Nature Podcast 9 August 2007

This is a transcript of the 09th August 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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Chris Smith: This week, could it be that behind every successful female, there is a repressed male just trying to get out?

Catherine Dulac: We observed a mutant mouse that is affected for a gene called Trpc2 and astonishingly the female mouse then behaves exactly like a male.

Chris Smith: Meaning presumably that they stopped doing the washing up, tiding up the nest or putting the bins out. Also, with new insights into the relationships of our own distant ancestors.

Fred Spoor: We just discovered that habilis and erectus came into fossil record at about the same time and that we see here in East Africa they overlapped for almost half a million years and habilis then disappeared, then in other places we know that Homo erectus continued on.

Chris Smith: So, Homo habilis and Homo erectus were actually long-term neighbours. We will also be hearing how researchers have blown the dust off one of Isaac Newton's old experiments.

Henry N. Chapman: Well, we did an experiment that was actually carried out over about 300 years ago by Isaac Newton and that was called the dusty mirror experiment. So, we decided to do the same experiment, but we modernized it a bit in one respect, which was to use extremely intense and very short pulses of x-rays from an x-ray free-electron laser.

Chris Smith: But what did they actually see? Well I would not spoil the surprise for you. You will have to keep listening to find out. Hello, I am Chris Smith. This is the Nature Podcast. First this week, news that will shock feminists everywhere because it looks like there is a male mind lurking inside every female, at least in mice. Catherine Dulac told Kerri Smith how on activating a single gene can turn sedate females into sex-craze man-eaters. Nature advance online publication 5 August 2007

Catherine Dulac: Everyone observing animal behaviour knows that the biggest difference in individual behaviour is between males and females. So, these comprise productive behaviour, maternal behaviour, social behaviour, very large array of different behaviours. And so people for many many years have been looking at the brain and tried to identify what is the nerve origin of these behavioural differences between males and females. To everybody's surprise those differences have been extremely minor and so we observed the behaviour of a mutant mouse that is affected for a gene called Trpc2, which encode an ion channel that is required for a sensory organ called the vomeronasal organ to function. And astonishingly the female mouse then behaves exactly like a male and so that tells us that within the female brain exists a perfectly functional male behaviour circuits and that the circuit is actually repressed in the normal animal. So, the male and female brains are probably very similar in nature and that is why people did not see those distinct differences and what makes male and female behave differently are sensory input that makes sure that the right gender behaviour is activated and wrong one is repressed.

Kerri Smith: Let us go back to this gender behaviour then. How do you mean they act like males?

Catherine Dulac: They act like a male in two types of behaviours, one is sexual behaviour. So, basically the female attempts to mount other mice exactly like male. They have pelvic thrust exactly like male. You would see the video of these mice. You would not be able to distinguish from a behavioural point of view, the behaviour of these mutant female to the behaviour of a genuine male. So, the sexual behaviour is identical to that of a male and what we call the courtship behaviour is also the one of a male. So, the courtship behaviour if you wish is the way a male normally approaches a female. It raises its snout and over the body of the female it made ultrasound vocalization typically and very specifically to a female. So, all these very male-specific traits are all shown and displayed by the mutant female.

Kerri Smith: And this is the doing of just one mutant gene then, this Trpc2 gene that you were telling us about. What does this gene do when it is working normally?

Catherine Dulac: It is essential for the sensory function of a set of olfactory neurons that are located on the tip of the nose in a structure called vomeronasal organ. In rodents and in animals that have a functional vomeronasal organ, this structure was thought to be essential for detecting pheromones. Pheromones are chemicals that are used among animals to communicate with each other to provide social cues. In the male, we have found that it is essential for sex discrimination so for an animal to be able to tell apart a male from a female and our story that we are publishing now shows that in the female it is also essential to ensure that the female behaves like a female.

Kerri Smith: Now, that was going to be my next question. You have found that female mice with these Trpc2 mutations, or would have their vomeronasal organs removed acted just like males, is the reverse true that males with this mutation act like females?

Catherine Dulac: We already know that the Trpc2 mutant males are much less aggressive. So, you could say this could be a trait that resembled that of a female, but overall, you know, I think the expectation is that, it is very likely that what we found in the female also exists in the male because from a developmental point of view it makes total sense. Instead of building a male brain and then the female brain, you build a mouse brain and then there is a sensory switch that makes sure that the animal behaves appropriately according to its gender. So, what we found in the female presumably also exists in the male.

Chris Smith: And I have heard there is a human homologue for that knock out. It is called alcohol and going to university. Harvard's Catherine Dulac has found how to de-repress the circuits for male behaviour in the brains of female mice. Well, from what makes females behave the way they do to what made modern mankind. One of our earliest ancestors was Homo habilis, which everyone thought turned into Homo erectus, but now Fred Spoor and his colleagues have uncovered new evidence that they two actually coexisted for at least half a million years in the same lake basin in what is now North-Eastern Kenya and this was all happening over one-and-a-half million years ago as Fred explained to Mike Hopkin. Nature 448, 688–691 (9 August 2007)

Fred Spoor: Two fossils were found, one is a beautifully preserved Homo erectus skull, it is actually a skull without a face that we technically refer to as calvaria, the brain case in essence, but it is beautifully preserved. The other fossil is Homo habilis jaw bone of the upper jaw and both of them have a story of their own to tell as well as in combination with each other. The habilis fossil turned out to be remarkably young and is by far the youngest habilis fossil now on records and it shows that how habilis is a species that survived for a long time in this area jointly with Homo erectus, in other words they did morph into Homo erectus and Homo erectus skull, on the other hand, is a tiny thing. It is a really small skull, more of Homo habilis size, so in fact when it was first found we thought that it would be Homo habilis until we studied it well, and it shows that there is a lot of size variation in Homo erectus, which has all kinds of implications for behaviour of the species.

Mike Hopkin: So, what does it mean for our view of human evolution, if these two species did actually live side by side?

Fred Spoor: Well, I think what it very much does is change the perception of how the origin of our own genus, the genus Homo, how that actually appeared. In many people's view, it was very much a linear progression. Homo habilis either gradually or relatively rapidly morphed into Homo erectus and then later on, on to Homo sapiens and now that we actually have better dates for how long Homo habilis lived and also really we considered what the earliest evidence, good evidence for Homo habilis and then we just discovered that habilis and erectus came into fossil record at about the same time and at least here in East Africa, they overlapped for almost half a million years and habilis then disappeared and in other places, we know that, that Homo erectus continued on, but what it emphasizes is that there are multiple human ancestors, they are not strict human ancestors, but types of human ancestors evolved at the same time.

Mike Hopkin: Do we still think that Homo erectus is the best candidate for our direct ancestor, you know, as Homo sapiens?

Fred Spoor: I think so that that is the case, yes. From what we know there is a transition, some people will recognize that transitional species as a separate species, but there is something of a transition from Homo erectus to Homo sapiens of some sort and then by about 200,000 years ago we were really recognized a species or fossils that are not significantly different from us as we are today and erectus remains the best candidate for that.

Mike Hopkin: What was Homo erectus actually like, because I gather that the new skull actually changes our idea of what sort of a species that actually was?

Fred Spoor: Yes, very much, I mean, it is interesting that the variation in size of the skulls that we find is really rather substantial coming close to what you see in gorillas and why do we note that because the Homo erectus skull that we described in the Nature article is very small, in fact it is the smallest one on record so far and if we add the two samples that we have in East Africa then we find that enormous size range. Now, size range that large usually expresses at least to some extent that males and females are really different in size; small females, large males. If we then look at the primates as we know them today and look at those species that have such large differences between males and females those are species where you have a very particular social interaction between the sexes. It usually involves one dominant male with multiple female mates. That is a group setting i.e., as we find it in the earliest Homo erectus as well.

Chris Smith: UCL's Fred Spoor describing to Mike Hopkin how he has uncovered two new fossils of Homo habilis and Homo erectus, which show that the two early hominids did not exist sequentially, but instead lived side by side for over half a million years.

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Chris Smith: On the way, a modern spin on Newton's dusty mirror experiment. It is helping researchers to make movies at an atomic level of resolution, but first down to Earth and the question of what happens to the crust material that gets swallowed up in subduction zones. Does it disappear into the Earth's interior never to be seen again or does it resurface later? Well lava emerging in Samoa says that it comes back and the giveaway is a chemical fingerprint written into the sea floor. Here is Matt Jackson. Nature 448, 684–687 (9 August 2007)

Matthew G. Jackson: When two plates collide inevitably one plate ends up being subducted into the mantle and ocean plates are covered with sediments, sediments in years that are tens to hundreds of meters thick. So, this sediment ends up going into the mantle with the rest of the plate. So, we would expect then that if these plates go down into the mantle, into the deep Earth to depths of 3000 kilometres or so, if these plates are ever recycled back to the surface again in mantle plumes that we would be able to see the sediment signature.

Chris Smith: I guess that is what you are saying then that you have obviously got evidence here that there is recycling of this subducted material from deep in the Earth back up to the surface?

Matthew G. Jackson: You would see in that way, but given the large volumes of sediment that we see entering the Earth's mantle subduction zones in the present day and approximately a half a cubic kilometre sediment annually is thought to be subducted into the mantle, there should be a large mass of sediment that has accumulated in the Earth's mantle over geologic time, you know, over four billion years of subduction for example, you would expect to have a reservoir of sediment in the Earth's mantle that is nearly has third of the mass of the Earth's continents, but the sediment reservoir just does not seem to surface very often in oceanic volcanism. It is incredibly rare.

Chris Smith: So, where do you think it is going?

Matthew G. Jackson: And, that is a big question, no one really knows. One possibility is that the sediment we think is going into the mantle subduction zones might actually be short-circuited very quickly back up into the subduction zone volcanoes. The other alternative is that the sediment ends up forming a reservoir somewhere deep in the mantle that is somehow stable and isolated from mantle convection, maybe even it has been suggested that perhaps beneath the continents there is a stable region where volumes of sediment could hide out and effectively be isolated from mantle convection so that we would not see it surface in hotspots.

Chris Smith: So, what is special do you think then about Samoa?

Matthew G. Jackson: You know, that is a great question. We do not why this striking recycle sediment signature is showing up in Samoa. There are dozens of hotspots in the world's ocean basins and many of these hotspots are thought to be fed by deep-seated mantle plumes like Samoa, but why these other mantle plumes that feed other hotspots so rarely exhibit a recycled sediment signature is, it is really unknown. We do not know why.

Chris Smith: So it sounds to me like you could be looking at an exception in Samoa and in fact there might be something else different going on elsewhere around the world then?

Matthew G. Jackson: Absolutely! It has often been thought that the mantles are really dynamic environment, even though 99% of the mantle is solid rock, on 100-million-year geologic time scales the mantle convects and these convecting currents are to, perhaps, homogenize heterogeneities that are injected into the mantle, its subduction zones. So, perhaps what we are seeing in Samoa is an example of sediments that has been injected into the mantle and miraculously survived this big blunder, this homogenizer that we call the mantle.

Chris Smith: Hot stuff, well, at least we know now what is going on in one part of the mantle. Matt Jackson is at the Woods Hole Oceanographic Institution. Finally this week, to an experiment that baffled even Newton, the rings he saw when light reflected from his dusty mirror onto a screen that was interference, but now Henry Chapman and Janos Hajdu have used the same idea to develop an incredibly powerful imaging technique that might even be capable of visualizing the atomic structure of whole cells. In their experiment, they arranged polystyrene balls on a mirrored surface and then fired very short, but intense pulses of x-rays at them. The balls exploded due to the x-ray energy hitting them, but not until after the x-rays had already passed through them and bounced off the mirror. The x-rays then passed back through the exploding bolt to produce a hologram of the structure of the particles. Here is Henry Chapman. Nature 448, 676–679 (9 August 2007)

Henry N. Chapman: Well, we did an experiment that was actually carried out over about 300 years ago by Isaac Newton and that was called the dusty mirror experiment. This was actually the first recorded observation of interference, which is the effect where one light wave can cancel out another light wave and give you bands of darkness, which is the same principle used in noise cancelling in headphone.

Chris Smith: So, what did Isaac Newton see when he did this?

Henry N. Chapman: He had shown light on a mirror that inadvertently had dust on it and he shown the light back up onto a screen and he saw these dark and light bands, which he did not understand at all and that took another 100 years to figure out. That was done by Thomas Young, the famous guy who did the Young slits experiments and he said that this must be due to the fact that light can be described by waves that adapt or cancel each other out at certain places and that these interferences were caused by light scattering both of dust that's on the mirror interfering with the light scattered from its image. So, we decided to do the same experiment, but we modernized it a bit in one respect, which was to use extremely intense and very short pulses of x-rays from an x-ray free-electron laser.

Chris Smith: So, Janos, how did you actually do it, take us through the experimental technique that you used?

Janos Hajdu: The basic idea is to capture an image from an object before the object has time to respond to the extremely high radiation intensity that hits it. So, there is a fundamental limit in imaging and imaging get very high resolutions is murderous to the sample because some of the interactions is the probes that one is using for imaging that get the electrons, neutrons, protons, you name it, will deposit energy into the sample so that the sample is moved by these interaction slightly so that when the next photon, electron or neutron arise it sees a slightly altered sample.

Chris Smith: So, effectively you are going to get a blur and you are also going to see something different?

Janos Hajdu: Exactly, and this can go all the way to the point when the sample that is at the beam at the moment has nothing to do anymore with the sample with what one started with. So, that is the reason why we do not have for instance an atomic resolution structure for a living cell, we cannot do it. Even if we cool it to cryogenic temperatures, these interactions will cause the sample to move.

Chris Smith: So, how did you get around the problem in this experiment?

Janos Hajdu: So, this experiment is actually a follow up to an earlier problem and the earlier problem was how to expand resolution at very high photon intensities and try to outrun radiation-induced changes in the sample by putting in an extremely high number of photons and extremely short period of time, in femtoseconds, and by doing that one could obtain an image from the sample before the sample really has time to respond. If you blew up and turn into plasma, but not before the image is formed and then, Henry and his daughter, Catherina came up with the idea, which was amazingly simple through a visit to a planetarium of Berkley.

Chris Smith: So, Henry, what did you do?

Henry N. Chapman: Well, it was a demonstration that showed exactly Newton's experiment and what it was, was a long tube that you could look down and down the bottom was a mirror that had spores of a plant covering it. These pores are too small to see, but as you peer down there and shown a little flashlight into it you saw these interference rings and so, there playing around with my daughter and I figured out that it was due to this interference between the object and its image and it suddenly struck me, if we look at this dusty mirror experiment with an extremely short pulse, one that is much shorter than the actual gap between a little object and the mirror we could have a second look at it with the pulse that actually reflects back from the mirror.

Chris Smith: Why is that important than having a second look?

Henry N. Chapman: Because the pulse is so short that nothing really happens for a particular experiment within the first pulse that is what we saw with our first imaging experiment, but as we go down to the shorter x-ray wavelengths required to image at the near atomic resolution, then this is going to get very important. So, we have these models that describe how the explosion proceeds, but the only way to really compare with the models is to follow the evolution of the explosion afterwards, and then see anything we can do now. So, we initiate the explosion with the pulse coming in and then we essentially probe it, we see how much the ball has exploded in the time that it took the light to travel, just 100 microns or so out to the mirror and back, and that is a very brief amount of time, I mean, we are interested in time scales of picoseconds and lower and smaller, which is the time light takes, you know, to travel sort of less than a width of a human hair.

Chris Smith: So, Janos, what did you actually see when you did this and what are the implications for how we now understand, how to probe these things and now they change when we try and look at them?

Janos Hajdu: This idea of Henry provides the means of shooting a movie with a femtosecond time resolution on an x-ray-induced explosion process for the first time and, let us say, we have this dusty mirror. It was covered with uniform balls of polystyrene. One can put these on the surface of the mirror at a slant so there is a slight slope and you can work through with the x-ray laser hitting various points on the surface and starting the explosion, hitting the mirror, reflecting back from the mirror surface, hitting the sample again with the time delay defined by the distance to the mirror surface and then, this time delay can be changed as you work up the slope. This way one could produce a movie of the explosion and that showed that it took place in about few picoseconds. So, much, much longer time scale than the laser pulse, which was then femtoseconds, about thousand times shorter than that the time scale which it took for the explosion to manifest it visibly.

Chris Smith: And when you do this, what does the information, the picture that comes back on, how does that tell you about the thing that you have blown up?

Janos Hajdu: This technique is a method of doing extremely fast movies, on something that is exploding and it is exploding not by our design, but because of the necessity that we wish to look at very high-resolution structures and this is not possible without sacrificing the sample and so eventually we would like to use the method for imaging interesting biological structures like cells, single virus particles, maybe eventually single micro molecules or single molecules at some point when hard X-ray lasers become available in the next few years.

Chris Smith: So, if you ever need an example of a contribution to science museums and children can make to science, this has to be it. That was Uppsala University's Janos Hajdu and Henry Chapman, who is at the Lawrence Livermore National Laboratory, have come up with the X-ray equivalent of Newton's dusty mirror experiment. That's it for this week. Thanks for listening. Please send any feedback to mailto:podcast@nature.com. Next time, windy stars, glasses made from metal, and the ageing process. For more science in the meantime this week's edition of the Naked Scientist Podcast explores nature's chemical arsenal including a scorpion sting that can highlight cancer, how funnel-web spiders are helping farmers to fight off insect pests, marine cone snails that harbour a painkiller ten thousand times more powerful than morphine, and how a snake bite can help to prevent a heart attack. That is this week's Naked Scientist Podcast, which is on the http://www.nakedscientists.com, and as I had mentioned last week voting is ongoing for the Naked Scientist, who received two nominations in this year's podcast awards. So, if you would like to show them your support by registering a vote for them, details of how to do that are on their website, http://www.nakedscientists.com. This week's show was produced and presented by me, Chris Smith. Good-bye.

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