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Unexpected allies aid ants at war with “zombifying” parasite

May 4, 2012
Courtesy of Penn State University
and World Science staff

Trop­i­cal ants plagued by a par­a­site that turns them in­to “zom­bies” have un­ex­pected al­lies in their strug­gle: oth­er par­a­sites, sci­en­tists say.

A group of “zomb­i­fy­ing” par­a­sites known as Ophio­cordy­ceps are fun­gi that hi­jack ants' brains. The fun­gus them al­ters an­t's brain mech­a­nisms so that the in­sect march­es to its death, all in an or­ches­trat­ed pro­cess that fa­cil­i­tates the par­a­site's re­pro­duc­tion.

A dead "zom­bie" ant with the brain-mani­pu­lating fun­gus Ophio­cordy­ceps uni­lat­er­alis s.l. grow­ing out of its head. Its stalk is in turn par­a­si­tized by an­oth­er fun­gus, the whitish-yellow ma­te­ri­al, which sci­en­tists say cas­trates it. (Cred­it: Da­vid Hughes, Penn State U.)


But the fun­gal ty­rant suf­fers op­pres­sion in turn from oth­er fun­gi—which bas­ic­ally cas­trate it chem­ic­ally and dis­rupt its re­pro­duc­tion, ac­cord­ing to the re­search­ers. The counter-par­a­sites keep the first one in check and help pre­vent it from over­run­ning en­tire ant col­o­nies.

It's truly a case where “bi­ol­ogy is strang­er than fic­tion,” said Da­vid Hughes of Penn State Uni­vers­ity, who led a re­search team that pro­duced the find­ings. The re­search was pub­lished May 2 on­line in the sci­en­tif­ic jour­nal PLoS One.

A par­a­site of a par­a­site, called a hyperpar­a­site, “ef­fec­tively cas­trates the zom­bie-ant fun­gus so it can­not spread its spores,” said Hughes. This saves not a few hap­less ants from a hor­ri­fy­ing death. Af­ter com­man­deer­ing their brains, the zomb­i­fy­ing fun­gus makes them march to a mass ant grave near the ants' home, where it drops dead. The fun­gus then promptly sprouts out of the an­t's head­—form­ing a stalk from whence it spreads its fet­id spores to claim more vic­tims.

But the bat­tle—which the re­search­ers stud­ied as it plays out in the At­lantic rain­forests of Bra­zil but which al­so oc­curs in oth­er parts of the world—is­n't so sim­ple. Far from rest­ing easy in its ca­dav­er­ous new home, the zomb­i­fy­ing fun­gus falls prey to its own tor­men­tors. These par­a­sites spread on­to its own stalk and make their home there. “Be­cause the hy­per­par­a­sit­ic fun­gi pre­vents the in­fected zom­bie-ant fun­gus from spread­ing spores, few­er of the ants will be­come zom­bies,” Hughes said.

As part of their re­search, Hughes and col­leagues cre­at­ed a de­tailed mod­el to re­veal de­tails of the in­ter­ac­tions be­tween the fun­gus-in­fected ants and the par­a­site-in­fected zom­bie-ant fun­gus. Sci­en­tists pre­vi­ously had known that ants de­fend their col­o­nies against mi­cro­scop­ic en­e­mies such as fun­gal spores by ef­fi­ciently groom­ing each oth­er. In this stu­dy, the re­search­ers al­so mod­eled the ef­fect of such ant be­hav­ior on lim­it­ing in­fec­tion. 

“Interest­ingly, be­yond the well-known ef­fect of de­fen­sive ant be­hav­ior, 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,” Hughes said.

The sci­en­tists re­port that only about 6.5 per­cent 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 neigh­bor­hood, only a few of the spores of the zom­bie-ant fun­gus will be­come ma­ture and able to in­fect healthy ants,” Hughes said. “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.”


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Tropical ants plagued by a parasite that turns them into “zombies“ have unexpected allies in their struggle: other parasites, scientists say. A group of “zombifying“ parasites known as Ophiocordyceps are fungi that hijack ants' brains. The fungus them alters ant's brain mechanisms so that the insect marches to its death, all in an orchestrated process that facilitates the parasite's reproduction. But the fungal tyrant suffers oppression in turn from other fungi—which basically castrate it chemically and disrupt its reproduction, according to the researchers. The counter-parasites keeps the first one in check and prevents it from overrunning entire ant colonies. It's truly a case where “biology is stranger than fiction,“ said David Hughes of Penn State University, who led a research team that produced the findings. The research was published May 2 online in the scientific journal PLoS One. A parasite of a parasite, called a hyperparasite, “effectively castrates the zombie-ant fungus so it cannot spread its spores,“ said Hughes. This saves not a few hapless ants from a horrifying death. After commandeering their brains, the zombifying fungus makes them march to a mass ant grave near the ants' home, where it drops dead. The fungus then promptly sprouts out of the ant's head—forming a stalk from whence it spreads its fetid spores to claim more victims. But the battle—which the researchers studied as it plays out in the Atlantic rainforests of Brazil but which also occurs in other parts of the world—isn't so simple. Far from resting easy in its cadaverous new home, the zombifying fungus, known as Ophiocordyceps, falls prey to its own tormentors. These parasites spread onto its own stalk and make their home there. “Because the hyperparasitic fungi prevents the infected zombie-ant fungus from spreading spores, fewer of the ants will become zombies,“ Hughes said. As part of their research, Hughes and colleagues created a detailed model to revealed details of the interactions between the fungus-infected ants and the parasite-infected zombie-ant fungus. Scientists previously had known that ants defend their colonies against microscopic enemies such as fungal spores by efficiently grooming each other. In this study, the researchers also modeled the effect of such ant behavior on limiting infection. “Interestingly, beyond the well known effect of defensive ant behavior, 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,“ Hughes said. The scientists report that only about 6.5 percent 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 neighborhood, only a few of the spores of the zombie-ant fungus will become mature and able to infect healthy ants,“ Hughes said. “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.“