Friday, June 15, 2012

Unexpected allies aid ants at war with “zombifying” parasite

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 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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