for probably hundreds of years, since the founding of the SI systemand even since the introduction of the metric system during the FrenchRevolution.

Interviewer: Benjamin Thompson

Fantastic. Well, Lizzie, I knowyou were tweeting up a storm during the event. Where can people find yourthread?

Interviewee: Lizzie Gibney

@LizzieGibney on Twitter.

Interviewer: Benjamin Thompson

Perfect. Alright, well let’s moveon to our second story, and it also involves measuring something, but mygoodness, I think the scales couldn’t be more different if we tried. What havewe got for this one?

Interviewee: Lizzie Gibney

That’s right, so this is thefirst automated system for getting an early warning about volcano eruptions, atleast at this stage a particular eruption which is of Mount Etna which is avolcano in Sicily.

Interviewer: Benjamin Thompson

I mean I know on the podcastbefore we’ve talked about volcanic eruptions. What’s been going on that’sallowed them to kind of do this?

Interviewee: Lizzie Gibney

This has been a study looking atthe very low-frequency sound waves, infrasound, so these are waves that peoplecan’t hear but that travel for thousands and thousands of kilometres, and thisis something that scientists have realised that they can actually use in orderto not only detect eruptions but to predict them.

Interviewer: Benjamin Thompson

Okay, and how does that work?

Interviewee: Lizzie Gibney

The idea is that as gas comes outof the magma, of the lava, ahead of an eruption, it causes the air within thecrater to kind of move back and forth, to slosh, in a way that creates soundwaves a bit like they would in an instrument, and just like in an instrumentyou can use those waves to figure out the geometry of the space inside whichthey’re moving. So, what scientists did was, starting back in 2010, theystarted to look at eruptions and listen out for these infrasound signals, andto figure out whether they could actually use that infrasound in order topredict when an eruption was going to happen.

Interviewer: Benjamin Thompson

And did they manage it?

Interviewee: Lizzie Gibney

They did. So, in a period ofabout eight years, the system was successfully able to predict 57 out of 59events, which means it actually sent messages to the scientists an hour beforethe eruption took place.

Interviewer: Benjamin Thompson

I mean that’s really clever, andI know with these kind of disastrous events, I mean an hour is so useful,right?

Interviewee: Lizzie Gibney

Exactly, so often you needexperts to vet information if they see an eruption might be on the cards – thattakes time. This is an automated system that can work faster than that, whichis really important in all the situations where time is really of the essence,so if you’re a community that lives near a volcano or maybe you’re a planethat’s flying into a region near a volcano, this is exactly what you need toknow and fast.

Interviewer: Benjamin Thompson

I know that all volcanoes aren’tnecessarily created equal and there’s different sorts. Is this something thatcould be used in other locations?

Interviewee: Lizzie Gibney

So, scientists hope that thesystem will work in other kinds of open vent volcanoes – a particular kind thatexists. So for instance, one is Mount Pavlov in Alaska, so that might beanother test in the future for this kind of early warning system, and they’realso actually already employing sensors in order to see if this will work inIceland.

Interviewer: Benjamin Thompson

Great stuff, thank you, Lizzie.That’s it for this week’s News Chat and listeners, that’s it for this week’sshow. As always, if you’d like to find out the latest news from the world ofscience, head over to nature.com/news.

Host: Shamini Bundell

And don’t forget to check out ourvideo of the silent ion plane. You can go to nature.com/news oryoutube.com/NatureVideoChannel to see it in action. I’m Shamini Bundell.

Host: Benjamin Thompson

And I’m Benjamin Thompson. Seeyou next time.

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