About 100 million years ago, the bacterium Wolbachia came up with a trick that has made it one of the most successful parasites in the animal kingdom: It evolved the ability to manipulate the sex lives of its hosts.
"When it developed this capability, Wolbachia spread rapidly among the world's populations of insects, mites, spiders and nematodes, producing the greatest pandemic in the history of life," says Seth Bordenstein, assistant professor of biological sciences at Vanderbilt, who is studying the relationship between this parasitic bacteria and Nasonia, a genus of small wasps that prey on various species of flies, including houseflies, blowflies and flesh flies.
Bordenstein is a member of the Nasonia Genome Working Group, a collaboration of scientists who published the complete genomes of three species of Nasonia in the January 15 issue of the journal Science. In the paper the group identifies several genes that the wasps appear to have picked up from the bacteria.
This new genetic information has allowed Bordenstein to identify one of the key tools in the bacteria's bag of tricks. It causes a gene in the wasp's immune system to produce less of the protein responsible for detecting bacterial intruders and issuing the chemical alarm signal that activates the wasp's various defense mechanisms. This hijacking of the immune system allows the bacteria to invade the bodies of its hosts with relative impunity, he proposes.
Exactly how the bacteria alters its hosts' reproductive systems to its advantage remains a matter for future study. But scientists have identified the bacteria's basic strategies. Depending on its host, the bacteria either:
- Kills infected males;
- Feminizes infected males so they develop as females or infertile pseudo-females;
- Induces parthenogenesis: the reproduction of infected females without males;
- Makes the sperm of infected males incompatible with the eggs of uninfected females or females infected with a different Wolbachia strain.
Wolbachia favors female over male offspring because they are present in mature eggs, but not in mature sperm. As a result, only infected females pass the infection on to their offspring.
"This makes them the ultimate feminist weapon," Bordenstein quips.
Although the bacteria's parasitism is limited to arthropods – animals with exoskeletons instead of backbones like insects, spiders and crustaceans – its prevalence means that it has a major impact on the biosphere. According to one study, more than 16 percent of the insect species in South and Central America, Mexico, the Caribbean Islands and southern Florida are infected and as many as 70 percent of all insect species are potential hosts.
Recognition of Wolbachia's capabilities has made it a promising candidate for genetic engineers looking for more effective ways to fight human diseases spread by insects. "Once we understand how Wolbachia works, we should be able to add some genes that allow us to control insects that vector human diseases like malaria and dengue fever," says Bordenstein. "There is already a number of research projects supported by the Gates Foundation and the National Institutes of Health pursuing this idea."
Although the ubiquitous bacteria cannot trick the human immune system, it does have an adverse impact on human health. For example, it infects many species of nematodes, including the filarial nematodes that infect more than 200 million people worldwide, causing debilitating inflammatory diseases, such as river blindness and elephantiasis.
In the last 10 years scientists have realized that it is actually the bacteria, not the nematode, that is responsible for most of the symptoms produced by these illnesses. Although Wolbachia can only survive about three days in the human body, the parasitic nematodes act as a continuing source of the bacteria that cause most of the damage. This surprising insight into the disease pathology has improved the treatment of these illnesses: They are now treated with an antibiotic that kills the bacteria and is less toxic than anti-nematode medications.
Bordenstein's research was supported by a grant from the National Institutes of Health and the genome sequencing was funded by the National Human Genome Research Institute. Additional details about the research conducted in the Bordenstein lab is available at http://bordensteinlab.vanderbilt.edu.
[Note: A multimedia version of this story is available on Exploration, Vanderbilt's online research magazine, at http://www.vanderbilt.edu/exploration/stories/nasonia.html].
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Science