The More the Mer­rier

by Elio

Some­thing there is that likes to mull over sym­bioses. In­deed, what is more en­joy­able to contem­plate than the re­la­tion­ship be­tween two or­gan­isms? The an­swer? One that in­volves three, or four, or even more. Not that this is un­usual, just that it's al­ways ex­cit­ing. Here are two ex­am­ples.

Cer­tain in­sects, no­tably leaf-cut­ting ants, live off a fun­gus that they cul­ti­vate in house on mas­ti­cated and trit­u­rated plant ma­te­r­ial they bring into their nest. They have a prob­lem, to wit, un­wanted fungi can also grow on this fer­tile soil. How to make sure that only the de­sired crop, and not weeds, grows?

Leaf cut­ting ants. Source

The an­swer – the same as more re­cently de­vised by hu­man farm­ers – is to use a chem­i­cal weed-killer (but a nat­ural fungi­cide, phew!). Such a com­pound is made by an­other part­ner in the deal, an actin­o­mycete that the ants carry in abun­dance on their back. This part­ner­ship has been fine-tuned over mil­len­nia; the fungi­cide is highly spe­cific for the num­ber one crop pest, the par­a­sitic mi­cro­fun­gus Es­cov­op­sis. Re­cently, yet an­other player was added to the ros­ter, a black yeast that is an an­tag­o­nist to the actin­o­mycete. The yeast thus re­duces the amount of fungi­cide pro­duced, to the detri­ment of the ant colony. Let's see, is that four play­ers, or five?

SEM mi­cro­graph of adult south­ern pine bee­tle. The ar­row in­di­cates the lo­ca­tion of a myc­etangium used to trans­port En­to­mo­corticium spp. Source

Now along comes an­other story from the lab of Cameron Cur­rie, the dis­cov­erer of the leaf cut­ter ant multi-ac­tor troupe. Some bee­tles, for ex­am­ple, the South­ern pine bee­tle, tun­nel through the bark of trees, even­tu­ally girdling and killing them. Their lar­vae sub­sist on a fun­gus, En­to­mo­cor­ti­cium spp., whose pres­ence is en­sured by the par­ent. Adult bee­tles carry this fun­gus in a spe­cial­ized sac called a myc­etangium, ready at hand for in­oc­u­lat­ing into the bark.

Just as in the case of the ants, there is a com­pet­ing fun­gus, Ophios­toma mi­nus, that can out­grow the de­sir­able one. Again, an actin­o­mycete comes to the res­cue by mak­ing an anti-Ophios­toma an­tibi­otic. This an­ti­fun­gal, called My­cangimycin, is a novel 20-car­bon car­boxylic acid with lots of dou­ble bonds and a five-mem­bered en­doper­ox­ide ring.

The struc­ture of my­cangimycin, a seven-con­ju­gated dou­ble bond chain and a five-mem­bered en­doper­ox­ide ring. Source

One would guess that in­sects who have close relation­ships with fungi (and which don't?) are be­ing "mined" by drug-seek­ers for new, and per­haps spe­cific, an­ti­fun­gal an­tibi­otics. It would be nice if ba­sic stud­ies of sym­bio­sis led to such prac­ti­cal dis­cov­er­ies.