Nature Podcast 6 April 2006

This is a transcript of the 6 April edition of the weekly Nature podcast. Audio files for the current show and archive episodes can be accessed from the Nature podcast index page (http://www.nature.com/nature/podcast), which also contains details on how to subscribe to the Nature podcast for FREE, and has troubleshooting top-tips. Send us your feedback to mailto:podcast@nature.com.

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This week Tiktaalik and what it can tell us about how early life invaded the land, mole rates, or fat cats, how one species' lot in life is to not do very much, the genome of waste water-consuming anammox bacteria gets cleaned up, moon dust sheds a little light on the composition of the sun, and the first example of a parasite that can escape the clutches of its hosts' predators.

Chris Smith: Hello, I'm Chris Smith. Welcome to the 6th April edition of Nature's podcast. First this week we're winding our watches back about 380 million years to the time when the first animals substituted feet for fins and began to heave themselves out of the sea and onto land. Although we know this must have happened, fossils of animals undergoing this transition have been hard to come by but now, working in Canada in what would've been a prehistoric swampy river delta, the University of Chicago's Neil Shubin and his colleagues have uncovered specimens of Tiktaalik, a primitive fish with a head like a crocodile, ribs and fins like feet. (Nature 440, 747-749; 2006 | Nature 440, 757-763; 2006 | Nature 440, 764-771; 2006).

Neil Shubin: The transition from a fish that lives in water to an animal that's able to live on land is one of the great transitions in the history of life. We know from a variety of different lines of evidence that this transition happened around 380-365 million years ago. We also know the likely players involved; the different kinds of fish that are likely to have given rise to land-living animals. Now, remember, we're talking about land-living animals. We're talking about a whole branch of the tree of life, that branch that includes amphibians, reptiles, birds, mammals, even us, so this is really a part of our own, albeit very ancient, evolutionary past. The discovery is really important because it's the discovery of a new kind of fish that blurs the distinction between fish and land-living animals. It has a mosaic of features seen in both. Like a fish it has scales and fins and a very primitive jaw, but like a land-living animal it has ribs that fit together. It has a neck where the head can move around separately. It has a flat head like primitive amphibians and, importantly, when you look inside the fin it has a number of bones that compare very closely to the bones in our own wrist, in our own hand. And so, in that sense, it's a real mosaic and tells us, to a great extent, how parts of our skeleton evolved.

Chris Smith: And where was it found?

Neil Shubin: We found it in a place called Ellesmeer Island which is one of the northernmost islands in Arctic Canada, several hundred miles south of the North Pole. When we work there in the summers it's daylight 24 hours a day, there are polar bears walking around, there are mustocks. It's a classic arctic landscape with ice and glaciers. But between the ice and glaciers is bedrock and that bedrock is Devonian age rock of about 375 million years old and the environments that are contained in those rocks reveal an ancient river or delta ecosystem, and within that delta ecosystem we're finding a variety of different kinds of fish of which this new kind is just one.

Chris Smith: But a common criticism in the past has been that there are hardy any of these fossils ever found, these transitions from one state to another.

Neil Shubin: Yes exactly, and this really puts a lid on that because not only do we have a beautiful transitional form, but we have multiple skeletons of it, and when you compare this creature, the new one, to other creatures which we've known about before, a creature called Panderichthys or Eusthenopteron which are creatures that are known from the Baltics or Russia, compare them to the earliest known land-living animals, a creature known as the Acanthostega. When you put this whole series together it's truly one of the remarkable transitional series between different kinds of life in the history of the earth.

Chris Smith: How do you think it came by these really interesting adaptations in the first place, though?

Neil Shubin: Let's think about how this thing lived. What it has is a flat head with eyes on top, much like a crocodile. It has nostrils on the side. It's a very flat animal. It has an appendage or a fin that's able to bend its elbow and its wrist. It has a ribcage which suggests that it was able to support itself in gravity. What you have is an animal that's clear specialising for life on the water bottom, in the shallows or even out in the air for periods of time. So, if we look at the geology of the site this thing came from, it came from a very small stream in a large delta environment, a giant swampy environment, and it's within these small streams which we believe that we find the locus for the evolution of many creatures that later forms used to evolve on land.

Chris Smith: Neil Shubin from the University of Chicago introducing Tiktaalik which is pointing the way to the origins of the tetrapod limb. Now, taking up the story on how this animal came by its unusual adaptations his co-author, Ted Daeschler, from the Academy of Natural Sciences of Philadelphia

Ted Daeschler: The rocks that we find tictalic in are clearly the deposits of meandering stream systems, so flood plains, and what we can deduce is that these features enabled these animals to exploit new opportunities in these habitats along these flood plains. And, of course, these animals had no intention of eventually coming on land, they were existing and doing well with the habitat that they were specialised for, but as often happens in evolution, features that developed to be useful in those sorts of habitats set the groundwork for features that would eventually be helpful for animals to actually move onto land.

Chris Smith: Does the geology give you any clues as to what might have lured these animals onto land?

Ted Daeschler: The geology itself doesn't particularly tell us why these animals moved onto land. In the case of Tiktaalik really I can't imagine that it did spend much time on land, but if we do look at the history of life, we can be very clear that plants were diversifying in the late Devonian, there were invertebrate sorts of animals, arthropods in particular, scorpions, millipedes, spiders, which were also specialising for these late Devonian flood plains. And so, in many ways, the story with the fish-tetrapod transition wouldn't have been written if it weren't for the changes to life with plants and invertebrate animals as well.

Chris Smith: Ted Daeschler. But what questions still remain unanswered? Neil Shubin again.

Neil Shubin: There are a number of unresolved issues with the story of the transition to land. We haven't found a single vertebra yet, which is a bit of a mystery, actually, because if you're thinking about a creature that is supporting its body, the backbones are really important, and it may very well be that the backbones are preserved but only in cartilage. One of the things we want to do, in going back this year, is to search for evidence of the vertebrae.

Chris Smith: Now you know who's to blame if you get tennis elbow. That was Neil Shubin introducing Tiktaalik , the fish that found its feet.Now, someone who's looking back even further into the past is Australian National University's Trevor Ireland who's been analysing fragments of the solar wind embedded within samples of moon rocks retrieved by the Apollo 11 mission. The idea is to try to work out the composition of the protoplanetary soup that spawned our solar system, but instead of shedding light on the Sun's chemistry, the new results have thrown an isotopic spanner in the works because they show far less Oxygen 16 than other models and measurements would've predicted. (Nature 440, 776-778; 2006).

Trevor Ireland: We have very little accurate information about the chemical and isotopic composition of the Sun, and the issue is here is that that composition defines a lot of our models of how the Solar System formed, so we set out to try and establish what the starting composition of the Sun might be. The model suggests that the Sun should be very O16 rich and what we found was something completely different. It's a composition was not enriched in oxygen 16, it was actually depleted or enriched in Oxygen 17 and Oxygen 18, and if this composition is correct, it disagrees with all the present models for the solar system.

Chris Smith: So how did you set out to solve the puzzle in the first place? What did you actually do?

Trevor Ireland: What we did was to look through one of the lunar return samples that was collected by Apollo 11 and separate out grains which didn't have any oxygen in them in the first place, and so what we were after was the solar wind composition.

Chris Smith: And you can infer that the solar wind, because it's coming from the sun, must be a pretty good reflection of what the Sun's made of?

Trevor Ireland: That's right. The idea is that the solar wind is actually implanted below the surfaces of grains and so you can avoid all the chemical alteration that might be taking place. As soon as we got rid of the top 20 nanometres of surface material on these grains, the oxygen showed a distinct composition which is different from anything else that's ever been measured before.

Chris Smith: Do you actually think that the solar wind isn't going to be enriched for one particular isotope of oxygen that might be misleading? We're not measuring someone else's solar wind, for example?

Trevor Ireland: Not as far as we know. The solar wind bombards the lunar surface and the lunar surface is actually gardened by little micro-meteorite impacts, so grains at the top eventually go down below and are removed from the solar wind so we suspected it's just the grains right at the top of the surface which will be the ones we'll need to find out the solar wind composition.

Chris Smith: And what are you going to do to now resolve the problem you've created?

Trevor Ireland: This is a good question because it's something that we didn't really set out to find, and so this one's very strange and obviously needs a lot more research, so what we're going to try and do is see how prevalent it is around the surface of the moon and do a number of different samples to see if it's a common signature, or if we've just missed the point entirely and it's a very localised signature from something which we don't understand either.

Chris Smith: A scientist with a mystery on his hands. I guess we'll just have to watch this space for the answer. Trevor Ireland from the Australian National University.

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Chris Smith: Coming up shortly the two-tier rain driven lifestyle of the mole rat and the world's first example of a parasite adapted to escape from its host's predators. But first, with this week's news, Anna Lacey talks to Nature's Alex Witze.

Alex Witze: Thanks. We've got two stories this week actually dealing with the South Pacific. The first one sounds like paradise but not quite. We had a reporter go to the small South Pacific nation of Tuvalu, which is only a few metres above sea level, to go and discover whether, in fact, the nation is drowning, so he went down there and talked to some of the researchers who are trying to figure out how long they have before Tuvalu actually gets flooded out. ( Nature 440, 734-736; 2006).

Anna Lacey: What's the extent of the flooding?

Alex Witze: It varies from year to year so this year, in fact, they had some of the worst tides in quite some time. The tides came to 1.5 metres above sea level and since a lot of the island is just a few metres above sea level, as you can imagine, it tends to inundate large parts of the island.

Anna Lacey: But in 2000 there were reports that the sea level was actually falling on this island so what's all that about?

Alex Witze: There have been reports, yes, that it was falling, but there is basically only one tide gauge on the whole nation and there are questions about how that data is analysed and whether it's showing a rising or a fall. The intergovernmental panel on climate change has estimated that you've got a 1-2mm rise per year over the past century, and depending on how you crunch these numbers from this single gauge on Tuvalu one groups gets a rise of about 1.2mm which would be sort of right. But, yes, it's an ongoing debate.

Anna Lacey: But seeing as Tuvalu is being used an example, saying that climate change is really happening, we've got sea level rises and they've got one gauge, how can scientists justify using that data?

Alex Witze: That's an excellent question and scientists, of course, would say they need more data, more gauges, whenever possible. It is very much the frontlines of climate change and people are talking about trying to really instrument nations such as Tuvalu and get the data that they need to really understand what the threat is, not only to the country obviously, but to the people as well.

Anna Lacey: But beaches are known to be pretty dynamic places. Is it really climate change?

Alex Witze: Right, so that's the $64,000 question as always. Scientists say yes, and something is going to happen. What exactly that is we don't know yet but this is the place to look.

Anna Lacey: And so the people of Tuvalu aren't planning to leave then?

Alex Witze: Some of them are. There are big discussions about the local climate refugees. Many people, of course, don't want to leave, but the notion of climate refugees and whether other countries will have take them in is definitely a touchy political subject, and whether most people have to resettle remains to be seen.

Anna Lacey: Well, now let's move from people wanting to stay in the South Pacific to artefacts going AWOL.

Alex Witze: That's right. We've got a story that's really quite interesting. There is a new museum in San Francisco in California that recently opened after renovation to replace its earthquake damaged structure, but one of the centrepieces of its exhibits is thousands of artefacts from Melanesia. In particular there is a collection from a very prominent art collecting family that has been donated. There are, within that collection, something like 400 artefacts from Papua New Guinea, so you've got these fabulous funerary masks, ritual figures, carvings, sculptures, large gongs, our reporter found that at least nine out of 400 have a questionable background. They are, in fact, listed as part of the national cultural property of Papua New Guinea, which is essentially a government designation that says, hey, it's something that matters to us as a country. (Nature 440, 722-723; 2006).

Anna Lacey: So what's the problem with them being returned then?

Alex Witze: It depends who you ask. Some of the challenges might be whether the native country, in the case of Papua New Guinea, might have enough money to take care of them. Do they have enough money to keep, for instance, the air-conditioning running, to keep the security systems running so that nothing might be get stolen? Our reporter talks to some of the authorities at the museum back in Port Moresby who acknowledge that, yes, money is constantly a problem, they just aren't as well funded as they want to be. So, one issue is whether they would be well taken care of if they were returned. And if you think about a country such as Papua New Guinea, which is not very politically powerful, does not have a lot of money towards trying to recover some of its lost patrimony, these issues become really important for it.

Anna Lacey: So what now, then?

Alex Witze: What now? The director of the museum has taken these questions quite seriously. He says he's going to investigate and look into where these artefacts came from. They have in the past returned objects to other countries if it turns out that they have been questionably acquired. There is a cultural ambassador from Papua New Guinea who is actually coming to the United States this week and will be looking into it as well, so investigations are underway.

Chris Smith: Nature's Alex Witze talking to Anna Lacey.

All the stories we're discussing this week are available online at http://www.nature.com/nature, and you can access them with a personal subscription to Nature or through an institutional site license. The details of both of these options are available from http://www.nature.com, and do please send us your comments or feedback. The address to write to is mailto:podcast@nature.com.

Now, African mole rats. These social rodents live together in cooperative underground colonies, but they don't seem to share the housework very equally. There appears to be a two-tier system with the population divided into highly active frequent workers who do 95% of the work, and a fatter population of infrequent workers who do almost nothing for most of the time. But when it rains these fat cats suddenly begin to dig furiously and they head off to found a new colony. (Nature 440, 795-797; 2006).This suggests that this energy conserving lifestyle is a way of building up body reserves to assist in dispersal. Mike Scantlebury is from the University of Pretoria.

Mike Scantlebury: What we found is that there are differences in energy expenditure between what we think are different castes of mole rates living in the Kalahari in South Africa, and mole rats are rodents, they eat tubers and vegetables, and they burrow deep into the sands and you can hardly see them. Nobody knows where they go and what they do, but we did have some idea from laboratory studies that within a colony, which is consisting of one breeding queen and maybe one or two breeding males and lots of non-breeding individuals, different animals were specialised for doing different things. So, there is a parallel here with the social insects where they have specialised castes.

Chris Smith: Do they actually have a pecking order then, and one animal will slowly ascend that pecking order as it becomes more senior within the colony?

Mike Scantlebury: Not really because, in terms of breeding, there is one queen and it doesn't change until that queen dies, and when colonies get founded it's settled by one breeding female and one or two or three breeding males, but the offspring they produce are not reproductive. So, the interesting thing that we've found is that we notice that after rain there is an awful lot more digging around colonies of mole rats, so we measured energy expenditure in four individuals in wild colonies of mole rats and found that there was a difference in energy expenditure between different types of mole rats that we found.

Chris Smith: And the big ones do less work than the small ones by the look of it?

Mike Scantlebury: Most of the time when it's dry, which is 355 days of the year, or something, the big ones do almost nothing. Their energy expenditure suggests that they're just maintaining themselves, which means that they don't even walk around, they just eat and sit around. But the smaller guys which we've termed them frequent workers, their energy expenditure was much higher.

Chris Smith: When it comes to founding a new colony, how is that actually achieved, and do these large ones essentially build up reserves so that they've got something to start a new colony with? Is that the game plan?

Mike Scantlebury: Well, yes, the theory is that when they do disperse which can only happen after large amounts of rainfall which may only happen once or twice a year in the Kalahari, the ground is sufficiently wet, and then there's lots more digging, and we've measured that the infrequent workers suddenly wake up and start digging furiously. The anecdotal evidence is that there is a very high mortality rate amongst individuals that do try and disperse away from the colony, say, over 70% of the animals.

Chris Smith: But why do they die? Is that hunger?

Mike Scantlebury: To a large extent it could be hunger because where they live food is plentiful but it's fairly infrequently found, so part of the argument for having a large colony is that you have many individuals looking for food so when they do find a food item it's a large food item that's able to feed many mouths. But if you're just one guy then your probability of finding a food item by digging a random tunnel in a random direction is quite small. So there is, we think, a large mortality because of starvation which, if they're fat, then that could help them, but I think there is also many predators as well such as mole snakes and jackals which dig up the mole rat tunnels.

Chris Smith: And as far as you're aware are these mole rats the only example of a mammal behaving in this way, as yet discovered?

Mike Scantlebury: Naked mole rats are something similar, which live in East Africa, but we've got physiological evidence that there are difference in behaviour when dispersal conditions become possible.

Chris Smith: Mike Scantlebury from the University of Pretoria helping to unearth the secret lifestyle of the mole rat.

Next up, anammox bacteria, bugs that get their energy by anaerobically metabolising ammonia. Scientists are interested in them because they may hold the key to clearing up nitrogen containing compounds in sewage. To get to the bottom of how these bugs do this, Mike Jetten and his colleagues have sequenced their genome, but it took quite a while because Kuenenia stuttgartiensis , the organism they've sequenced, grows extremely slowly. (Nature 440, 790-794; 2006). So slowly, in fact, that it took a year to culture enough of them just to be able to analyse them.

Mike Jetten: We work on very important bacteria, the anammox bacteria, that turned up in many waste water treatment systems and in the ocean where they helped to remove the excess ammonia, and therefore elucidated the blueprint of this bacteria to cover all it's DNA and all the genes in it.

Chris Smith: So what was involved in doing the sequencing? How did you do it?

Mike Jetten: We invested a lot of time in cultivating these organisms. We have developed special systems where we can keep the bacteria as they grow, so we can make reasonable amounts of these bacteria. The actual sequencing was done at Genoscope in France.

Chris Smith: What did the sequence reveal? Were there any surprises that leapt out at you when you did the work?

Mike Jetten: Yes, many surprises. We thought it would be a very, very specialised bacterium only capable of a very few tricks, that it could only use carbon dioxide as its diet, but it turns out that it can use many more compounds as food, it can use organic acids, acetate, propionate, also the occurrence in many oceans. Colleagues who go out on the sea find them everywhere. That is a very big surprise. And if we calculate them then maybe the air that we breathe, half of that is made by these anammox bacteria.

Chris Smith: What do these bacteria do in the wild? What's their role? What sort of chemistry are they involved in, and where do you tend to find them?

Mike Jetten: You find these bacteria everywhere where there is plenty of ammonia and very limited amounts of oxygen. There the bacteria can do their job in converting ammonia into harmless dinitrogen gas.

Chris Smith: And is that the reason why people are interested in them for cleaning up waste water?

Mike Jetten: Yes it is a very good alternative. Normally you would need a huge amount of oxygen to remove the ammonia in waste water, but by using these bacteria you would need much, much less so you could clean up much more waste water with much less costs and chemicals.

Chris Smith: But given that they grow so slowly, are they actually going to be efficient or are you going to be borrowing biology and stealing from their Genome in order to make other bugs which can use the same techniques that these bugs would but just do it more efficiently?

Mike Jetten: That would be a very nice future dream but we have a long way to go to accomplish that. Trying to express the genes of this bacterium and other bacteria is quite difficult. We have only succeeded in doing so in one or two. But we have now the first full scale plant of these bacteria in operation in Rotterdam in the Netherlands. There is actual use to clean up the waste water so we have learned a lot of things, then we can use this knowledge and also the biomass we have there, that's 75,000 litres of these bacteria to seed new waste-water treatment plants and hope that they can be started up more rapidly.

Chris Smith: From the Netherland Radboud University's Mike Jetten, who has deciphered the genetic sequence of the anammox bacteria, Kuenenia stuttgartiensis , which might end up powering water clean-up plants of the future.

Now, we finish this week with a story of how the hunter almost becomes the hunted yet still manages to escape. Frederic Toma from the National Centre of Scientific Research in France, together with Fleur Ponton and her colleagues, have described for the first time a parasitic worm that‘s evolved the ability to escape from animals that prey upon its insect host. (Nature 440, 756; 2006). This gordian work, called Paragordias tricuspidatus infects land-living insects like crickets but it needs to return to water to complete its lifecycle. To ensure that this happens, it forces its host to commit suicide by leaping into the nearest pond so that the worm can escape. But what happens if the arrival of a tasty treat like a cricket arouses the interest of nearby fish or frogs?

Frederic Toma: What we found for the first time is the escaping response of a parasite when the host is eaten by a predator. So, in this case, the host is an insect, a cricket, and the parasite is a worm inside the cricket. This parasite finishes its lifecycle in water and after several months of development inside the cricket, the parasite alters the behaviour of the cricket making him commit suicide by jumping into the water. The problem for the parasite is that when the host jumps into the water, if there is a predator like a fish or a frog it can be the end for the parasite.

Chris Smith: So, what happens when the fish eats the cricket? How does the parasite, the worm, living inside, how does it get out?

Frederic Toma: The parasite has first to leave the host inside the digestive tract of the predator and then the parasite has to crawl until the mouth of the predator and then the worm, by a special movement, is able to escape.

Chris Smith: Can you tell us a bit about the worm?

Frederic Toma: It is a group of parasite poorly known called Nematomorpha. Adult male and females are free living in aquatic environments and doristreal insects become infected with hairworms when they ingest parasitic larvae, and Nematomorpha grows from a microscopic larvae to a large worm who's size exceeds the length of the host by a considerable amount. In our case, the cricket is about 7 or 8mm, and the worm is about 12cm. When the parasitic development has been completed the worm occupies most of the host cavity with the exception of the head and the legs, and worms are only ready to emerge once they reach this stage.

Chris Smith: Tell us how you actually did the study.

Frederic Toma: I decided to keep at the laboratory frogs and several species of fish and we collected crickets just before they jumped into the water. We provided these crickets to the predator at the laboratory and we had a prior prediction that something should occur to prevent the worm from dying with the host following the prediction. And it was, of course, incredible for us to see the worm that was escaping the predator.

Chris Smith: How do you think it's evolved to have this incredible behaviour to be able to escape?

Frederic Toma: It is well known that an insect at the surface of the water is very easy to catch and very attractive for predators, so this suggests that there has been strong selective pressure to favour the emergency of an anti-predator strategy.

Chris Smith: Frederic Toma from France's National Centre of Scientific Research with the first description of a parasite that's evolved to escape not only from its host but also from its host's predators. Fleur Ponton, who carried out the study with Frederic, tells me that they're now looking at exactly how the parasite achieves its Houdini-style escape.

Well that's it for this week, and thanks for listening. Remember that Nature and all of the journals and resources produced by the Nature Research are available online including through institutional site license access. There is more information about this on our website at http://www.nature.com.

Next week we are fighting malaria and finding out how parasites can regain their independence. But for those of you in the mood for more science in the meantime, have a listen to this week's Naked Scientist podcast where we'll be looking at the science of brain development, brainwashing and the neurological basis of pain. That's the Naked Scientist podcast which is freely available from http://www.thenakedscientists.com.

Production on this week's Nature podcast was by Anna Lacey in the Department of Pathology at Cambridge University, and I'm Chris Smith.

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