Science Made Cool

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

Deep Time

A German archaeologist has discovered something truly awe-inspiring. In southeastern Turkey, there's a site called Gobekli Tepe -- it's a collection of standing stones on a barren hilltop. It's not buried or hidden or anything, it's sitting out plain as day. And according to Dr. Kurt Schmidt, the site's been sitting there for about 11,000 years.

That's a long time. That's a really really long time. When the Great Pyramid was built in Egypt, those stones in Turkey were older than the Pyramids are today. When those stones were carved, parts of Scandinavia were still covered by glaciers. Mammoths and Mastodons still walked the Earth. The Sahara was a grassy savannah. According to current theories, agriculture hadn't really been invented yet when humans carved the stones at Gobekli Tepe. In fact, Dr. Schmidt's hypothesis is that sacred sites like Gobekli Tepe may have led to permanent settlements, trade, and farming -- rather than the reverse, which has long been the assumption.

You could drop the entire written history of humanity from about 3,000 B.C. to the present into the gap between the building of Gobekli Tepe and the first writings in the Fertile Crescent. What happened in those long dark millennia? Were there civilizations lost to us? Cruel empires? Wise and noble states? Profound thinkers? We don't know. We think we know all there is to know about our world and our universe, but the humbling fact is that we are still utterly ignorant. We don't even know about most of our own history. It may be forever unknowable.

Inevitably, one thinks about the future. In the year A.D. 13,000, will any of our works survive? The plaques left by the first explorers on the surface of the Moon, perhaps? What will the people of that era think of us, if they bother to think of us at all?

 

Happy 350th, Royal Society!

In 1660, a group of  men devoted to the radical idea of  actually testing their ideas about the way the world worked formed a club of sorts -- meeting once a week, not just to drink heavily, but to try out scientific experiments (and presumably, drink heavily afterwards). The King approved, and within a couple of years their Society became Royal. 

Members kept a running record of their experiments, and were soon publishing reports they received from other natural philosophers. Over the centuries, the journal and its descendants have seen the first appearance of all manner of scientific discoveries, including the first description of microorganisms, a demonstration that lightning is electrical,  equations describing electromagnetism, and evidence for plate tectonics, as well as cautionary tales and the occasional paper about sex. As part of their anniversary celebration, the Royal Society has put up an interactive timeline of major discoveries, and it's a lot of fun to play with. 

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. 

 

Rocket vs. Atmosphere

Few things can show that air is a fluid as well as this movie of the February 11th launch of the Solar Dynamics Observatory. As the Atlas V rocket raced upward, it pushed the  air in front of it aside and threw it into ripples like the ones you see when you throw a pebble into a lake. You can't see the air ripple when NASA launches  into a clear blue sky, but February 11th was cloudy, and the ice crystals high in the air were enough to show off the waves. There's about a minute of rocket climbing before you see the effect -- wait for it.

A tip of the hat to Alex Irvine for pointing out the video. 

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

Professor Pangloss In Space

New Scientist has an editorial up about future directions for NASA now that the Constellation programs have been (for all practical purposes) shut down. The piece is notable for its tone of lunatic optimism. Somehow, freed from the bother of actually being able to launch anyone into space, NASA can develop cool new technologies like an asteroid rendezvous mission or fuel depots in orbit. It all hinges on one tiny little "if" -- so tiny that the author palms it like a magician doing a trick with coins. The key is "IF" Congress decides to keep funding NASA at current rates despite cancelling the entire project to replace the aging Space Shuttle.

Does anyone actually believe that will happen? I sure don't. There aren't any voters in orbit. In a time when Federal tax revenues are in steep decline and Federal expenditures are going up faster than a rocket to Mercury, gutting NASA is a convenient way for legislators to seem fiscally responsible without feeling any political pain.

So here's my predictions on the next decade for NASA:  the agency will put out study after study after study for interplanetary missions they'll never get funded. They'll rent seats on Russian rockets to send people to the International Space Station. And when the next human being steps out on to the surface of the Moon, he won't be carrying an American flag.

Are Wolves Returning To Colorado?

Maybe. Michelle Nijhuis lays out the evidence and possible environmental consequences in an article in the High Country News. Definitely worth a read. 

Seeing Red

Pluto, which isn't a planet, is acting funny. Specifically, the remote dwarf planet is turning red. Marc Buie, a planetary scientist in Boulder, used Hubble Space Telescope images to study Pluto and noticed a distinct reddening of the non-planet's surface some time about the year 2000. Since Pluto wasn't kicked out of the planet club until 2006, it can't be blushing from embarassment, as some have suggested.

It's possible that some phenomenon is releasing reddish methane compounds, though exactly how that could happen on a world as cold as Pluto is unclear. Perhaps the New Horizons probe will reveal more about what's changing Pluto's color. We'll find out in five years.

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