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Last Update: 2012-07-03   Source: Research Headlines
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How ants fight zombie-ant fungus

An international team of researchers has discovered what gives ants the means to fight infestations by the zombie-ant fungus: a hyperparasitic fungus. The zombie-ant fungus attacks the ant's brain and forces it to die at a mass grave near the ant colony. There, the fungus spores erupt out of the ant's head. The study, presented in the journal PLoS ONE, was funded in part by the ANT FUNGI EP ('From ecology to mechanisms of the extended phenotype') project, which clinched a Marie Curie Action grant worth more than EUR 214 000 under the EU's Seventh Framework Programme (FP7).

A zombie-ant with the brain-manipulating fungus (Ophiocordyceps unilateralis s.l.) having been castrated by a hyperparasite fungus (white with yellow material) © D. Hughes, PSU
A zombie-ant with the brain-manipulating fungus (Ophiocordyceps unilateralis s.l.) having been castrated by a hyperparasite fungus (white with yellow material)
©  D. Hughes, PSU

'In a case where biology is stranger than fiction, the parasite of the zombie-ant fungus is itself a fungus – a hyperparasitic fungus that specialises in attacking the parasite that turns the ants into zombies,' said Professor David Hughes from the Center for Infectious Disease Dynamics at Penn State University in the United States and senior author of the study. 'The hyperparasitic fungus effectively castrates the zombie-ant fungus so it cannot spread its spores. Because the hyperparasitic fungi prevent the infected zombie-ant fungi from spreading spores, fewer of the ants will become zombies.'

The team developed a model able to show previously unknown details of interactions between the fungus-infected ants and the parasite-infected zombie-ant fungus. Past studies determined that ants fight off microscopic enemies, including fungal spores, by efficiently grooming each other. In this latest work, the researchers modelled the effect of ant behaviour on limiting infection.

'Interestingly, beyond the well known effect of defensive ant behaviour, our new research reveals the added effect of the castrating actions of the hyperparasite fungi, which may result in significantly limiting the spread of the zombie-ant fungus,' said Professor Hughes.

It should be noted that just 6.5% of the spore-producing organs of the zombie-ant fungus were viable. 'Even though there are a lot of dead and infected zombie ants in the neighbourhood, only a few of the spores of the zombie-ant fungus will become mature and able to infect healthy ants,' Professor Hughes pointed out. 'Our research indicates that the danger to the ant colony is much smaller than the high density of zombie-ant cadavers in the graveyard might suggest. This complex interaction between ant colonies, their brain-manipulating parasites, and other fungi capable of lending assistance to the colony underscores the need to study social insects under natural conditions.'

According to Professor Hughes, the team is working on fuelling its efforts and 'remains focused on following the exciting theatre played out on the rainforest floor.'

Experts from the University of Copenhagen in Denmark and the Federal University of Vicosa in Brazil contributed to this study. Funding for this study also came from the Danish National Research Foundation, Brazilian National Council for Research and Penn State University.


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