Blogs / The Loom

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…

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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.]

These biologists are holding out on me.

I’ve been writing about biology for quite some time now, and sometimes I think I’ve got a pretty good sense of the scope of life. Neurosurgeon wasps–got it. Eels with alien jaws–check. And then I stumble across something new, or should I say, new to me.

This week’s revelation is androgenesis. Androgenesis is what happens when kids get all their genes from their father. Normally humans and other animals produce offspring by combining DNA from both mother and father, an arrangement that’s often the case in plants as well. Both sperm and eggs only get one of the two copies of each chromosome in the genome. When male and female gametes combine, they have enough DNA together to make a new organism. There are exceptions. Hermaphrodites can be fathers or mothers. Then there’s parthenogensis, in which a female animal’s eggs begin developing into embryos without any sperm carrying Dad’s genetic delivery. I thought that this represented the outer edge of strange kinds of reproduction. And then I came across androgenesis.

At first I couldn’t even imagine how androgensis can work. How can you start growing an embryo from a sperm, I wondered? And while I could at least imagine that a man might bear a child if an embryo was implanted in his gut, I could not understand how this could happen in any animal naturally. But of course, I should know never to rely on my feeble powers of imagination to guess at the maximal weirdness nature can deploy. (I probably shouldn’t have posted this on April 1, because people may think I’m joking. I’m not.)

Androgenesis, it turns out, transforms fatherhood into a parasitic invasion. It begins like normal fertilization, with a sperm fusing to an egg. But then the egg’s DNA gets hurled out of its nucleus, so that the sperm’s genes are the only ones left in the egg. The egg begins to develop into an embryo, but only after it has lost the mother’s DNA.

Androgenesis is rare in the natural world, but it’s not obscure. Asian clams (Corbicula) came to the United States several decades ago, possibly by Chinese immigrants for food. Today they’re a major pest. And two of the species that have come to our shores use androgenesis to reproduce. These particular Asian clams are hermaphrodites. Along with eggs, they also make sperm–but these are odd sperm, with a full supply of DNA. Once their sperm has fertilized the egg of another clam, the female DNA is ejected, and the clam embryo starts to develop. The mother clam broods father’s clones in her gills, where they can probably thrive on the food that gets trapped there.

To be precise, the eggs of Asian clams don’t lose all their DNA. Most of the DNA in animal cells is in the nucleus, but there are also dozens of little energy-generating factories in the cell called mitochondria that carry their own bits of DNA as well. Fathers evict the maternal DNA from the nucleus, but the mitochrondial DNA stays behind. This only makes sense (insofar as androgenesis makes much sense at all), because a cell without a way to generate energy would have a hard time growing into an embryo.

Scientists can build evolutionary trees from DNA, using it to determine how individual animals are related to one another. But when they build the tree of invasive Asian clams, the mitochondrial and nuclear DNA reveal different genealogies. It looks as if the two invasive species have been carrying out androgenesis on each other.

The ability Asian clams have to use other species to make near-clones of themselves may help resolve one of the paradoxes of androgenesis. Once androgenesis arises in a species, it should be able to spread quickly because the fathers can reproduce so much faster than more conventional clams. Taking the female side of this deal is a losing proposition, because females can’t pass their DNA down to the next generation. So androgenesis ought to lead to swift extinction, like some sort of paternal plague. Even if androgenetic animals could somehow avoid this doom, they would bear another burden: mutations. Without the benefit of sex to mix up their DNA, they would accumulate harmful mutations over the generations that could threaten the survival of the entire population.

The huge success of Asian clams in the US is proof that androgenesis is not a one-way ticket to oblivion. Androgenesis may survive in part because the fathers can make other species their victims, too. Another species can offer an extra supply of eggs, as well as an extra supply of DNA. Studies on the DNA of Asian clams in the US suggest that on rare occasion, some of the mother’s DNA stays behind in the egg. This fresh influx of genes may rescue androgenetic clams from some of their harmful mutations.

For more information on androgenesis, check out this new paper in the journal Evolution. And if any biologists are holding out on any other bizarre biology, it’s time to come forward.

In the comment thread for my post about Microcosm’s rave review in Publisher’s Weekly, outeast writes,

There’s been something I’ve been dying for, and here’s as good a place as any to mention it: real coffee-table editions of your books, meaning lavishly illustrated throughout rather than with a couple of meagre (though nice in themselves) wedges of pictures in the middle. When I’m reading about the different stages parasites go through and so on I want to see it - I want to see the flukes pouring from the toad and all that. And I want books that visitors will ohh and ahh (and eww) over, books that will last for years and that my kids will stumble across a decade from now and show to their fascinated and horrified friends… Pretty pretty please, do tell your publisher!!

First off, everyone should know that writers love this sort of stuff. It keeps us going. Look at your bookshelf, go find the web sites of your favorite living writers, and email them to let them know their books matter to you. Left to our own devices, we get mopey and complain to each other about writer’s block and the death of the novel and all that.

As to outeast’s suggestion, that would be lovely. It would also be a good way to keep guests from eating too much at dinner…

But there is, of course, the matter of economics. It’s very expensive to publish books with full-color pictures throughout–not just the ink, but the high-quality paper it has to be printed on. My book, Evolution: The Triumph of an Idea had lots of color pictures in the original hardback and paperback. When those ran out, my publisher reissued the book without pictures. (My text does well enough without them, in my humble opinion, but those were some nice pictures…) Monster best-sellers, like A Short History of Nearly Everything, make that equation easier to solve.

On the other hand, in 2010 it will be 10 years since Parasite Rex came out…maybe time for a revision… hmm

I’ve got some more talks coming up that I want to let you know about–especially those of you around Lincoln, Nebraska or Sarasota, Florida–as well as those of you who like to go to meetings about parasites…

1. DARWIN DAY: I’ll be doing my part to celebrate, at the University of Nebraska. My talk will be this Friday. I’ll be talking about what bacteria could have taught Darwin about evolution–drawing in part from my upcoming book, Microcsom. Here’s the UNL link with details (Facebook event).

2. THE ORIGIN OF WHALES: Mote Marine Laboratory runs a great series of public lectures. On March 10 I’ll be talking about the origin of whales–and how scientists keep filling in the gaps (despite what certain creationists might claim as they lose debates with PZ Myers). Details here. (and at Facebook)

3. PARASITES IN POPULAR CULTURE: I’ll be speaking June 28 at the American Society of Parasitologists in a special symposium called, “Parasitology: Public Awareness Through Literature, Art, and Film.” (pdf flyer) I’ll be talking about the intense fascination people have about parasites, writing a book about it, and, of course, blogging about parasites. I’m also gathering examples of parasites in recent films, etc., so any tips would be appreciated. Since I’m sure not all of you will be at this meeting, I’ll do a little live-blogging to fill your need for parasite news.

More events to come, as details crystallize. You can always check my talk page or my Facebook page for the latest info.

Bora and his hard-working crew have picked the entries for the next anthology of science blogging, Openlab 2007. My entry on how tapeworms evolved into parasites made the cut. See the full list here.

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.

The Center for Science Writings at Stevens Institute of Technology has rolled out the “Stevens Seventy,” the seventy greatest science books since 1900. If you click all the way through to Z, my 2000 book Parasite Rex ends the list. Many thanks.

As the introduction to the list points out, these things are always arbitrary, so judge for yourself. Did they leave any classics off? Did they honor an unworthy title?

My talk with John Horgan on bloggingheads.tv is up. I’m sure the fact that the label “weird life” appears directly over my head was an accident (right, John?). Anyway, we had a good talk about parasites, aliens, and how to handle hype in science. I may have made some mistakes–feel free to fact-check in the comment thread. Unfortunately, I can’t strike out my spoken errors, but we can make the best of it. (If you hanker for more scienceblog/bloggingheads encounters, see these talks by PZ Myers and Chris Mooney.)

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]

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Science Made Cool writes from Tokyo, describing the world’s only parasite museum. Someday I’ll get there…

Sadly, the keychain with the sushi worm embedded inside is not for sale online…

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.