Podcast: Leapfrogging speciation, and migrating mosquitoes

Host: Nick Howe

Welcome back to the Nature Podcast. This week: sex and speciation in frogs…

Host: Shamini BundellAnd migrating mosquitoes. I’m Shamini Bundell.

Host: Nick Howe

And I’m Nick Howe.

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Host: Shamini Bundell

First up, what makes a frog attractive? For the strawberry poison dart frog, the amphibians they’re most attracted to often resemble their parents. This sexual sorting may give an insight as to how new species could form. Reporter Geoff Marsh has the story.

Interviewer: Geoff Marsh

You might have thought that 160 years after Darwin’s explosive book The Origin of Species, we’d be done with the question of where species come from, but it turns out that many questions still remain. This week in a paper in Nature, a US team have been working in the lab with some very colourful Central American frogs to probe an intriguing new way for species to leapfrog to speciation. We’ll get to the frogs in a minute, but first it’s worth having a bit of an evolutionary theory refresher. For a bit of background, I called Machteld Verzijden, an evolutionary biologist from Aarhus University in Denmark.

Interviewee: Machteld Verzijden

The idea behind the evolutionary theory is that there is selection on individuals. There is variation between individuals and natural selection can come from predation, food availability, climate, and some individuals might be better able to cope with the circumstances than others. Another type of selection is sexual selection where some individuals are better in getting matings, being selected by partners or being able to produce more offspring.

Interviewer: Geoff Marsh

So, a trait in a male, for example, could be completely influenced just by how sexy, I suppose, that is to the females of the species.

Interviewee: Machteld Verzijden

Exactly, so you have these amazing pictures of birds of paradise, for instance, with these ridiculous feathers that look amazing to us but seem completely impractical for flying, and the idea behind that is that they are selected through females liking them.

Interviewer: Geoff Marsh

So, sexual selection can promote certain traits in a species, but there’s a debate amongst modern evolutionary biologists about whether or not sexual selection alone can lead to a new species because if one sex does have a strong preference for a particular trait, the genes for that trait would presumably spread quickly through the population, erasing the genetic variation that needs to exist for new species to arise. So, there needs to be some sort of advantage to being different and a reproductive barrier to stop the different types of individuals in a population being mixed back in to the same crowd. One potential mechanism to provide both these ingredients could be sexual imprinting, where offspring inherit their parents’ sexual behaviours through learning.

Interviewee: Machteld Verzijden

Imprinting is very widespread among species that have parental care, such as birds, a number of mammal species, a couple of fish species and well, now this paper also shows that it’s happening in frogs with parental care.

Interviewer: Geoff Marsh

We’ll come back to Machteld. But first, allow me to introduce you to the strawberry poison dart frog. You’re listening to males calling out from their aggressively protected territories. They’re also wonderful parents. Both males and females get involved in parenting in this species, but more so the mother. When she arrives to visit her tadpoles, they vibrate against her body to beg for food, which she supplies in the form of unfertilised eggs. Not only is that objectively adorable – this quality time provides the perfect window for behavioural imprinting to happen. Here’s first author Yusan Yang from the University of Pittsburgh.

Interviewee: Yusan Yang

So, there are these tiny little frogs in Panama, and basically on different islands you can see all these different colours of frogs.

Interviewer: Geoff Marsh

So, they’re all the same species but within the species there are these really distinct colour variations.

Interviewee: Yusan Yang

Exactly.

Interviewer: Geoff Marsh

What kind of colours are we talking about?

Interviewee: Yusan Yang

We have yellow, we have green, red, orange, blue, and then they also have different spotting patterns. The females, when they’re picking mates, usually they have certain preferences, so they will like one colour over the others, but why do the red frogs like red frogs? Is it because it has the genes that encodes the preference for red or is it because the individual sees red sometimes during its lifetime and then associates red with something that’s familiar?

Interviewer: Geoff Marsh

Right, and the only way to do that then is to bring these different colour morphs together into the lab and then have a kind of fun cross-breeding and fostering programme. Tell me about your setup?

Interviewee: Yusan Yang

Yeah, I think it’s a pretty interesting experimental setup. So, basically, we’re taking the tadpole that came from a pair of red frogs and then we take that tadpole and give it to a pair of parents that are of a different colour, say a pair of green frogs.

Interviewer: Geoff MarshSo then that red fostered frog then goes on to be attracted to green males.

Interviewee: Yusan Yang

Yeah, so that’s our finding. So, it’s pretty surprising that it is not a genetic behaviour but instead it is something that they learn from the tadpole stage. So, basically, the offspring learning the parents colour when it’s a juvenile and then using that reference when they’re adults and doing things like mating. This is the first time, at least to my knowledge, that this has been demonstrated in a frog.

Interviewer: Geoff Marsh

Cool, and it wasn’t just the females imprinting on their parents. It was happening with the boy frogs too, wasn’t it, about who they were going to get into fights with?

Interviewee: Yusan Yang

Yeah, exactly. So, these tadpoles grow up to be a more aggressive when they’re engaged in fighting with a rival that has the same colour as their mother.

Interviewer: Geoff Marsh

Now, how do those two different imprinted behaviours kind of play out in an evolutionary scenario? I mean I know you can’t wait around for millions of years to watch this happen, but presumably you can plug it into a computer model and see what happens, right?

Interviewee: Yusan Yang

Yeah, that’s exactly what we did. So, the females, because a tadpole from a red mum will inherit genes that make their skin red, and then at the same time because they are learning from their mum, they will also have this preference for red, so we get this association between the colour of the frog itself and its behaviour. That will, I guess, facilitate the red frogs mating amongst themselves and then the green frogs mating amongst themselves because the green frogs will also have a green bias, so that will reduce the breeding between the red frogs and the green frogs. And so, on the other side of things, the male competition actually keeps both green frogs and red frogs around because when the males are competing with each other and then they’re competing more with the colour of their mother, then that basically means that if we have a lot of the red frogs in the population then we will have a lot of male offspring that are fighting with the red ones and so, in this case, being a green frog, being a minority, actually has an advantage. Then we have a kind of a balancing selection for us on to keep both the colours around and then so that will be a raw material which evolution can act on and I guess it will be a required precursor for any speciation happening within the same area to happen.

Interviewer: Geoff Marsh

Do you think that could be a kind of common precursor to new species, reproductive isolation and then new species? Do you think that’s more common than we have perhaps given credit for?

Interviewee: Yusan Yang

I think there is potentially a lot of different animals that have some sort of parental care or some sort of contact with other individuals of the same species when they’re growing up and then the learning in that period has a lot of potential to influence their behaviours later on in life, and so I do think that it is probably a lot more widespread than we considered.

Interviewer: Geoff Marsh

Back to Machteld, who you heard at the beginning.

Interviewee: Machteld Verzijden

I found this paper really interesting because it so beautifully links the maintenance of polymorphism, so the different colour types, and also explains at the same time reproductive isolation through female imprinting. I worked on a different set of species known as cichlid species, and I am interested in imprinting behaviour and speciation, so I looked at similar questions in those species and I found very similar answers. I find it super interesting that these frogs species that also show some really remarkable maternal care also do this behaviour and have similar effects on the sexual selection.

Interviewer: Geoff Marsh

While sexual selection seems to be capable of maintaining these different types of individuals in the population, does that mean they’re eventually heading for speciation?

Interviewee: Machteld Verzijden

It is enough? I don’t know. This is really hard to say because we don’t know all of the natural selection pressures around, and so especially the predation is a big factor, I think, that will affect whether these will be different species or not. So, there is no predators in the model, there is no other ecological pressures in the model. If there is natural selection pushing them to become more uniform, I don’t know that they can maintain their polymorphism over a very long time.

Host: Shamini Bundell

That was Machteld Verzijden of Aarhus University in Denmark. You also heard from Yusan Yang from the University of Pittsburgh in the US. You can find Yusan’s paper over at nature.com.

Host: Nick Howe

Later in the show, I’ll be talking to Nisha Gaind about the pressure to cite superfluous studies – that’s coming up in the News Chat. Now though, it’s time for the Research Highlights, read this week by Dan Fox.

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Dan Fox

A team of researchers have created one of the world’s tiniest traffic lights. The multifunctional nanoparticles radiate red from one end when exposed to one wavelength of infrared, but shine another specific wavelength on them and they make green from the other end. These so-called ‘upconversion nanoparticles’ could have medical applications as they can stimulate tissues from within. In fact, the team successfully used the particles to control the beat of heart muscle cells by triggering light-sensitive cellular structures called ion channels. This could give doctors a new tool for correcting abnormal heart rhythms. Shine a light on that research over at Nature Communications.

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Dan Fox

If you like to listen to the Nature Podcast while driving, you might be in luck. A team of physicists have tackled the classic motorist’s conundrum of where to park. The researchers compared three typical strategies of finding a parking spot to determine which saves the most time, at least in their simplified model car park. The researchers calculated that on average a ‘prudent’ strategy, where you pick the first gap between two cars, is the most efficient. This was slightly head of an ‘optimistic’ option, where you drive straight to the destination and then backtrack to find a spot. But bad news for drivers using a ‘meek’ strategy, where you park in the first available space next to a car, as this finished a distant third. Read the full paper after you’ve parked in the Journal of Statistical Mechanics: Theory and Experiment.

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Interviewer: Nick Howe

Next up, I’ve been looking into malaria and mosquitoes. Malaria kills hundreds of thousands of people a year and causes illness in millions. One of the most effective ways to fight it is to control the mosquitoes that spread malaria, but that requires a really good understanding of the mosquito lifecycle. First, female mosquitoes lay eggs at the surface of water –

anything from a puddle to a lake. Next, once the eggs become submerged, the larvae hatch and feed in the water. Then after about a week, they complete their lifecycle and take to the air as adults. That is when all the biting, swatting and mating takes place, which leads us neatly back to the egg-laying again. Malaria is spread during the adult phase but crucially, in order to get there, mosquitos need water. This Achilles’ heel, as it were, means that in areas like Northern Africa, where there are dry seasons and water dries up, mosquitoes disappear. But when the rains return, mosquito numbers surge and they do so very quickly – too quickly, in fact. For decades, scientists haven’t been able to work out how mosquitoes can return in such numbers after rains come. Where do they come from? Well, this week in Nature, there may be an answer. We’ll hear from one of the researchers involved in that study a bit later, but first, here is Nora J. Besansky, a researcher in malaria and its vector – the mosquito – with a bit of background.

Interviewee: Nora J. Besansky

You might think, well, maybe even though the pools and puddles are dried up, the mosquito eggs or various mature stages, maybe they can sort of live in suspended animation, and once you add water they pop back into life. Well, that doesn’t happen, according to a lot of research, so we can rule that out. Now, the other thing you might consider – suppose a mosquito gets trapped in someone’s car, right, and gets transported somehow by humans. This happens. We know it does. But that can’t explain the data because you don’t get such a surge in numbers by this kind of accidental mode of human transport. So, that doesn’t work either. Maybe these mosquitoes persist locally in a state of dormancy that we call estivation. The problem is that people have spent months and years searching for mosquitoes in this animal burrows and tree holes, wherever, and they cannot find them in great numbers.

Interviewer: Nick Howe

So, if you discount these unlikely theories, what is left?

Interviewee: Nora J. Besansky

So, maybe the most obvious idea is that adult mosquitoes can fly back and recolonise from a place where water is present year-round.

Interviewer: Nick Howe

But as far as scientists can tell, mosquitoes can’t really fly for more than 5 kilometres, which isn’t really far enough to repopulate some more remote areas.

Interviewee: Nora J. Besansky

So, that leaves really only one other notion, which would be, well, suppose these mosquitoes are migrating on high-altitude winds.

Interviewer: Nick Howe

And that is exactly what a team lead by Tovi Lehmann is suggesting in a paper in this week’s Nature, based on data from a dry region of Mali. Now, testing this hypothesis is pretty tricky. Tovi and his team needed to find evidence of tiny mosquitos travelling at high altitudes. But to do it they used a fairly simple method. Here’s Tovi to explain.

Interviewee: Tovi Lehmann

We have used large helium-filled balloons that are tethered to the ground and on the line that tethers into the ground we hung sticky nets.

Interviewer: Nick Howe

Nora was sceptical of the idea when she first heard about it.

Interviewee: Nora J. Besansky

At the time, a number of us thought it was fairly insane. Think about it – a tiny insect like a mosquito travelling in an air stream, and you’ve got just a single balloon with a net. Even if mosquitoes do this, even if they travel this way, what’s the chance you’re going to sample them with a single balloon that has three sticky nets tethered to it?

Interviewer: Nick Howe

But find mosquitoes, they did.

Interviewee: Tovi Lehmann

Instead of finding only one or two species that might do this crazy thing of windborne long-distance migration, we have found tens of species. Essentially, we found that the majority, over 50% of the species of mosquitoes in Mali, engage in such movements and with this limited effort and I would say very ineffective sampling technique, finding such diversity was our first surprise.

Interviewer: Nick Howe

Now, you may wonder if just catching mosquitoes really means that they are migrating. Could they not just be caught up in the breeze? Tovi didn’t directly show that mosquitoes were going from one place to another, by tagging them or otherwise, but the timing was consistent with mosquitos moving between wet areas with the seasons. And this method of migration might not actually be that outlandish. From butterflies to aphids, there are many other examples of insects using high-altitude winds to migrate. Nora certainly seemed convinced.

Interviewee: Nora J. Besansky

Without question, I’m totally convinced.

Interviewer: Nick Howe

Tovi suggests that the mosquitoes can migrate up to 300 kilometres in this way. So, what does all this mean for controlling malaria? Well, let’s do some maths. Tovi collected over 3,000 mosquitoes with his balloons. Of those, 235 were species that are capable of carrying malaria. But Tovi’s sample size was still very small. Extrapolate it out, and he predicts that in the dry region of northern Africa, the Sahel, millions of mosquitoes could be migrating and of those, hundreds of thousands could be carrying malaria. If they were, these numbers are high enough to re-infect whole areas that are free from malaria. It could also facilitate the spread of insecticide resistance. But as it happens, even though Tovi found plenty of mosquitos capable of carrying, malaria none of them were actually infected with the disease. This could just be a quirk of the small sample size, or it is possible that something about being infected could prevent mosquitos from migrating on the winds. As yet, scientists don’t know. But this discovery is still very important for the field – here’s Nora.

Interviewee: Nora J. Besansky

I think the main findings are, I would go so far as to say, transformative for the field. I mean even if it’s uninfected mosquitoes, the idea that they can migrate 300 kilometres a night is something that I think we never dreamed of before this. It really is sort of revolutionising our understanding of the basic biology of these mosquitoes.

Interviewer: Nick Howe

That was Nora J. Besansky of the University of Notre Dame in the US. You also heard from Tovi Lehmann of the National Institute of Health, also in the US. You can find Tovi’s paper over at nature.com.

Interviewer: Nick Howe

Last up on the show, it’s time for the News Chat. I’m joined in the studio by Nisha Gaind, Nature’s European Bureau Chief. Nisha, hi!

Interviewee: Nisha Gaind

Hi, Nick.

Interviewer: Nick Howe

So, for the first story this week, we’re talking about the parasite Guinea worm. Before we get into the news from that, Nisha, what exactly is Guinea worm?

Interviewee: Nisha Gaind

Yeah, so Guinea worm is this pretty horrible parasite that historically has affected Asia and Africa and it comes from drinking water that carry Guinea worm larvae and these can enter the body, and they grow a sort of stringy worm that can be up to 90 centimetres long and it often erupts through the skin on the leg and it’s the journey of the worm out of the body that becomes extremely painful and can completely incapacitate people and there’s no drug or vaccine to treat it.

Interviewer: Nick Howe

So, it’s a pretty horrible disease and because of this, the World Health Organisation actually targeted it for extinction by 2020.

Interviewee: Nisha Gaind

Yeah, that’s right. Only one human disease has been eradicated so far— that’s smallpox – but the World Health Organisation had been hoping that Guinea worm would be the second disease to be eradicated and it had been hoping to do that by 2020.

Interviewer: Nick Howe

But you say hoping there – they’re not actually going to aim for that anymore.

Interviewee: Nisha Gaind

That’s right. So, that’s what we have heard in the past few days. The World Health Organisation has rather quietly pushed back its ambition for stamping out this disease and they’ve pushed it back to 2030 and there are a few sort of worrying but quite complex reasons for why they’ve decided to do that.

Interviewer: Nick Howe

So, what are these reasons then?

Interviewee: Nisha Gaind

So, part of the problem is that researchers are still finding out quite a few new things about Guinea worm. For a long time, they thought that it only infected humans, but in the past few years there have been a number of discoveries that have suggested that it also infects other animals and also that the routes of transmission might not be as simple as they thought they initially were. Researchers initially thought they could just target drinking water and use quite simple measures to try and get rid of these Guinea worm larvae, but that doesn’t seem to have worked. So, as part of this, they found that infections are soaring in dogs, there are also some infections in baboons, and all of these are raising questions about how Guinea worm is transmitted.

Interviewer: Nick Howe

So, the deadline for eradicating this disease has been pushed back, so what is the new strategy?

Interviewee: Nisha Gaind

So, the WHO says that this new date of 2030 is intended to allow time to stop the transmission of Guinea worm but also for a certification committee to verify that the disease is truly gone, and that process is something that takes three or more years because the commission has to certify that there are no people with an infection or no animals with an infection and that takes quite intense surveillance.

Interviewer: Nick Howe

For our next story then, there’s been a poll about pressure to cite. Nisha, this is actually going back to a story we published a few weeks ago about manipulation of citations, but what was that story about?

Interviewee: Nisha Gaind

So, that story was about the Dutch publisher Elsevier and they had conducted quite a rigorous analysis of peer reviewers who work for their journals and they had found that a small proportion of them seem to be consistently using the peer review process to perhaps boost their own citations, and that’s a rather frowned upon thing to do. They were effectively exploiting peer review for their own benefit.

Interviewer: Nick Howe

So, as a follow up to that, Nature’s done its own poll about this pressure to cite or citation manipulation. Nisha, what were the results from this?

Interviewee: Nisha Gaind

Yeah, that’s right. So, we became interested in this question of coercive citation and that’s the name that we give to this practice that could be, for example, peer reviewers or journal editors perhaps pressuring study authors in the review process or in the publication process to cite particular citations and we asked our readers whether they had ever felt pressured by peer reviewers to cite seemingly superfluous studies in their work, and the results were quite interesting. We found out that about 66% of our readers, and there were thousands of readers who answered this poll, about 66% of them said that yes, they had felt pressure to cite kind of unnecessary studies in the peer review process.

Interviewer: Nick Howe

And that’s quite a high proportion of readers then. How does this compare to other polls that have looked at this?

Interviewee: Nisha Gaind

It is a really high proportion of readers and it was something we were quite surprised by and it’s much higher than other surveys have previously found. One survey found that a fifth of researchers reported that they had been asked by journal editors to include some superfluous citations and another found the rate to be about 14%. But these are different surveys. They have been differently conducted and looked at people in different fields, so our poll is much more of a general question just to get a flavour of what readers and what the community might be thinking about, but it certainly suggests that this issue of coercive citation does seem to be emerging in the research community and it’s something that readers and researchers are increasingly aware of.

Interviewer: Nick Howe

And so, what can be done about this phenomenon?

Interviewee: Nisha Gaind

There are suggestions that journals should be doing more about this because they are the ones who are ultimately in control of the peer review process, and perhaps journal editors could be more aware of whether peer reviewers are suggesting that study authors add more citations perhaps to the peer reviewer’s own work during the review process, so there’s a suggestion that there should be more scrutiny of the request to add references in peer review.

Host: Nick Howe

Thanks, Nisha. Listeners, head over to nature.com/news for more on these stories and all the latest from the world of science.

Host: Shamini Bundell

That’s all for this week. If you’ve liked what you’ve heard, why not leave us a review? The best place to do that is on the Apple Podcasts app. I’m Shamini Bundell.

Host: Nick Howe

And I’m Nick Howe. See you next time.