parasitoid

Parasitic wasps hitchhike on butterflies by smelling for chemical chastity belts

edyong — Fri, 19 Feb 2010 07:30:36 +0000

Parasitic wasps hitchhike on butterflies by smelling for chemical chastity belts

It's not every day that you hear about spy missions that involve a lack of sex, but clearly parasitic wasps don't pay much attention to Hollywood clichÃ©s.

These insects merge the thriller, science-fiction and horror genres, They lay their eggs inside other animals, turning them into slaves and living larders that are destined to be eaten inside-out by the developing grubs. To find their victims, they perform feats of espionage worthy of any secret agent, tapping into their mark's communication lines, tailing them back to their homes and infiltrating their families.

Two species of parasitoid wasp - Trichogramma brassicae and Trichogramma evanescens - are particularly skilled at chemical espionage. They've learned to home in on sexual chemicals used by male cabbage white butterflies. After sex, a male coats the female with anti-aphrodisiac that turns off other suitors and protects the male's sexual investment. These chemicals are signals from one male to another that say, "Buzz off, she's taken."

But the wasps can sense these chemicals. They feed on the nectar of the same plants that the cabbage white visit and when they do, the wasps jump her. They are tiny, smaller even than the butterfly's eye (see the image below), and they hitch a ride to the site where she'll lay her eggs. There, they lay their own eggs inside those of the butterfly. Amazingly, the wasps use the same trick for different species of cabbage white butterflies, which secrete very different anti-aphrodisiacs. They can even sense when the anti-aphrodisiacs are wafting among the general scent of a freshly mated female. It's all part of a sophisticated "espionage-and-ride" strategy.

The wasps' tactics were discovered in 2005 by Dutch researchers led by Martinus Huigens. Now, he has found that they parasitise two species of cabbage whites in very similar ways, despite different chemicals and different rewards.

The large cabbage white lays large clutches of 20-50 eggs and uses the benzyl cyanide as an anti-aphrodisiac. The small cabbage white uses a different chemical - methyl salicylate - and it lays just a single egg. For a wasp, the large butterfly is clearly the better investment, as a hitchhike would lead it to a gigantic larder for its young. But the small cabbage white is far more common, making it a worthy target nonetheless. The wasps certainly have no preference for one species over the other.

By using Y-shaped tubes and blowing different airstreams down either arm, Huigens could work out which of two smells was most alluring to the wasps. The smell of mated females was a stronger pull than clean air or the smell of virgins. But if Huigens dabbed virgin butterflies with the right odours, they suddenly became attractive luxury vehicles.

Neither wasp is drawn to the anti-aphrodisiac alone; both need to smell the trigger as part of a sensory milieu given off by a mated female. Huigens thinks that the chemicals themselves - benzyl cyanide and methyl salicylate - aren't actually attractive in their own right. Instead, they change the wasp's perceptions so that the aromatic blend of its host becomes even more attractive.

Of the two species, only brassicae knows how to parasitise the cabbage whites from birth. The other, evanescens, has to learn the espionage trade. Only after hitching a ride on a female butterfly and only after successfully impregnating her eggs will it learn to recognise benzyl chloride with the smell of reproductive success.

It's possible that this difference depends on how picky the wasps are. Evanescens is far more Catholic in its tastes, parasitizing a very wide range of butterflies and moths. For such a generalist, it pays to behave in a more flexible way. What the butterflies do in retaliation is anyone's guess.

Reference: Huigens, M., Woelke, J., Pashalidou, F., Bukovinszky, T., Smid, H., & Fatouros, N. (2010). Chemical espionage on species-specific butterfly anti-aphrodisiacs by hitchhiking Trichogramma wasps Behavioral Ecology DOI: 10.1093/beheco/arq007

Images: copyright of Nina Fatouros

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edyong Fri, 02/19/2010 - 02:30

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animal behaviour

animals

Butterflies and moths

insects

"Two species of parasitoid wasp - Trichogramma brassicae and Trichogramma evanescens - are particularly skilled at chemical espionage. They've learned to hone in on sexual chemicals used by male cabbage white butterflies."

You mean: home in.

I presume the research was done in Europe. Any idea if these wasps also occur in North America on our Cabbage Whites here?

If it's so Catholic in its tastes, why doesn't it prefer the Cardinal?

This is sooo cool! A little nitpicking: the Small White does not lay a single egg, but the eggs - many - are laid singly, that is, isolated from each other, not in clusters as in the Large White, but there can be several on each cabbage leaf a few cm from each other.

Virus and bacteria team up to save aphid from parasitic wasp

edyong — Fri, 21 Aug 2009 08:00:47 +0000

Virus and bacteria team up to save aphid from parasitic wasp

Viruses and bacteria often act as parasites, infecting a host, reproducing at its expense and causing disease and death. But not always - sometimes, their infections are positively beneficial and on rare occasions, they can actually defend their hosts from parasitism rather than playing the role themselves.

In the body of one species of aphid, a bacterium and a virus have formed a unlikely partnership to defend their host from a lethal wasp called Aphidius ervi. The wasp turns aphids into living larders for its larvae, laying eggs inside unfortunate animals that are eventually eaten from the inside out. But the pea aphid (Acyrthosiphon pisum) has a defence - some individuals are infected by guardian bacteria (Hamiltonella defensa) that save their host by somehow killing the developing wasp larvae.

H.defensa can be passed down from mother to daughter or even sexually transmitted. Infection rates go up dramatically when aphids are threatened by parasitic wasps. But not all strains are the same; some provide substantially more protection than others and Kerry Oliver from the University of Georgia has found out why.

H.defensa's is only defensive when it itself is infected by a virus - a bacteriophage called APSE (or "A.pisum secondary endosymbiont" in full). APSE produces toxins that are suspected to target the tissues of animals, such as those of invading wasp grubs. The phage infects the bacteria, which in turn infect the aphids - it's this initial step that protects against the wasps.

Oliver set up populations of pea aphids that were all genetically identical. Some of them carried H.defensa in their bodies while others did not, and of the infected individuals, some were also ridden with the APSE-3 phage. He unleashed the wasp A.ervi upon all of these individuals, and waited.

The results clearly demonstrated the importance of having both virus and bacteria - with the full alliance intact, less than 10% of the aphids fell victim to the wasp. If one or both of the partners was absent, the wasp successfully parasitized 90% of its potential hosts. Without the phage, the aphids might as well not have been infected by H.defensa at all, for all the good it did.

The phage bestows protective powers on the bacteria thanks to a single gene that encodes a toxic protein. It also makes a lot of it - the toxin-producing gene is almost three times as active as one of the most active genes in the bacterium's own genome. How exactly this poison kills the wasp is unclear and it's worth noting that H.defensa's has toxins of its own. The most intriguing possibility is that somehow the viral and bacterial poisons work together to kill the wasp, but that's a theory to be tested in the future.

In lab conditions, the alliance between bacterium and virus is a very stable one, but in the wild, H.defensa has a bad habit of spontaneously losing its all-important phage lodger. Even within a single aphid, only some bacteria have viral genes integrated into their own. No one knows why the virus should disappear so frequently. Perhaps it's not an entirely welcome tenant and its presence carries some sort of cost that occasionally outweighs its benefits. Perhaps only phage-free H.defensa can be passed on from aphid to aphid.

Regardless, case of the pea aphid is a great example of the role of phages as vehicles for genes, shuffling them between bacteria and providing the carriers with extra superpowers. In almost all cases, these gene transfers affect the ability to cause disease. It's a phage that carries the CTX toxin that the bacterium Vibrio cholerae needs to cause cholera. It's a phage that carries the Shiga toxin that turns harmless strains of Escherichia coli into a bug capable of causing severe food poisoning.

The APSE-3 phage is different - its toxin, far from turning its host into a more effective parasite, actually turns it into an anti-parasite defender. Faced with an enemy that threatens them all, the virus, the bacterium and the aphid have formed an evolutionary alliance, with infection as its foundation.

Of course, not all parasitic wasps are foiled by viruses - some have managed to turn them into biological weapons, stealing their genes and using them to subdue their caterpillar prey...

Reference: Science 10.1126/science.1174463

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edyong Fri, 08/21/2009 - 04:00

Minor correction, Ed - although Dr. Oliver might not consider it so minor:

"She unleashed the wasp A.ervi upon all of these individuals, and waited."

Actually, he unleashed the wasp. You linked to his faculty bio page, but apparently never looked at it, because Kerry Oliver is awfully rugged-looking and bearded for a 'she.'

How is the virus transmitted? Seems like there would be selection for protecting the host only if it's transmitted to host offspring, presumably inside with the bacteria. If it ever evolves the ability to infect and reproduce in the wasps, I'd expect it to suppress rather than strengthen host defenses.

Oh God - n00b error! You know, I once sent a female scientist a write-up of their work for fact-checking and referred to them as "he" throughout. She sent it back with a couple of technical corrections but didn't spot the gender-bending one.

Ford-
My impression is that the phage is usually transmitted along with the infected facultative symbiont from parent to offspring, though it can be lost (rendering the Hamiltonella worse than useless). Since this is a phage, there would have to be a susceptible microbe in the wasps for it to infect.

Ed, you use the construction "H.defensa's" in a way that seems odd to me more than once in the article. Is it just a cut'n'paste anomaly, or am I missing some meaning?

Once again I am led to doubt that the phage should really be treated as an organism independently from the bacterium. You don't always wear your hat, but it's still your hat.

Self-medicating caterpillars use toxic plants to kill parasites

edyong — Fri, 20 Mar 2009 07:30:24 +0000

Self-medicating caterpillars use toxic plants to kill parasites

There are so many fascinating stories about parasitic wasps that they have become a regular feature in this blog. Usually, their prey come off poorly in these tales, with caterpillars being reduced to little more than living, paralysed larders for macabre wasp grubs. But not always - some hosts don't take the invasion of their bodies lying down. This post is an attempt to redress the balance between parasite and host, by telling the story of the caterpillar that fights back... with medicine.

One species of tiger moth, Grammia incorrupta, has a fuzzy caterpillar called the woolly bear. Like most other caterpillars, it's exploited by several species of parasitoids including flies and wasps. If these body-snatchers lay their eggs inside a caterpillar, its menu changes and it develops a preference for a group of plant toxins called pyrrolizidine alkaloids (PA).

These have no nutritional value and they clearly come at a cost, for woolly bears that eat a PA-rich diet grow more slowly than their peers. And yet, infected caterpillars gulp down these poisons by the leaf-ful. They are the medicine that the caterpillar uses to kill its unwanted hitchhikers.

Michael Singer from Wesleyan University discovered the benefits of the caterpillars' questionable diet by feeding them on plants that were either rich or poor in PA, and then exposing some of them to parasitic flies. If caterpillars weren't carrying fly eggs, their odds of survival fell by 16% if they ate PA-rich food. But if they were infected, those that munched on PA-rich plants were 17% more likely to survive than those that didn't. The toxic chemicals killed the developing flies, and far fewer of them made it to adulthood if their hosts were loaded with PA.

Singer found that parasitized caterpillars wolfed down more than twice as much PA-rich food than uninfected ones. But when he gave infected caterpillars a choice of food, he found that they have subtly different strategies for coping with parasites, depending on how many eggs they've been saddled with.

He infected caterpillars with anywhere from 0-3 fly eggs and gave them a choice a PA-rich diet with precious few other nutrients, or a nutritious block of food without any protection toxins. If the fly had laid just one, the caterpillars that survived were those that ate more of the nutritious food but not the PA-rich one. Wesleyan believes that in consuming more protein and carbohydrates, the caterpillars gave their immune systems the fuel they needed to fight off the invaders.

If individuals were laden with two eggs, those that survived used a different tactic, eating more of the PA-rich food and not the general nutritious variety. With the extra invader, a boosted immune system wasn't enough and extra medicine was needed. If there were three parasite eggs (which hardly ever happens in nature), nothing did any good. Neither eating more nutrients nor more PA improved the caterpillars' survival - this degree of infection overwhelmed both of their defences.

Singer's study is the first definitive example of an insect using medicine to treat its own infection, where it's clearly doing so to improve its chances of survival. There's only one possible other example - the caterpillars of another species of tiger moth (Platyprepia virginalis) switches from tasty bush lupine to poisonous hemlock when it's invaded by parasites.

However, in this case, the poisons seem to bestow the caterpillar with tolerance rather than resistance and in many cases, both it and its parasites survive. As such, it isn't clear whether the caterpillar is medicating itself, or whether it's all part of the parasite's manipulations.

Being simple invertebrates, Singer's woolly bears also divorce the practice of self-medication from its typical association with high intelligence. Many other animals, from chimps to sheep, can learn (or be trained) to use specific plants to treat poisons and parasites but the woolly bears show that learning isn't necessary.

They eat PA-rich plants as a matter of course; when parasites are afoot, all that changes is how much of this food they consume. Singer thinks that the caterpillar's immune system recognises the presence of parasites and changes its taste system to make it more responsive to the tang of PA. Indeed, other studies have found that parasitized woolly bears respond more strongly to the taste of PA than uninfected ones.

Reference: Singer, M., Mace, K., & Bernays, E. (2009). Self-Medication as Adaptive Plasticity: Increased Ingestion of Plant Toxins by Parasitized Caterpillars PLoS ONE, 4 (3) DOI: 10.1371/journal.pone.0004796

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edyong Fri, 03/20/2009 - 03:30

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Animal defences

animals

Butterflies and moths

A perfect example of adaptive evolution that demostrates the balance of nature. With time many species evolve to survive perils of their environ.

Great website. I love the variety of blogs.

It gave me such a thrill to see this post pop up in Google Reader! I worked in this lab as an undergrad for three years. Michael Singer's research is interesting and he was a fantastic mentor. Thanks for the fond memories :)

Fascinating! It's interesting to hear about the kind of defenses that involve in hosts.

On a tangent course... Many years ago I was a summer stupid in a lab that studied leps and their parasites. We collected huge numbers of lep larvae. Whenever we caught a really unusual lep, it was heavily parasitized. I speculated that the host's response in this case was to maximize its (and the parasites') likelihood of being eaten. This idea impinges on the (largely discredited) concept of group selection, but any increase in parasite mortality benefits all (including relatives) at zero cost. Nobody ever followed up on this idea.

This was a very interesting change from the usual story of the hosts coming off second best!

Aphids hide from parasitic wasps among the corpses of their peers

edyong — Thu, 19 Feb 2009 07:30:42 +0000

Aphids hide from parasitic wasps among the corpses of their peers

It's a scene straight out of a horror film - you look around and see dead bodies everywhere. They haven't just been killed either, they've been hollowed out from the inside-out leaving behind grotesque mummified shells. What would you do if you were confronted with such a macabre scene? Flee? Well, if you were an aphid, you'd probably just feel relieved and go about your business. Aphids, it seems, find security among the corpses of their peers.

Aphids, like almost all insects, are the targets of parasitic wasps that implant eggs inside their bodies. On hatching, the wasp grubs use the aphid as a living larder and eat their way out, leaving behind a mummified aphid-shaped husk.

These husks ought to be (quite literally) a dead give-away that parasites are afoot, valuable intel for any animal. But far from treating these bodies as a sign of danger, aphids actually see them as a reason to stick around. As Fievet says, "In human history, mummies had long been known to protect the dead; our study shows that in nature, mummies can also protect the living."

When food is scarce or danger abounds, aphids respond by producing winged offspring that can fly off to better, less threatening homes. They certainly react in this way when directly faced with parasitic wasps. Virgil Fievet from BIO3P wanted to see if they would do the same when surrounded by the mummified remains of their peers.

They placed individual grain aphids (Sitobion avenae) in cages, either alone or surrounded by mummies produced by the body-snatching wasp Aphidius rhopalosiphi. Unexpectedly, the aphids produced fewer winged offspring in the presence of the corpses, and the more corpses, the fewer fliers. Even if the mummies were placed on an adjacent plant, the aphids still behaved in the same way, investing in young that stayed put rather than left home.

It turns out that the aphids are not the only ones that glean information from the mummies. They're a message to the wasps too and apparently, they say "It's not worth it." When Fievet placed female wasps in a glass cage with aphid-infested plants, he found that they abandoned plants with 5 mummies and 10 healthy aphids more quickly than those with just 10 healthy aphids. While searching for their prey, the wasps bumped into fewer living aphids when the mummies were around, although whenever they found one, they were just as likely to attack it.

All in all, the aphids were actually safer among their deceased peers. In a group of 10 healthy individuals, an average of 75% were injected with eggs before the wasp flew away. If 5 mummies were present, just over half of the aphids were parasitized.

Fievet thinks that from the wasp's point of view, the dead aphids are a sign that this particular plant had already been mined for bodies. To avoid competition for other individuals, they soon flew off to find another group of aphids to attack. Perhaps, they were even trying to avoid the dangers of hyperparasites - wasps that attack other species of parasitic wasps.

The aphids meanwhile do a simple risk assessment and conclude that it's better to hide among the bodies of former peers than to find new, uncharted territory. Fievet has named this phenomenon the "dead zone effect" and he suggests that it may be found elsewhere in nature.

Wolves, for example, leave behind the carcasses of elk they have killed. Wolves are territorial hunters and to other elks, the presence of a fresh kill might indicate that the local hunters are temporarily satisfied. It might actually be better for them to stay close by than to move to an area where the local wolves may be hungrier. That will obviously need testing. For the moment, there is just one good example of the dead-zone effect but it's a strong one.

Reference: V. Fievet, P. Le Guigo, J. Casquet, D. Poinsot, Y. Outreman (2009). Living with the dead: when the body count rises, prey stick around Behavioral Ecology DOI: 10.1093/beheco/arp014

More on aphids: Aphids get superpowers through sex

edyong Thu, 02/19/2009 - 02:30

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Very cool research! And another brilliant summary.
It's nice to have some research to go along with an image like this one.

As a predator biologist (though primarily marine predators), I love stories like the wolf and the elk carcass. These are not mindless killing machines- they are far more devious than we give them credit for.

This is very interesting. I was at first confused as to why they didn't produce fliers, but it really does make since to stick around the dead. It's kind of like in movies, like in 28 Days Later where the main fellow hid amongst corpses while the military folk were trying to kill him. I've always though that if I were in a war and felt like I was about to be killed, the best thing to do would be to pretend I'm already dead. Is this something that become instinctual amongst aphids, or are they actually capable of this kind of logical induction?

Fantastic stuff. I would also wonder if it's not just that corpses indicate an area is "done", but if the search efficiency of the wasps is reduced in a minefield of corpses. So even if they wanted to find a juicy aphid, it would be harder to find. From the aphid's perspective, they get the benefits of sociality (reduced predation) without the cost (competition for resources).

Eric - what a pic! I was trying to find something to go with this article to no avail. But that's beautiful...

Dallas - I think it's fairly safe to say that they *aren't* capable of that level of logical induction.

Dan - I think a bit of both; it's probably that the wasp finds it harder to track down a live aphid and that it has less motivation to do so.

aren't aphid exuviia hollowed out shells? and if wasps see shells they know they're too late for egg laying and move to another plant.

Ed: Many thanks for your brilliant summary. You really did a great job! I found the metaphor of the horror film particularly attractive.

Dallas: Thank you, I was trying to find a movie scene to illustrate our results. I really have to improve my SF background. I agree with Ed, itâs safer to not consider aphids capable of logical induction. But who knows, they are so surprisingâ¦

Alex: Muratori had recently found that exuviae are recognised and attacked by wasps at the same level as aphids when both are present in the patch. Parasitoids wasted time in patch with exuviae but they are still motivated in searching for aphids. Muratori proposes however that aphids gain in protection by encounter-dilution effect when leaving their empty shells in the colony (http://www.biomedcentral.com/1471-2148/8/338/abstract/).
Parasitoids reacted to empty shells and mummies differently.

Virgil, thanks so much for coming here and taking questions from people. I wish more scientists would take this approach to public engagement - it really does help to foster interest in the area to be able to directly chat to the people doing the research. Bravo.

Cool. I saw this and was reminded of this paper on a similar issue:
http://www.biomedcentral.com/1471-2148/8/338

Muratori FB, Damiens D, Hance T, Boivin G.
Bad housekeeping: why do aphids leave their exuviae inside the colony?
BMC Evol Biol. 2008 Dec 19; 8:338

Wasps use genes stolen from ancient viruses to make biological weapons

edyong — Thu, 12 Feb 2009 13:00:21 +0000

Wasps use genes stolen from ancient viruses to make biological weapons

This is the seventh of eight posts on evolutionary research to celebrate Darwin's bicentennial. It combines many of my favourite topics - symbiosis, horizontal gene transfer, parasitic wasps and viruses.

Parasitic wasps make a living by snatching the bodies of other insects and using them as living incubators for their grubs. Some species target caterpillars, and subdue them with a biological weapon. They inject the victim with "virus-like particles" called polydnaviruses (PDVs), which weaken its immune system and leave the wasp grub to develop unopposed. Without the infection, the wasp egg would be surrounded by blood cells and killed.

The wasps' partners in body-snatching are very different to all other viruses. Once they have infected other cells, they never use the opportunity to make more copies of themselves. They actually can't. To complete their life cycles, viruses need to package their genetic material within a coat made of proteins. In most cases, the instructions for building these coats are encoded within the virus's genome, but polydnaviruses lack these key instructions entirely. Without them, the virus is stuck within whatever cell it infects.

It's such a weird set-up that some scientists have questioned whether the polydnaviruses actually count as viruses at all or whether they are "genetic secretions" from the wasps themselves. Where on earth are those missing coat genes?

Annie Bezier form Francois Rabelais University has found the answer and it's an astonishing one. The viruses' coat genes haven't disappeared - they've just been relocated to the genomes of their wasp hosts.

In this way, the wasps and the viruses have formed an unbreakable alliance, where neither can survive without the other's help. Without the virus, the next generation of wasps would be overwhelmed by the defences of their caterpillar larders. Without the wasp, the virus would never be able to reproduce. Some viruses may be able to live happily alongside their host with little ill effect; others may even be beneficial in some way. But this is the first example of a virus co-evolving with its host in a compulsory binding pact.

Secret origins

The polydnaviruses were first observed in 1967 within the ovaries of parasitic wasps. From this storage facility, these biological weapons can be easily injected into a host, for the wasp's sting is just a modified version of her egg-laying tube. Bezier knew that when the viruses are first produced in the wasp ovaries, they come complete with coats, so the genes for building these must lie nearby.

She searched for virus-related genes that were switched on in the ovaries of two parasitic wasps from the braconid group - Cotesia congregata and Chelonus inanitus. It wasn't long before she found some - 22 potential genes that strongly resembled genes from another group of viruses called nudiviruses. Sure enough, some of these 22 genes encode proteins that form viral coats, and others encoded proteins responsible for assembling the virus particles together. These initial results strongly implied that the polydnaviruses are descendants of the nudivirus family .

Nudiviruses infect insects, so Bezier needed to make sure that the genes she was picking up hadn't just come from an infection that her wasps had picked up in her laboratory. Fortunately for her theory, she managed to find some of the same genes in eight species of braconid wasp collected from all over the world. And when she sequenced large tracts of C.congregata's genome, she found that at least 10 nudivirus-related genes had been fully integrated into the insect's chromosomes. These were no temporary hitchhikers - they were clearly stable parts of the wasp's genome.

Bezier went on to show that these nudivirus-related genes, hidden among the wasp genomes, were switched on when the adult wasps were still tucked away in their pupae. That's exactly the time when the polydnaviruses are first assembled. The genes are even switched on the exact spot in the wasp ovaries where the virus particles are produced. And to top it all off, Bezier found that 20 of these ex-nudiviral genes encoded proteins that make up part of the polydnavirus coat. It was the final proof she needed. The wasp genomes clearly contain ex-viral genes, which complete the assembly process that polydnaviruses can't finish themselves.

Evolution of a partnership

The two wasps that Bezier studied hail from distantly related arms of the braconid family tree, which suggests that all the wasps of this group have nudiviral machinery incorporated into their genomes. Bezier thinks that the bond between wasp and virus was forged at least 100 million years ago, before the braconid wasp lineage had started to expand.

The genetic evidence suggests all the virus genes in today's wasps are the descendants of a single ancestral nudivirus that integrated itself into the genomes of an ancestral wasp. Many of these transferred genes are still incredibly similar across different species, suggesting that some aspects of the wasp-virus union are so important that they are resistant to change. Others have diverged more thoroughly and Bezier thinks that these are probably responsible for more specific interactions between virus and host.

In contrast, the polydnaviruses actually share very little of their own DNA. As an example, the strains used by C.congregata and C.inanitus hardly have any genes in common, even though their respective wasps use very similar coat-producing genes. Bezier thinks that many of the genes within the polydnaviruses aren't viral at all - they actually came from the wasp.

When the ancestral nudivirus infected the ancestral wasp, the two partners did a massive genetic swap, with wasp genes replacing viral ones and viral genes infiltrating the wasp genome. The virus has become a vehicle that allows the wasp to switch on its own genes within the body of a caterpillar. It's possible that this partnership was one of the reasons why the braconid wasps became such successful parasites and why today, there are at least 17,500 species of them.

Reference: Science 10.1126/science.1166788

Images: Photo by Alex Wild

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edyong Thu, 02/12/2009 - 08:00

Interesting post! I always like to look at discoveries like these in a backwards fashion - Like, instead of thinking about how these wasps 'took' genes from a virus, but more how something like a wasp developing proteins like this, then spreading them to a host could result in the formation of a virus (of course I have no backing to this whats-so-ever, but it's interesting to think of it this way). Or what if a wasp infected another wasp (which doesn't happen, at leased not that I know of) with a pseudo-virus, and that is able to 'absorb' (for lack of a better term or mechanism) a protein coat gene - creating a virus that is able to replicate. I'm sure there is much more evidence for the forward progression, but something like this could help explain how something like a virus could come to be.

This provokes the question as to where and how viruses originated. Might this be not a case of genes migrating from viruses to wasps, but rather a case of genes failing to migrate from wasps to polydnaviruses? If that is not plausible (for some specific reason) in this case, is there any reason it can't be in some more likely case, e.g. an archaeon or bacterium?

I am thinking, particularly, of bacteriophages, which seem to me more likely to have been hijacked by viruses than to have been invented by them. Parasites are generally better known for a tendency to degeneracy than for exquisite mechanical engineering. Has anyone even looked for instances of a bacterium using the phage structure as a carrier for communication or as a weapons delivery platform? Might not the phage virus qua virus itself originate as a weapon in inter-bacterial warfare?

The idea seems testable by identifying patterns of phage components in bacterial genomes.

AHHH COOOOOOOOOOOL!!!

What ERV said.

Another cool narration by Ed.

I'd much rather read posts like this rather than pointless battles between theists and atheists. Go Ed!

Wow. This is one of the coolest examples of horizontal gene transfer I've heard about. Thanks for posting this.

Stunning! This could make for an awesome sci-fi story.

Cyberworld is full of indecipherable sci-pseudo-jargon that is not very well written. I would like to thank you for inspiring me to be a better writer in terms of clarity and lucidity; for your ability to express complex subjects in a manner that makes it easy to understand is exemplary. You should be a spokesperson for the World Health Organization.

How diversity creates itself - cascades of new species among flies and parasitic wasps

edyong — Mon, 09 Feb 2009 07:30:57 +0000

How diversity creates itself - cascades of new species among flies and parasitic wasps

This is the second of eight posts on evolutionary research to celebrate Darwin's bicentennial.

What do you get when one species splits into separate lineages? Two species? Think bigger...

When new species arise, they can set off evolutionary chain reactions that cause even more new species to spring forth - fresh buds on the tree of life create conditions that encourage more budding on different branches.

Biologists have long suspected that these "cascades of speciation" exist but have struggled to test them. Enter Andrew Forbes from the University of Notre Dame - his team of has found a stunning new proof of the concept by studying a fruit fly called the apple maggot (Rhagoletis pomonella) and the parasitic wasps that use it as a host.

Contrary to its name, the apple maggot's natural host is not apples - it's hawthorn. The fly only developed a taste for apples about 150 years ago, when the fruit was first introduced to North America. This culinary switch has created two races of apple maggot - one that eats hawthorn and another that eats apples. Even though they are often found in the same place, the two races don't mix and they don't breed together. They are well on the road to becoming separate, genetically distinct species.

And so are their parasites. A wasp called Diachasma alloeum specialises in attacking apple maggots. It lays its eggs inside the fly larvae, and its grubs eat the victim from the inside out. Forbes found that the wasp has also started to form separate races that don't crossbreed with one another, even though they have overlapping ranges. By adapting to new host plants, the flies inadvertently set up barriers that separated their respective parasites from one another. Now, the wasp, like its hosts, are also on the way to becoming separate species. It's a fantastic example of diversity bringing itself about.

Forbes studied these changes by collecting thousands of samples of both flies and wasps from the eastern United States. His team studied both races of apple maggot along with two closely related species, the blueberry maggot (R.mendax) and the snowberry maggot (R.zephyria). You'd find it almost impossible to tell the difference between them by sight but these species are indeed genetically distinct.

Each maggot eats the fruit of a different plant, and each plant bears fruit at different times of the year. Because the flies have just one generation per year and live short lives, adults of one species rarely buzz around at the same times as those of the others. It's shift-working on a massive scale - all the flies may live in the same place, but time separates them and thus, their genes.

This chronological separation is boosted by a geographical one. The flies stick to the fruit that they ate as maggots, by following their distinctive smells. They mate there and lay their eggs there - another barrier that prevents the genes of different races or species from mixing.

The wasp D.alloeum targets flies from all four host plants and Forbes found that it is influenced by the same barriers that have cordoned off the flies into mutually exclusive populations. By sequencing DNA from various positions in the wasps' genomes (and those of their mitochondria), he showed that some genetic variations consistently turned up in some wasps but not others, depending on which flies they attacked.

The wasps had effectively splintered into apple, hawthorn, blueberry and snowberry populations, just as their hosts had done. The mitochondrial sequences revealed that these splits were relatively recent affairs, but even now, there is little genetic flow between the different sub-groups. And that was due to the same timing issues that have divided the flies.

By observing wild wasps, Forbes found that they do indeed mate and lay eggs near the fruits of their chosen hosts. They have a life-long affinity for the distinctive smell of their fruity birthplace. Using Y-shaped chamber with smells pumping down one arm, Forbes shown that D.alloeum is drawn towards the odour of its preferred fruit and no others. It's more likely that this preference is inherited rather than something the wasps learn as larvae. After all, they are laid inside the flies and pupate as soon as they burst out - they're never actually in contact with the fruit itself.

The wasps are not just separated in space, but also in time. The fruits ripen at different times, the flies emerge to synchronise with the fruit and the wasps emerge to synchronise with the flies. They too have short life-spans of about two weeks and just one generation ever year, so adults from populations based on different fruits have few opportunities to meet. Forbes even found that these timing differences in the wasp life-cycles may be influenced by the very genetics variations that he identified earlier.

The expansion of the Rhagolettis flies into species and races that eat different fruits has triggered a parallel expansion among the wasps that exploit their bodies. The smell and timing of different fruits act as invisible barriers that keep different populations of both flies and wasps apart.

Forbes even suggests that the effect may work both ways. It's possible that the wasps themselves have provided the impetus for the flies to exploit new fruits in the first place. Those that did so would find themselves with a big advantage in a new environment free of enemies. But that itself creates a fresh niche for the wasps to exploit, and they too diverge to make the most of the new riches.

This is the clearest example yet of a "cascade of speciation", of diversity creating more diversity. It is absolutely not the only one. Just think about these statistics. There are more plant-eating insects than any other life form on earth and they are plagued by hordes of parasites that co-opt their bodies. In fact, Forbes cites that one in five of all insects are probably parasitic wasps. Perhaps these ripples of divergence are the very reason why these insects groups are so remarkably varied. As Forbes says, "There is a world of opportunity for sequential speciation in nature."

Reference: A. A. Forbes, T. H.Q. Powell, L. L. Stelinski, J. J. Smith, J. L. Feder (2009). Sequential Sympatric Speciation Across Trophic Levels Science, 323 (5915), 776-779 DOI: 10.1126/science.1166981

Images: Fly by Rob Oakleaf; others by Andrew Forbes

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edyong Mon, 02/09/2009 - 02:30

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Wow, that is really neat!!

Hi Ed. Thanks for the article! We think its an exciting finding and you've captured the idea of the paper perfectly. Cheers!

Parasites are so cool! Its really interesting seeing more and more how they've shaped diversity

This article is really neat and really talks about how evolution is here to stay. These species have truly evolved and adapted in so many ways. I think this was a brilliant idea in capturing the essence of natural selection. Great findings.

Brilliant!

(Typo: search on "test them".)

Evolution is such a big part of nature! I sometimes forget how much it effects earth and its creatures. This article definatly reminded me. Great find :)

This article was really interesting and intriguing! Great findings on the article Dan. Evolution is a major part to everyone and the effects of it are just amazing to read about and see with your own eyes. The way these species have adapted, have become very resourceful for themselves. This was a very essential, educational article!

I think this article is so cool. It all makes sense, it's just something you never really think about until someone else brings it to your attention. I have heard of wasps that lay their eggs in catepillars but this wasp that lays its egg in various fruit eating flies is very smart. it's awesome how all these different organisms sync up their life patterns to ensure survival.

This was a crazy find. It's amazing to see how much evolution takes toll on species and that it is still going on everyday and has facts as incredible as this to prove it.