– João Araújo on the New York Times. Some are looking for the origin of our species. Others hunt for the origin of birds.
On October 24, a team of researchers reported important new insights into the origins of zombies ̵
1; in this case ants zombated by a fungus.
Here's how it works: Sometimes an ant that is out in the open enters a fungal spore. It clings to the ant's body and invades a fungal cell.
The fungus called Ophiocordyceps feeds on the ant from the inside and multiplies into new cells. But you would not know, because the ant lives on and is looking for food to bring back to the nest. Meanwhile, the fungus continues to grow until it accounts for almost half of the ant's body mass.
When Ophiocordyceps no longer feeds from its host, the fungal cells accumulate in the ant's body. They form a mat and push needle-shaped protrusions into the muscle cells of the ant. The fungal cells also send chemical signals to the ant's brain, causing the host to do something strange. The ant leaves her nest and climbs on a nearby plant.
In the tropics, where many species of Ophiocordyceps live, the fungus drives ants up to a leaf above the ground. The ant bites, the jaws lock when dying.
The fungus emits sticky threads that stick the body to the leaf. And now it is ready to take the next step in its life cycle: A giant stalk bursts from the ant's head, showering spores on the ant paths below. "The ants walk across a minefield," said David Hughes, an expert on Ophiocordyceps at Pennsylvania State University.
Naturalists published their first reports on Ophiocordyceps more than a century ago. But only in recent years have researchers investigated how these fungi zombify ants.
It turns out that this is an extremely complicated process in which researchers leave many questions unanswered. Scientists do not even know which chemical gets into the host's brain and it leaves the nest and ascends.
"We still have not found the smoking weapon," Hughes said. In 2010, Hughes and his colleagues identified a 48-million-year-old fossil of a zombie ant with a death grip on a leaf. The fossil has shown that zombies have been around for a long time. But there was no indication of how the fungus developed from its ordinary ancestors.
"You think," Where the hell did that come from? "Said Hughes.
He suspected that an answer lurked in the diversity of the living mushrooms. However, before the age of DNA sequencing, researchers had difficulty classifying ophiocordyceps.
In 2013, one of the Hughes PhD students, João Araújo, began sequencing the DNA of mushrooms in scientific collections. He also undertook his own expeditions and turned pages to find zombified ants. If it was a species he had not seen before, Araújo photographed it and removed the tiny bodies to bring home various ant species or infest other insects.
These species all belong to a much larger group of mushrooms. Many of their relatives feed on dead plants, while some infect insects – mostly a group called Hemipteren, which includes aphids and cicadas.
Araújo, now a Research Fellow at Ryukyu University in Japan, analyzed the DNA of more than 600 of these related species. When comparing the genetic sequences, he was able to draw a mushroom lineage.
The tree showed that all Ophiocordyceps species are descended from a common ancestor. But this ancestor did not infect a Hemipteran. The scientists concluded instead that initially the beetle larvae were infected.
The beetles affected by the fungi live in rotting trunks. When the beetle eggs hatch, the larvae crawl alone in the trunk and chew on wood.
Charissa de Bekker, an integrative biologist at the University of Central Florida who was not involved in the new study, said it was intriguing that Ophiocordyceps' ancestors choose such a host.
"It's unlike Ants are mainly loners with a very different life story. "
When a beetle larva comes into contact with a spore, it invades a body in the insects and feeds on its muscles.
The fungus grows its stem and spreads spores around the dead body, and other larvae crawling in the tree trunk are then infected.
Araújo and Hughes assume that millions of years ago the fungi were sometimes picked up by ants that also lived in tree trunks new ant hosts had the mushroom already the ability to feed on muscles, stalks to breed and spread.
But also Ants presented the parasites with a great challenge. Unlike beetles, ants live in crowded nests. Diseases can wipe out an entire colony, causing the ants to ruthlessly attack any individuals with signs of disease. "They throw them out of the nest or kill them and tear them up," Araújo said.
As a result, Ophiocordyceps was unable to spread the way beetles did by simply killing its host and sending out spores.
Natural selection must have favored fungi that can keep ant hosts healthy as they are parasitized. When the mushrooms were ready to leave the ant's body, they had to get him alive from the nest. It became necessary to zombify the ant and do some things she would not normally do.
By climbing onto a nearby plant, the dying ants were also able to infect new ants. Ophiocordyceps "had to develop a way to leave the host, but not so far, as he still had to shoot spores and infect new hosts," Araújo said.
The transition from mushrooms to ants triggered an evolutionary explosion. After Ophiocordyceps had developed to live in an ant species, it began to jump to new species.
De Bekker said that knowing the development of Ophiocordyceps could help scientists figure out which genes use different species to turn ants into zombies.
"The behavior is very complex, so it will not be a gene responsible for it," she said. "This study helps us to see what comparison we should make."
The new study also shows that the fungus has jumped into the ants in the tropics. Over the course of millions of years, it then spread to the Poles. When the mushroom moved to a new home, he faced new challenges.
Zombie ants could no longer bite on leaves as they fell off in the fall before mushrooms spread to new hosts. Instead, infected ants began to hug twigs.
Hughes predicts that the first zombified ants lived in small colonies in rotting wood and the infected ants barely came out of their nests before death.
It is even possible that some mushrooms belonged to this early date ancestry line still manipulate ants today. Hughes and Araújo suspect that hundreds of other species of Ophiocordyceps still need to be discovered.
"Every time I visit the same reserve I find new species," Araújo said. "I think the description of new species will be an infinite task for generations."