A tapeworm starts off as an egg which then develops into a cyst. Inside the cyst is a ball-shaped creature with hooks that it can use to crawl around its host before growing into an adult. Many species are made up of dozens or hundreds of segments called proglottids. Each proglottid may be equipped with both eggs and sperm-making organs. As an adult, a tapeworm also grows a head-like end often equipped with suckers or hooks of its own. This strange organ is called the scolex. (The shark tapeworm in this photo is displaying its fearsome scolex.) While the tapeworm lives in the gut of its host, it uses its scolex to clamp down in place, although it may swim around to find an ideal spot from time to time.
And from time to time, a proglottid breaks off from the body of the tapeworm and ends up leaving the body. Actually, it can leave under its own steam. Proglottids have muscles and nerves that they can use to crawl out the back end and along the ground.
Tapeworms evolved from free-living flatworms, but they have undergone some radical changes. Along with the evolution of their proglottids, they also abandoned their digestive tract, opting instead to slurp up their food directly through their skin. They’re so weird now that scientists haven’t even been sure which end is which. Some people have suggested the scolex is the head of the tapeworm. But others have pointed out that while the tapeworm is still in its cyst, the hooks actually form on the other end of its body. What’s more, in related flatworms with recognizable heads and tails, the sperm-organs are closer to the head than the ovaries. In tapeworms, the ovaries are closer to the scolex.
This morning at the second day of the American Society of Parasitologists, Peter Olson from the Natural History Museum in London, offered a potential solution to the puzzle. All animals–including us–use a set of master genes to determine the head-to-tail anatomy of developing embryos. The precise DNA sequence of these genes is different from species to species, but they show clear evidence of having evolved from a set of genes in a distant ancestor. Scientists have carried out most of their research on these genes in well-studied species like fruit flies and mice. Only recently have scientists started to look at how these so-called Hox genes work in other animals. Olson is studying the genes in tapeworms that live in mice, called Hymenolepis.
One key gene Olson described is called Post-2. It corresponds to genes that defines the tail end of insects and mammals. When tapeworms develop into little balls with hooks, Olson has discovered, Post-2 becomes active on the end of the ball with the hooks. That suggests that the hooks are growing at the tail-end of the animal, before it has yet grown a tail.
Later, when the tapeworm develops into an adult with proglottids, Post-2 becomes active at one end of each segment. It becomes active at the end furthest from the scolex. So the tail end of the animal appears to be pointing away from the scolex–in other words, the scolex really is the head after all. It may not be a head you’re familiar with–sans teeth, sans eyes, sans taste, although not quite sans everything. But when the tapeworm grew its head, it knew where it was the same way you knew when you grew yours.
[Edited a bit to address questions from commenters.]
It feels like a homecoming: I’m among hundreds of people who live for parasites.
I arrived in Arlington Texas this afternoon to attend the annual meeting of the American Society of Parasitologists. I’m going to give a talk tomorrow about the public awareness of parasitology, talking about my long-term relationship with the beasties in books, articles, blogs, and beyond. But till then, I get to hang out with parasitologists. I’ve met a lot of the people here over the years, like the leech-master Mark Siddall, and I’ve read the work of a lot of people I’m just meeting (work on things like how lice jumped from gorillas to human ancestors).
And I’m also hearing new people talking about research I’ve never heard before–”nice and weird,” as one parasitologist described the species she studies. I heard about a parasite in Nebraska, a flatworm called a trematode (Halipegus eccentricus), that scientists discovered living in the ears of bullfrogs. But the trematodes in their ears are all adults. Matt Bolek from the University of Nebraska described how he and his colleagues had figured out the rest of the parasite’s life cycle. The parasites release their eggs from the frog ears, which then get scarfed up by snails, where they hatch and start to develop. Then they leave the snails and swim in search of little aquatic invertebrates called ostracods. The ostracods get eaten by the larvae of damselflies, which then mature and fly into the air, only to be devoured by frogs. The parasites escape the damselflies and move through the bodies of the frogs to their ears. One trematode, four hosts.
And you thought your commute was long.
Tomorrow I’ll blog about more of these marvelous beasts.
[Image courtesy of Matthew Gilligan]
[Update: In answer to commenters--that's an invertebrate known as a isopod that's eaten the fish's tongue and is now sitting where the tongue used to be. Nice and weird, baby.]
In a couple weeks I head to Texas to the annual meeting of the American Society of Parasitologists to talk about parasites in pop culture. The symposium is called, “Parasitology: Public awareness through literature, art, and film.” Our panel has lots of creepy movie clips in store, plus other sorts of media including books and this humble blog. But maybe we’ll need to tack on “music” to the end of that list in the symposium title. Inspired by a post of mine on the gorey glam of Ampulex compressa , the emerald cockroach wasp, a band called Super Duper has composed a song. The video is below, and the lyrics below that…
A caterpillar’s life is not an easy one. The plants that it eats make toxins to make it sick. Birds swoop in to pluck it away and feed it to their chicks. But the most horrific threat comes from wasps that use caterpillars as hosts for their young. These parasitoid wasps are among my favorite creatures (see my post on the emerald cockroach wasp, which attacks cockroaches like a neurosurgeon). So it was with eye-popping delight that I read a new paper in PLOS
Biology One about how another species of wasp in Brazil attacks another caterpillar. Glyptapanteles glyptapanteles is more than just cruel to its host. It also gives its host an extreme case of Stockholm syndrome.
The fun begins when a female Glyptapanteles wasp comes across a potential host–a moth known as Thyrinteina leucocerae. The wasp inserts a stinger-like probe into the caterpillar’s
gut body cavity and injects dozens of eggs. The eggs hatch and grow into wasp larvae, which feed on the still-living host as it continues munching on leaves. The caterpillars even moult and pass through three or four stages with the parasites lurking inside them. Finally, when the wasps have finished their living feast, about 80 of them drill escape holes and crawl out of the caterpillar. They move a few inches away, where they spin cocoons on a twig or leaf, where they will develop into adults.
Many species of wasps exit their hosts this way, and in many cases the hosts promptly die. After you’ve just spent a couple weeks with dozens of parasites sucking your insides dry and then drilling their way out of your body, you’d probably feel like dying too. But in the case of Thyrinteina, death waits. The caterpillar stops feeding and crawling and simply sits, still alive, next to the wasp cocoons. When other insects come by, they wave their heads violently around, so violently they can knock the other insects off the tree. (You can download a movie of this behavior here.) Once the adult wasps emerge from the cocoons, the caterpillars finally expire.
You may be thinking that the caterpillars were turned into bodyguards for their parasites, protecting them from predators. And if it were ture, it would be a particularly striking addition to a long list of cases in which parasites manipulate their hosts for their own well being. I wrote about this manipulation at length in my book Parasite Rex, and have also written about it here on the Loom (suicidal rats, neurosurgical wasps, etc).
When scientists find a host acting weirdly, it’s a reasonable hypothesis that they’re being manipulated by their
host parasite. But it’s just a hypothesis, one that cries out for testing. There are other possible interpretations, after all. W,hat looks like a clever adaptation that boosts the parasite’s reproductive success may in fact just by an incidental byproduct of being sick. And a peculiar behavior that scientists observe in hosts they keep in a lab may not be terribly important out in the wild. Parasites sometimes need to go from one host to another to develop–in many cases, traveling from prey to the predators that eat them. If a parasite makes its host an easier target for predators, its host may get eaten by the wrong species of predator, one in which the parasite will die.
To test the bodyguard hypothesis on Glyptapanteles wasps, scientists ran experiments on the animals outdoors, on real guava trees. They observed how parasitized caterpillars behaved around the cocoons compared to healthy ones, and they observed how much protection the infected caterpillars really provided from predators by removing some of them.
The scientists found that almost all the parasitized caterpillars huddled close to their parasite cocoons, often actually crouching over them. When the scientists put cocoons next to unparasitized caterpillars, they simply went on wandering the leaves and twigs, feeding away. When stinkbugs came along, the parasitized caterpillars almost always lashed out, while the healthy caterpillars ignored them. Sometimes the stinkbugs and wasps fell off the tree, and sometimes they just gave up. This flailing made a big difference to the survival of the wasps, the scientists found. Removing infected caterpillars from the neighborhood of the cocoons double the death rate for the wasps.
So the bodyguard hypothesis looks good. But it naturally raises a question: how do the wasp larvae turn the caterpillars into their bodyguards? The catepillars only start fending off predators a couple hours after the wasps had left their bodies. It’s possible that they left behind some chemicals that altered their hosts’ behavior. But when the scientists dissected caterpillars three or four days after the wasps had left, they would find one or two living wasp larvae still inside. Scientists have found other parasites that have stayed behind in their hosts. Ants, for example, are infected by lancet flukes that drive them up to the tops of blades of grass, where they can be eaten by grazing livestock. It appears that they are driven by a few flukes that form cysts in the brains of the ants and can not pass on into their new host like the rest of the flukes. Perhaps the bodyguard caterpillars are being piloted by one or two wasps that stay behind, defending their siblings from predators while surrendering their own lives. Who knew such vicious parasites could be so heroic?
(Photograph by Prof. José Lino-Neto. Covered by a Creative Commons Attribution License. Any use should include citation of the authors and paper as the original source.)
[Note: Thanks for the fact-checking and copy-editing. I've fixed the text accordingly.]
It’s fun to write about discoveries, but mysteries are important too. In my latest column for Wired.com, I explore the mysterious death of honeybees, and the trouble scientists are having pinning down a culprit.
Last year I wrote about the emerald cockroach wasp, Ampulex compressa, which injects venom into cockroaches to turn them into zombie hosts for their parasitic offspring. (More posts on Ampulex here.) The scientists I wrote about have been trying to figure out what exactly the venom does to the nervous system of their victims, and they’ve discovered that it interferes with a neurotransmitter called octopamine. New Scientist has an update. And they also have a link to a YouTube video that offers more than you may want to see of this awesome parasitic manipulation.
It is a day to write about Giardia, and I am happy to say that I cannot do so from firsthand experience. Friends of mine have suffered infections of Giardia in their gut, but they haven’t been terribly forthcoming about the details. It’s not fun, they assure me, and it can last for months. Unpleasant as it may be up close, though, Giardia is one of the most fascinating, most enigmatic creatures on the planet (from a safe distance). Scientists do not yet quite know what to make of this single-celled parasite, but one possibility is that Giardia holds secrets to some of the key steps in the evolution of our own ancestors billions years ago. [cont. below the fold]
Science Made Cool writes from Tokyo, describing the world’s only parasite museum. Someday I’ll get there…
Update: Mark asks whether there’s an American museum in Maryland. It’s a collection, not a museum. I write about my visit there in Parasite Rex. A wonderfully creepy place, but no parasite-entombing keychains for sale.
Parasitoid wasps (or rather, one group of them called the Ichneumonidae) are the subject of one of Charles Darwin’s most famous quotations: “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars.”
Scientists have learned a lot more about parasitoid wasps since Darwin wrote about them in 1860, and their elegant viciousness is now even more staggering to behold. Not only do they devour their hosts alive from the inside out, but they also manipulate the behavior of their hosts to serve their own needs (see my post on zombie cockroaches for one particularly startling example).
To be fair, though, parasitoid wasps are not just vicious to their hosts. They can be just as nasty to other parasitoid wasps. Some wasp larvae can only mature inside other parasitoids, turning their host into a grotesque Russian doll. And, as I write in tomorrow’s New York Times, some wasps turn their caterpillar host into a battlefield, waging all-out war with other wasps. They kill other species of wasps, and will even kill their own siblings by the thousands. (Be sure to see the diagram of the sci-fi life cycle of the wasp Copidosoma floridanum. By the end of it, the caterpillar is a mummified mass of pupae.)
These creatures are certainly bizarre, but bizarre in an scientifically interesting way. Scientists have found that the evolutionary forces that shape other animals can also explain these wasps. As I explain in the article, the warfare among the wasps probably arises thanks to the peculiar way they develop. A single egg (like the one being laid inside a host egg in the picture) gives rise to thousands of genetically identical siblings. Up to a quarter of them become vicious soldiers, while the rest become passive feeders. The soldiers are sterile, lacking any sex cells. In a way, they’re not even really individuals. In a genetic sense, they’re like disembodied organs. Imagine you could send your liver off to kill your enemies.
The soldiers benefit their siblings by killing–either killing competitors or even killing other siblings. Evolutionary trade-offs between conflict and cooperation are also at work in the families of other species, from cooperative honeybee hives to cub-killing packs of lions to humans. What would Darwin make of these wasps? I’ll leave that for more theologically-minded folks to speculate.
[For those interested in all things Web 2.0-ish, I'm pleased to say that this is the first article I've written for the Times that includes a couple links to relevant scientific papers. It's downright bloggy.]
[For more fun with parasitoid wasps, you can also check out my book, Parasite Rex.]
Reports are coming out this morning on a new study on one of the Loom’s favorite organisms: Toxoplasma gondii, the single-celled parasite that lives in roughly half of all people on Earth and has the ability to alter the behavior of its host. I reported on the research last June in the New York Times, when the Stanford researchers reported their results at a scientific conference. It’s nice to finally get the results on paper, though.
The study is a fine example of an underappreciated part of science: replication. In 2000 British researchers carried out a study in which they put healthy and Toxoplasma infected rats in an outdoor enclosure and watched them nose around. They added odors to some of the corners of the enclosure; sometimes the odor of rats, sometimes of rabbits, sometimes of cat urine. They found that healthy rats were deeply affected by the scent of a cat, becoming less curious. Parasite-infested rats showed no fear. They proposed that the shift in behavior was an adaptation of the parasite for getting into its final host–cats. (I included a description of this study in my book Parasite Rex.)
It was a remarkable result, but even remarkable results may not be so significant as they seem at first. They need to be replicated by other researchers. That’s what the Stanford team has now done. They set up an enclosure, set both rats and mice loose in it, and observed a significant difference between infected and parasite-free hosts. The animals were actually attracted to the smell of cats. The Stanford team went beyond mere replication, however. They took a closer look at how the parasites manipulate their hosts.