Tuesday, 1 November 2016

Halloween Special Post: the zombie-making, mind controlling fungus

First of all, a slightly late happy Halloween to you all! I wanted to post something about a slightly spooky creature to celebrate this time of year, but the organism that I have chosen isn't a spider, or a vampire bat, or an equally well-known scary critter, but a fungus known as Cordyceps. This little ascomycete (sac fungus) may be small and seemingly unassuming, but through millions of years of evolution it has mastered the art of mind control. One of the best known species is Ophiocordyceps unilateralis. This parasitic fungus has made one species of ant its unwilling servant, thanks to a very clever trick of evolution, which some suggest may have been around for 48 million years or more.

The story begins with a Camponotus leonardi ant or one of its close relatives. It will be wandering around, minding its own business and working hard to keep its colony alive and well, when it will become infected - the spores of O. unilateralis will become attached to its body. They will stick to the hard carapace of the insect, but bit by bit, using enzymes and pressure, they will break through the exoskeleton and make it into the ant's body. This is when things start going really wrong for the insect. The fungus starts releasing compounds into the ant's body that affect its behaviour. The first signs are sudden convulsions, causing the ant to fall to the ground, only to recover, seeminly non the worse for ware. But underneath its exoskeleton, the fungus has the ant under its control. Suddenly the ant will begin climbing up nearby vegetation, at the command of the fungus, until it comes to a leaf, where it will bite down hard with its mandibles. The fungus will begin to work away at the insect's jaw muscles, destroying them so that the mandibles cannot be moved: the ant is trapped, biting down onto the leaf and unable to let go and escape. But now the ant has done its bit, it's got the fungus off the forest floor, now it is disposable. The fungus kills it, leaving it hanging there even in death, and the corpse will become a fortress, a sort of safe house from which the O. unilateralis will eventually be able to reproduce. The fungus will spread long, branching filaments, known as hyphae, thoughout the ant's body, fortifying the outer shell of the insect, making it safer. Mycelia - thread like structures - are sent out from the insect body to attach to the leaf, to secure it in place even further. Antimicrobials are released, protecting the corpse from microbes which would seek to break down the insect's body, which would ruin the fungus's plans completely.
Once it is ready, the fruiting body of the fungus will break out of the ant's head, a single stalk bursts out of the exoskeleton and gradually grows longer, until it finally completes its life cycle by releasing its spores. Now the reason for controlling the ant's behaviour becomes clear: by leading its host to a high up leaf, then from this vantage point the fungus can release its spores, which will have more chance of spreading than if they were released on the ground. This way they can drift down onto foraging ants, infecting them too, and so the cycle continues.

 2 dead ants infected with the fungus. They are biting onto the leaf and have the fruiting stage of O. unilateralis emerging from their body to spread spores.

The true story of the ant and the parasitic fungus should be thought of as an incredible example of how a little bit of evolution can achieve practically anything. So hats off to the Cordyceps fungus, for reminding us not only that whatever us humans can think up, nature's probably already done it, but also that we should never underestimate the small things in nature.

Happy Halloween, and until next time, keep on the wild side!