Fungal parasite grows by causing ants to die in the right spot

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Washington, August 12 (ANI): Harvard University researchers found that a fungal parasite causes ants into dying in just the right spot-one that is ideal for the fungus to grow and reproduce.

Led by David P. Hughes, the study shows just how precisely the fungus manipulates the behaviour of its hapless hosts.

Upon being infected by a fungus known as Ophiocordyceps unilateralis, a a carpenter ant remains alive for a short time.

However, the fungus compels the ant to climb from its nest high in the forest canopy down into small plants and saplings in the understory vegetation.

The ant then climbs out onto the underside of a low-hanging leaf where it clamps down with its mandibles just before it dies. There it remains, stuck fast for weeks.

The continues to grow inside the dead ant's body, and, after a few days, a stroma-the fungus's fruiting body-sprouts from the back of the ant's head.

After a week or two, the stroma starts raining down spores to the forest floor below, each having the potential to infect another unfortunate passer-by.

While this parasite's ghastly ability to turn unsuspecting ants into zombies has been known for over one hundred years, Hughes and his colleagues claim that they have chronicled the amazingly precise control the fungus has over its victim.

At a field site in a Thai forest, the researchers found that the infected carpenter ants were almost invariably found clamped onto the undersides of leaves that were 25 centimetres (about 10 inches) from the ground below.

They also observed that most of the dead ants were found on leaves sprouting from the northwest side of the plant.

According to them, temperature, humidity and sunlight in those spots were apparently optimal for the fungus to grow and reproduce.

When the researchers placed leaves with infected ants at higher locations, or on the forest floor, the parasite failed to develop properly.

"The fungus accurately manipulates the infected ants into dying where the parasite prefers to be, by making the ants travel a long way during the last hours of their lives," Hughes said.

But getting the ant to die in the right spot is only half the battle, as the researchers found when they dissected a few victims.

"The fungus has evolved a suite of novel strategies to retain possession of its precious resource," said Hughes.

After spreading within a dead ant's body, the fungus converts the insect's innards into sugars, which are used to help it grow. It, however, leaves the muscles controlling the mandibles intact to ensure the ant keeps its death grip on the leaf.

The fungus also preserves the ant's outer shell, growing into cracks and crevices to reinforce weak spots.

In doing this, the fungus fashions a protective coating that keeps microbes and other fungi out. At that point, it can safely get down to the business of claiming new victims.

Carpenter ants apparently have few defences against the fungus, the most important being to stay as far away from victims as possible.

Hughes say that that may be part of the reason why these ants make their nests in the forest canopy, high above fungal breeding zones.

He says that carpenter ants also seem to avoid blazing their foraging trails under infected areas, and that this may also be an adaptive strategy to avoid infection.

The researcher, however, admits that more research is needed to confirm it.

The mechanisms and cues the fungus uses to control an ant's behaviour remain unknown.

"That is another research area we are actively pursuing right now," Hughes says.

A research article describing the study has been published in The American Naturalist. (ANI)

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