The fungus now pushes long tubes through the back of its dead host. Each tube is a cannon, which shoots out fungal spores at up to 21 miles per hour. The spores rain down on other passing flies, and new cycles of death and puppeteering begin.
The natural world is full of such mind-controlling parasites that subvert the behavior of hosts. The rabies virus makes its hosts more aggressive, the cordyceps fungus turns ants into zombies, and the Toxoplasma parasite sends rats scurrying toward the scent of cats. Even microbes like Entomophthora, which lack brains and bodies of their own, can commandeer the nervous systems of more complex animals.
Eisen thinks there are countless other examples that we have yet to discover. “Microbes are the most innovative part of life,” he says. “Logic tells us that any time there are interactions between microbes and animals, which is all the time, some microbes will have figured out ways of manipulating the behavior of their host to their advantage.”
Fascinating though these puppeteers are, they’ve traditionally been hard to study because they and their hosts are hard to rear in laboratories. For that reason, Eisen has spent years searching for parasitic microbes that manipulate the fruit fly Drosophila—a mainstay of lab research. If successful, he could use all the slickest tools of neuroscience and genetics to work out exactly how the parasite works its mojo.
But most leads fizzled out. Eisen’s team tried to see if the yeast the flies eat could influence their actions. Nope. They checked if the microbes in their guts had an effect. Nada. That, ironically, was why Elya started leaving plates of fruit out. She wanted to collect wild Drosophila to check if their gut microbes differed substantially from the lab-bred insects. But when she discovered the Entomophthora-riddled carcasses, it seemed that she had inadvertently found the puppeteer that Eisen had been searching for.
Entomophthora was discovered back in 1855, but scientists had almost always found it in houseflies—a group that diverged from fruit flies back in the Cretaceous period. Elya became the first researcher to successfully get the fungus to infect lab-reared Drosophila, which allowed her and her colleagues to study it in earnest.
They began sequencing the fungus’s RNA—the molecule that’s produced when its genes are switched on. To their surprise, they found that around 10 percent of these sequences matched a virus, which had been discovered just three years ago. A team of British researchers had found its RNA among wild Drosophila, which they had collected from the English village of Twyford. “I thought, that can’t be a coincidence,” says Eisen. “We’re working on a fungus that infects Drosophila and here’s a virus from Drosophila.”
At first, Eisen didn’t even believe that the Twyford virus was actually a virus at all. Many viruses can insert their genes into those of their hosts and stay there permanently; around 8 percent of the human genome consists of domesticated viruses like these. Similarly, Eisen suspected Twyford was a former virus that had become part of the fungus’s genome. And the fungus, with those viral genes in tow, had gotten into the flies that the British team captured. Cute story, he thought, and promptly forgot about it.