Friday, March 4, 2011

Attack of the Zombie Ant!

A couple of years ago a study was published in The American Naturalist about an interesting fungal parasite known as Ophiocordyceps unilateralis. This fungus infects ants in the tribe Camponotini (carpenter ants) but does not kill them outright. Rather, the ant remains alive for a short time but the fungus is in control. The fungus compels the ant to crawl down from its nest in the high forest canopy down to the small plants of the understory. Then the fungus has the ant crawl onto the underside of a leaf, clamp down its mandibles, and then die. There the ant body will stay while the fungus continues to grow inside of its body, producing a hyphae and stroma (fruiting body) that grows right out of the ant's head. The stroma then releases spores on to the forest floor, spores waiting to infect the next unsuspecting ant passerby. You can see where the nickname "zombie ants" and "zombie fungus" came from. Now, this was not a previously unknown species of fungus but rather an unknown effect of the fungus on ants, a previously unknown part of the life cycle. What is truly amazing is the accuracy to which the fungus directed the ant. The ants always clamped on to the underside of a leaf and almost always on a leaf vein. The chosen leaf was about 25 centimeters above the ground, with 94-95% humidity, and between 20-30 Celsius. The fungus directs the ant to a location with the parameters that it needs to survive and reproduce.

Now a new paper in the journal PLoS ONE describes four new species belonging to the O. unilateralis species complex from the Atlantic rainforest in Brazil. The species are named according to their ant host species (specifically Camponotus rufies, C. balzani, C. melanoticus, and C. novograndadensis). Ultimately, this paper is just recognizing and naming new species. However, it helps to draw attention to the south-eastern region (Zona de Mata) of the State of Minas Gerais in Brazil, one of the most heavily degraded biodiversity hotspot on the planet. A total of 92% of this rainforest is gone, and four new species have just been discovered. How many more are there to find and how many have already been lost?

Want more zombie animals? Check out these:
The nematode-ant relationship in Central America
The emerald cockroach wasp (Ampulex compressa)-cockroach relationship in the Polynesian Islands.
The spider (Plesiometa argyra)-wasp (Hymenoepimecis argyraphaga) relationship in Costa Rica.

The list goes on and on; pill bugs and spiny-headed worms (Plagiorhychun cylin-draceus), grasshoppers (Melanoplus sanguinipes) and the protist (Nosema acridophagus), the fluke (Dicrocoelium dendriticum) and the ant, the wasp (Glyptapanteles) and the caterpillar, the distome (Leucochloridium paradoxum) and the snail, the barnacle (Sacculina carcini) and the crab, etc. Wasps, ants, and caterpillars tend to have a lot of parasite-host stuff going on (there's even a whole group of parasitoid wasps), although admittedly not all that much zombism. It is an ever-so-interesting evolutionary arms race!

Read more about zombie animals here:
and here:

The original zombie ant study (online version of the paper contains a video):
Andersen, Sandra B., et al. (2009) The Life of a Dead Ant: The Expression of an Adaptive Extended Phenotype. The American Naturalist: 174(3), 424-433. (DOI: 10.1086/603640)

The new study, and because it is published in PLoS ONE it is free access (yay!):
Evans, Harry C., Simon L. Elliot, and David P. Hughes. (2011) Hidden Diversity Behind the Zombie-Ant Fungus Ophiocordyceps unilateralis: Four New Species Described from Carpenter Ants in Minas Gerais, Brazil. PLoS ONE: 6(3), e17024. (DOI:10.1371/journal.pone.0017024)

Online stories on this paper:
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