Some species of parasitic fungi can only manipulate the behavior of the natural targets they co-evolved with. When that fungus encounters the brain of its preferred host ant, it emits a cocktail of zombifying chemicals. While the parasite could still infect other species, its mind-altering chemicals won’t get them to do its dirty work, according to the first extensive study of zombie ants in North America.
Fungus species from the genus Ophiocordyceps -- known as "zombie ant fungi" -- control their hosts by inducing a biting behavior. They've evolved a mechanism that causes the hosts to die while they’re attached by their mandibles to plants, providing a platform for the fungus to grow and eventually shoot spores from in order to infect other ants. "This is one of the most complex examples of parasites controlling animal behavior because it is a microbe controlling an animal -- the one without the brain controls the one with the brain," says David Hughes of Penn State in a news release. Last week, other members of his lab revealed a fungus that makes ants die on the doorstep of the colony.
"Fungi are well known for their ability to secrete chemicals that affect their environment," Penn State’s Charissa de Bekker says. They nourish themselves by secreting compounds to degrade molecules in their environment into smaller ones that can be taken up. "So we wanted to know what chemicals are employed to control so precisely the behavior of ants." Pictured below, a dead ant clinging to a twig in a South Carolina forest.
The team studied the chemical processes associated with metabolism in the newly discovered fungal species from temperate U.S., tentatively called Ophiocordyceps unilateralis sensu lato. They control the behavior of an ant species from the genus Camponotus. When the team infected nontarget hosts from the same genus and from another genus, Formica, they found that the fungus can infect and kill nontarget ants -- but it can’t manipulate their behavior.
To see how the obligate killer “knows” when it’s in the presence of its preferred ant, the researchers removed ant brains, keeping them alive in a special medium. Then they grew the fungus alongside brains from different ant species. "This was 'brain-in-a-jar' science at its best," Hughes says. "It was necessary to reduce the complexity associated with the whole, living ant, and just ask what chemicals the fungus produces when it encounters the ant brain.”
The team found thousands of unique chemicals, and among them were two neuromodulators previously implicated in brain disorders: guanobutyric acid and sphingosine. These were enriched when the fungus was grown in the presence of brains of its target species.
No single compound results in the exquisite control of ant behavior. "Rather, it is a mixture of different chemicals that we assume act in synergy,” de Bekker says. “Whatever the precise blend and tempo of chemical secretion," she adds, "it is impressive that these fungi seem to 'know' when they are beside the brain of their regular host and behave accordingly."
This work was published in BMC Evolutionary Biology this week.
Images: Hughes Lab, Penn State