There’s an arms race between a disease-causing fungus and its host, and it’s not the one depicted in HBO’s post-apocalyptic series The Last of Us. Scientists have found that with a little care, ants can help their buddies fend off a few fungal invaders—but that these fungi have also found a way to fight back. Such battles could influence the evolution of pathogens found in a wide range of species, including humans.
“The study captures evolution in action,” says Trine Bilde, an evolutionary biologist at Aarhus University who was not involved in the work. “That’s so thick. Ants, like humans, are social. They live in giant colonies, with each individual invested in the survival of the others. Worker ants don’t just sacrifice themselves for the queen, they treat each other for parasites, much like chimpanzees pick up fleas and ticks from their mates.
To find out whether such friendships help insects fight fungi, researchers at the Institute of Science and Technology in Austria turned to Argentine ants (Linepithema humile), a small brown species native to South America. In the wild, these ants live with thousands of other pathogens and are often infected with multiple fungi at once.
The researchers infected insects in the laboratory with six different types of related fungal pathogens. Then they watched how the different fungi evolved, infecting the ants over several generations.
When the insects were alone, one of the six fungal strains usually won out. The winning strain burrowed into the ant, infected and killed it to continue reproducing its spores while the other fungal strains died out. But with friends around, the balance of power shifted. As the insects modified each other, six species of fungi duked it out among themselves, but no clear winner took over, the team reports this month in Nature Ecology & Evolution.
The fungi also seemed to be evolving to become less lethal. But they didn’t give up the fight completely. Further experiments revealed that the pathogens release less of a molecule called ergosterol, which makes them visible to the ants. The less ergosterol the fungi produced, the less the insects treated each other.
This could allow the fungi to hide from the caring ants while developing new ways to defend themselves—a strategy not seen before, says Yoko Ulrich, an evolutionary biologist at the Max Planck Institute for Chemical Ecology, who was not involved in the study. Humans don’t cut each other like ants, he notes, but we engage in other behaviors to prevent disease, such as using hand sanitizer.
What impact might this have on the evolution of the microbial invaders that threaten us? Stay tuned for future science—or maybe the next HBO series.
