A Few Bugs in the System
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| Fruit flies are susceptible to Wolbachia parasites, which control their hosts' mating behaviors. (Jack Dykinga/USDA ARS) |
It creeps into your body, takes up residence in your sex organs, and from there controls who you can have children with, making sure you only have daughters—so it can live in them too! Sound like a script for the latest Hollywood horror film? Nope, it’s real and it can happen to you—if you happen to be an insect, crustacean, or arachnid. Let’s meet a few of the tiny parasitic bacteria that specialize in controlling the sex lives and genders of arthropods.
In the weedy, intertidal pools of Europe’s coastline live tiny shrimp called Gammarus deubeni. A protozoan parasite called Nosema granulosis inhabits the eggs of Gammarus. When the eggs hatch, Nosema stays with the baby shrimp—as long as they’re females. But what if Nosema finds itself inside a baby male? Simple: The parasite flips the shrimp’s gender, making it a fertile, functional female. No one knows how, but since Nosema infects only about a fifth of any given Gammarus population, neither creature is in danger of extinction.
Another microbe, Amblyospora californica, zips from one generation to the next through the eggs of North American mosquitoes (Culex salinarius). If Amblyospora ends up in a male larva, it kills its unlucky host. But this is not the end for the parasite, which curls into a hardy spore inside the larva’s floating corpse. When a female copepod, a common freshwater crustacean, dines on the dead male, she unwittingly swallows the Amblyospora too. The parasite then uncurls in the copepod’s ovaries, using them to churn out more spores, which infect only mosquitoes. The copepod lays the spores at the water’s surface, where they are sucked up by feeding mosquito larvae, and the whole drama begins again.
But the undisputed virtuoso of gender-bending host control are protozoans in the genus Wolbachia. Although they were discovered in 1924, scientists have only realized the scope and power of these diminutive Svengalis in the last ten years. One of their most intriguing characteristics is that they control their hosts’ mating altogether. Only animals infected with the same Wolbachia strain can reproduce with each other; all others, even members of the same species that carry a different Wolbachia strain, are biologically blocked. “It’s just sort of mind-blowing because it works at a much more subversive level than a lot of other parasites,” explains evolutionary biologist Marlene Zuk of the University of California, Riverside. “Wolbachia isn’t just getting in there and giving you a cough—it’s changing your genetic nature.”
It also suggests that Wolbachia are capable of creating new species this way and even directing the path of their hosts’ evolution. Big stuff. So big that last year a consortium of researchers, including scientists from the Smithsonian Tropical Research Institute, The Institute for Genomics Research, and the American Museum of Natural History, got a $5 million grant from the National Science Foundation to find out how Wolbachia perform their biochemical magic.
Today, scientists estimate these microscopic interlopers can be found in the ovaries and testes of 70 percent of all arthropod species (and likely more), making Wolbachia possibly the most common parasitic bacteria on Earth. Only mitochondria, the energy organelles in all animal and plant cells, come anywhere close to that distribution. Intriguingly, biologists say there is growing evidence that billions of years ago, mitochondria may have been bacterial parasites like Wolbachia that stopped being microscopic pirates and instead became part of all eukaryotic cells. One day, researchers say, Wolbachia may evolve into a ubiquitous cell organ too.
But parasites don’t always hold the aces in the mating game. In 1999, Ross Robertson of the Smithsonian Tropical Research Institute in Panama reported a different twist on the venerable host-parasite donnybrook. In the San Blas Islands off Panama, Robertson was studying a parasite called Kudoa ovivora that infects the eggs of the bluehead wrasse (Thalassoma bifasciatum). But to Robertson’s amazement, the protogynous female wrasses thwarted the offending parasite by changing into males. No more eggs, but lots of sperm. Voila! No more Kudoa infestation. Might this be another function of sex change? “The fact that these female wrasses were changing sex earlier than usual could mean they are in control,” says Robertson. “With Wolbachia, the bacteria are in control of things. But in this situation it appears exactly the reverse—the host is in control and changes sex to get rid of the parasite.”
—John Tidwell
ZooGoer 33(2) 2004. Copyright 2004
Friends of the National Zoo.
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