Tales From Planet Fun­gus

by Il­ham Naili

It was two years ago that I of­fered to my sis­ter for her birth­day the DVD set of Planet Earth (won­derful tales by David At­ten­bor­ough that I rec­om­mend to every­body), and I started to watch some episodes with her. A par­tic­u­lar episode was mem­o­rable for both of us, as we watched an ant be­ing lit­er­ally pos­sessed by an in­fect­ing fun­gus. It was such a strange no­tion, that this ant could be ma­nip­u­lated to leave its colony, climb up­wards on a stem, clamp its mandibles to se­curely hold the stem…and die there! The vi­sion of this fun­gus slowly grow­ing out of the ant's head was by far the strangest and most amaz­ing thing that I got to see on tele­vi­sion. My lit­tle sis­ter, who never got to close her mouth dur­ing this whole time, fi­nally looked at me with her big eyes and said, "This is SO cool!! How does it work?? Have you ever stud­ied this in one of your classes?"

(un­for­tu­natley, the video is no longer avail­able in 2014)
From David Attenborough's BBC Planet Earth. Pro­gramme 3: Jun­gles. 2006

"No"…but it is not true any­more.

Here I was, in our last In­te­gra­tive Mi­cro­bi­ol­ogy class, lis­ten­ing to our fun­gus mas­ter Elio as he started talk­ing about fungi and their abil­ity to ma­nip­u­late their hosts to en­sure their dissemi­na­tion. It still sounded like the cra­zi­est thing I have heard in mi­cro­bi­ol­ogy, un­til I made the connect­ion with the At­ten­bor­ough episode! So I de­cided to talk about this; many peo­ple in this class might do the same, but I want to take it more as an anec­dote.

Ant with a fruit­ing body of a Cordy­ceps-uni­lat­er­alis

The fungi re­spon­si­ble for this deadly be­hav­ior are mem­bers of the genus Cordy­ceps, which con­tains more than 400 dif­fer­ent en­to­mopath­o­genic fungi. Many of them have be­come spe­cial­ized and in­fect only one par­tic­u­lar in­sect, but the list in­cludes grasshop­pers, wee­vils, and many oth­ers. In the ex­am­ple of the ant, many of them can be in­fected by a Cordy­ceps. The car­pen­ter ant or the bul­let ant can be the un­for­tu­nate re­cip­i­ent of spores of Cordy­ceps uni­lat­er­alis. The spores at­tach to the exoske­leton of an ant and ger­mi­nate, a nec­es­sary step be­fore en­ter­ing the body of the ant. The ex­oskele­ton of the ant con­tains holes called spir­a­cles that al­low the ant to breathe; these spir­a­cles are the port of en­try of Cordy­ceps, which ar­rives in the tra­cheae of the ant. The fun­gus starts grow­ing in­side the ant's body cav­ity and forms mycelia while feed­ing on the soft tis­sues of the host; how­ever, it does not at­tack the vi­tal or­gans of the ant, thus it keeps it alive.

When the time to sporu­late has come, Cordy­ceps starts in­vad­ing the brain of the ant, which re­sults in a se­vere al­ter­ation of its be­hav­ior. The ant leaves its colony, and oddly enough, climbs up a stem. Even­tu­ally, it at­taches it­self to the stem by us­ing its mandibules; there, it slowly dies as the fun­gus feeds on its brain. Soon, from the gaps made by the joints of the ex­oskele­ton, the fun­gal fruit­ing bod­ies con­tain­ing the spores start to make their way out of the ant's head, and grow for up to three weeks. As the fi­nal step, the fruit­ing bod­ies re­lease spore cap­sules which, on their way down to the ground, ex­plode to un­leash all the spores to be dis­sem­i­nated. One can then ap­pre­ci­ate the strat­egy of the fun­gus; by ma­nip­u­lat­ing the ant's mind and forc­ing it to go to a higher al­ti­tude, the fun­gus en­sures dis­sem­i­na­tion over a large area, where it can in­fect again a mul­ti­tude of ants in the ground. One fun­gus is able to in­fect and dec­i­mate an en­tire colony through this strat­egy!

I tried to find in the lit­er­a­ture an ar­ti­cle that ex­plained how the fun­gus was able to ma­nip­u­late its host; as the be­hav­ioral change seems ex­tremely well reg­u­lated. Un­for­tu­nately, as Elio said, no­body seems to know much at this point. Also, in an ant colony, the work­ers have de­vel­oped a way to "de­tect" in­fected in­di­vid­u­als and then re­move them quickly from the colony. How have they learned to iden­tify the vic­tims? Is it some­thing they see or some­thing they smell?

So why did I choose this par­tic­u­lar ex­am­ple? Be­cause it is so fas­ci­nat­ing that I would like to share this story with the peo­ple around me, as Elio did with us. Be­cause my sis­ter, who is in Po­lit­i­cal Sci­ences, can still re­mem­ber how oddly beau­ti­ful these pic­tures were. And fi­nally, be­cause I think it only re­veals the many ques­tions that de­serve to be stud­ied and an­swered. Surely, if the mech­a­nism of this par­a­sitic re­la­tion­ship is en­tirely de­ci­phered, it will end up on this blog. I will keep look­ing there.

Il­ham is a mi­cro­bi­ol­ogy grad­u­ate stu­dent in the Joint Doc­toral Pro­gram of San Diego State Uni­ver­sity and Uni­ver­sity of Cal­i­for­nia at San Diego. This was her an­swer to an exam ques­tion: Dis­cuss a phe­nom­e­non char­ac­ter­is­tic of all or just some fungi that you have found par­tic­u­larly ex­cit­ing.