Science Made Cool

Not a Friday Parasite

There's an interesting press release out from MIT, describing some new discoveries about the widespread parasite Toxoplasma gondii. There's a good chance you've got Toxoplasma living inside you right now, especially if you own pets. Typically it forms cysts in your muscles or brain, and if your immune system is in good shape, your body can fight the parasite to a draw. It won't spread much, but you can't get rid of it, either. Meanwhile, it could be changing your personality, but what's a little mind control among friends?

Anyway, the new research is focused on figuring out how one strain of Toxoplasma causes inflammation in hosts -- which can lead to encephalitis and brain damage and all sorts of unpleasantness. They've tracked down the protein which produces the inflammation reaction, but it's still not clear why the parasite produces it. Is there some advantage to causing encephalitis? Or does the protein have another function? Stay tuned . . .

(Thanks to PhysOrg for spotting the story.)

Friday Parasite: Introducing the ALDF Tick App

It’s fall, and our property is inundated with deer ticks. Our chickens keep them under control in the yard, but our dog is a hot-blooded tick beacon during her daily walks through the woods. She loves to tear around in the underbrush, which means I’m guaranteed to find at least one tick hiding under her fur each evening. And although I’m much taller and wear clothes, occasionally one makes it onto my skin too. 

But the American Lyme Disease Foundation clearly doesn’t want me to panic about it. Their new iPhone/iPod app may have a disturbing title (Lyme Disease), but the material inside it is designed to teach users how to identify ticks, deal with their bites, and best of all, how to assess their risk for contracting the disease if they happen to get bitten. It includes a map developed by the Yale University School of Public Health that shows where deer ticks live in the United States (live in Wyoming? No deer ticks for you!), full-color life-sized images of the beasts in all their life stages, tips to prevent tick bites, a video demonstration of proper tick removal so you don’t wind up with mouthparts stuck in your skin, and in case you don’t remove a tick in time, a summary of Lyme disease symptoms with images of its characteristic rash that you can compare to your own skin and a search function for local physicians.

My favorite part of the app is the comparison chart that helps you estimate how long a tick has been attached by the amount of swelling in its abdomen. Since the chance of getting Lyme disease depends on how long the tick has been eating, the scorecard helps you decide whether or not you need to call the doctor.

It's also reassuring to learn that most disease is caused by ticks in their nymph form – because they’re small and hard to see, they get overlooked and remain attached for the 48 hours the Lyme disease bacterium needs to infect a new host. The ticks I’m finding right now are all adults: the males don’t bite, and the females are big enough to find easily so long as I keep up those daily checks. I'll be sure to hold what I find up to the app.

ALDF Lyme Disease iPhone/iPod app $1.99, Apple iTunes Store.

 

Friday Parasite: Comic Pointer

Matthew Inman of The Oatmeal brings you the life cycle of Dicrocoelium dendriticum.  Or, as he calls it (him?), Captain Higgins. Enjoy.

Thanks to David Shaw for the pointer!

Friday Parasite: Sugar Fiend

Fungi need to eat, same as you and me – they can’t make food out of air and sunlight like a plant can. But  they have no mouths, so they can’t bite and swallow their food like your average animal; instead, a fungus grows into its food source, releasing digestive enzymes to break down food and absorbing the results. The kinds of fungus you see on dead logs or making fairy rings in your yard are decomposers – they grow through (and eat) plant and animal tissues that are already dead. But there are plenty of species of fungus that chow down on living plant or animal tissues instead. These parasitic fungi face more of a challenge. Dead plants and animals can’t object to fungal digestion; living ones can and do, releasing a variety of cellular or chemical defenses to fight off an invading fungus. But a host only reacts if it detects the parasite after it moves in. A parasitic fungus has a lot to gain if it can keep its host from finding out that it’s arrived.

An interesting strategy for avoiding detection is found in a plant parasite known by the charming name ‘corn smut,’ described in this month’s issue of PLoS Biology. Corn smut fungus (Ustilago maydis) is a parasite of (unsurprisingly, given the name) corn (Zea mays). It infects living corn stalks, growing long, filament-like cells called hyphae through the tiny spaces between the corn plant’s cells. There’s plenty to eat   in those narrow alleys – sugars move through them as they leave the photosynthetic cells where they’re made and head for other parts of the plant. The predominant form of sugar moving through these spaces is sucrose -- a two-part sugar built of a molecule of glucose and a molecule of fructose. Your average fungus would dump digestive enzymes into the space between the cells, break the sucrose into glucose and fructose, then absorb the simpler molecules. But because there isn’t supposed to be a lot of simple sugar in that extracellular space, the glucose spike becomes a signal that tells the plant something is wrong and triggers defensive systems to attack the fungus.  The corn smut fungus avoids this state of affairs with a unique transport protein that lets it move sucrose directly into its cells.

The protein, called Srt1, sits in corn smut’s cell membranes and shuttles sucrose from the outside of its cells to the inside of its cells. That’s it. But what an effect it has! Srt1 lets corn smut digest sugar inside its body, hiding the glucose signal that would tell its host it has a parasite. Unmolested, corn smut uses the sugar it harvests to grow rapidly, forming hyphae-filled tumors all over the plant (most impressively inside corn kernels, where smut turns the seeds into giant, gray fungal breeding grounds). Mutant forms of corn smut lacking Srt1 can still colonize corn plants, but they don’t form tumors, suggesting that when the plant’s defense systems can identify the fungus they can keep it from growing out of control. Even more interesting, the gene for Srt1 is only expressed when the smut is inside plant tissue: otherwise normal corn smut hyphae didn't bother to build sucrose-transporting proteins when they were growing on agar plates in the lab. The signal that tells the fungus it's in a plant and starts it on its stealthy sugar diet is still unknown. 

Wahl, R., Wippel, K., Goos, S., Kämper, J., & Sauer, N. (2010). A Novel High-Affinity Sucrose Transporter Is Required for Virulence of the Plant Pathogen Ustilago maydis PLoS Biology, 8 (2) DOI: 10.1371/journal.pbio.1000303

Photo: © 2008 MAVI RODRIGUEZ GARCIA

Friday Parasite: Togetherness

A male and female schistosome worm in all their mated-pair glory. The male is the larger individual; the female can be seen peeking out of his gynacophoric canal -- a ridge along his belly that serves as her home.

SEM photograph by Bruce Wetzel and Harry Schaefer, National Cancer Institute. 

 

Friday Parasite: Entwined Under the Sand

At first glance, you might think the rounded mound sticking out of the sand was a mushroom cap. But look closer – those purple spots are flowers. It’s not a fungus, it’s a plant -- and as you might guess from the complete absence of green leaves, it’s a parasitic one.

 Sandfood (Pholisma sonorae) lives in the sandy deserts of southern California and Arizona. It grows in loose sand near its hosts, which are typically small shrubby desert perennials like arrowweed, burro-weed, or dyebush. The relationship between host and parasite isn’t obvious when you walk by, because the two are attached at the roots.

 A sandfood plant taps into its host’s root system and siphons off sugars and proteins to put toward its own growth. And can they ever grow – at the end of a growing season there can be a fleshy stem as long as a grown man hiding underneath that mushroomy flower. A full-grown sandfood can actually weigh more than its host plant, though a lot of that mass is water. P. sonorae doesn’t need a host for that part – it absorbs water through the modified scaly leaves covering its stem. And it’s possible that sandfood’s ability to store up fluid lets its hosts turn the tables on it during long droughts: sucking water out of the parasite to let them keep photosynthesizing in conditions that make other plants shut down their food factories. If that turns out to be true, it may be fair to ask which plant is actually the parasite. And the answer may be as changeable as the weather. 

Photo © 2008 Michael L. Charters, used with permission 

Source: Arizona Game and Fish Department Heritage Data Management

Friday Parasite: Sharing Dangers With Your Host

Becoming a parasite means that, one way or another, you’ve thrown in your lot with your host. Their food may become your food, their droppings (or flesh) the medium that moves your young to a new home, their habits a template for you to exploit. And an interesting paper published in the Journal of Experimental Biology last year shows that sometimes their dangers become yours, too.

 Ormia ochracea is a small fly that’s a parasitoid of crickets – adults may fly around in the environment, but their larvae have to grow up inside a field cricket. This means that a female Ormia needs to find a living cricket to feed her young. Fortunately for her, crickets advertise.

 Female Ormia exclusively target male crickets as host material. They find them by listening for their song: in fact, they’ve evolved specialized “ears” on their first pair of legs that are sensitive to the  high-pitched cricket song most flies can’t hear. So on a summer night, as a male cricket sings to attract a mate, gravid female Ormia are also flying toward his song. When a fly gets close, she lands, walks until she finds the singing cricket, then deposits larvae on his back and on the ground around him. The maggots burrow into their new host and spend the next 10 days chowing down on the cricket’s interior. When they emerge to pupate, the cricket dies.

Obviously, male crickets are singing to attract female crickets, not female parasitoid flies. But both the female crickets and the female Ormia fly toward the male’s song, and they can both wind up as the victim of the same voracious nocturnal predator: bats. Some bats also use cricket song – in this case to locate their prey. Above one calling male cricket you might find a female cricket flying toward a mate, a female Ormia flying toward a new host, and a bat flying toward a meal. Bats aren’t picky – one plump insect is as good as another – so all the bugs in this scenario are at risk. Tying their life cycle to the crickets, the flies also became vulnerable to their predators.

Flying crickets are known to perform evasive maneuvers when they’re hit by a bat’s sonar pulse, and Merri Rosen and her colleagues at Cornell University wanted to see whether Ormia had evolved a similar behavior. Bear in mind that most flies targeted by a bat would be toast – their ears aren’t sensitive to anything as high-pitched as bat sonar, so they’d never hear the bat coming. But Ormia has those specialized ears for tuning in cricket song: can they also use them to detect bats? And if the flies hear a bat, what do they do about it?

To find out, the researchers tested how the flies behaved when they heard high-pitched sounds, both while they were walking and in flight. They found that when the flies were on the ground, they walked toward any high-pitched sound. The flies clearly could hear the ultra-high batlike pitches just as well as the cricket chirps; they just didn’t seem to distinguish between sounds that meant ‘food for babies’ versus sounds that meant “DANGER!” All that changed when the flies took wing. In the air, the flies were more discriminating -- steering toward the lower-pitched cricketlike sounds, and away from pitches more like bat sonar. It’s a pretty bit of convergent evolution in which fly and cricket, linked by parasitism, exposed to the same danger, evolve similar plans for making a getaway.

Rosen, M., Levin, E., & Hoy, R. (2009). The cost of assuming the life history of a host: acoustic startle in the parasitoid fly Ormia ochracea Journal of Experimental Biology, 212 (24), 4056-4064 DOI: 10.1242/jeb.033183

PHOTO: Ron Hoy's lab: Cornell University

Friday Parasite: Changeling Factory

What’s wrong with this picture? The chicks should all look the same, but one nestling is big for its age, and has a much bigger and brighter mouth than its nestmates. That’s because it’s not really a baby Eastern Phoebe – it’s a chick from one of North America’s most widespread parasitic birds, the Brown-headed Cowbird (Molothrus ater).

Cowbirds are brood parasites: they don’t sneak under other birds’ feathers to grab a meal (they actually eat insects and seeds), but they leave their eggs in other birds’ nests. Each spring, when other birds are finding mates and building nests, female cowbirds are hiding in the shadows: spying on nesting birds and until they see a chance to rush in and lay an egg in their brood. Cowbirds are veritable egg factories: each female can lay more than 3 dozen eggs in a summer, spread one at a time into nests from about 140 different host species. And although a few hosts can recognize cowbird eggs and either eject them, cover them over, or abandon the nest completely to start over, most host species wind up raising a cowbird changeling along with their own chicks.

Interestingly, although a cowbird chick hatches first and grows faster than its nestmates, it doesn’t typically take the European cuckoo route of tossing its foster siblings over the side. Instead, it helps its nestmates make a fuss when the adults come by the nest – and the extra noise convinces the parents to go out and forage more than if the cowbird chick was home alone. But when they bring back the goodies, the cowbird’s extra big mouth and aggressive begging makes sure that the parents feed it the most – over half the feedings instead of a more equitable one-third, according to a 2004 study by Mark Hauber (now at Hunter College) and his colleagues at the University of Cambridge. This means that a cowbird chick with nestmates actually gets more food (and grows faster) than when it’s alone in a nest. And once it’s grown and fledged, it’ll fly off to take its own turn making changelings.

Photo by Mark Hauber.

Kilner, R. (2004). Brood Parasitic Cowbird Nestlings Use Host Young to Procure Resources Science, 305 (5685), 877-879 DOI: 10.1126/science.1098487

More than just the Friday Parasite...

…it’s the Daily Parasite! 2010 is the International Year of Biodiversity, and Susan Perkins – a parasitologist at the American Museum of Natural History -- is celebrating by highlighting a different species of parasite every day this year on her blog, aptly-named “Parasite of the Day.” She’s taking nominations, so if you have a favorite let her know. So far, it’s 22 down, 343 to go. 

(A tip of the hat to Carl Zimmer for pointing me to this new blog).

Friday Parasite: Under My Skin

This is a mite you don’t want to meet. Unlike dust mites, which merely live off the crud of your household and may never set foot on your skin, Sarcoptes scabiei prefers to get much closer to its food source. It also eats skin, but it doesn’t wait for you to shed it. Instead, the mites burrow into the outer layer of epidermis, making tunnels where they can live and lay their eggs. Where they set up housekeeping, they cause intense itching, and the scratching that ensues makes wounds that bacteria can thrive inside. We call the itching and lesions scabies (and related species of mite infesting our furry friends cause mange).

Image: Jens Mattssen, National Veterinary Institute, Uppsala, Sweden