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.

Leaf cutting ants. Source

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?

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 mycetan-
gium 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, e.g., 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.

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

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.

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.

Comments

[Ford]

Another interesting paper from the Currie lab. But there's still a big mystery about the fungicide-producing actinomycetes. Mutants that knock out fungicide production must occur all the time. Producing fungicide has a metabolic cost. So why don't nonproducing mutants outcompete fungicide producers on each individual ant? Sure, in the long run, spread of this mutant could hurt all the actinomycetes, as the ant colony collapses, but natural selection has no foresight. See link for a possible answer.

[Mark O. Martin]

It's like a Chinese Box: layers upon layers upon layers. And at the base? Microbial supremacy! And our microbial friends have much to teach us.

Not to wax Biblical, let alone Talmudic (too many years in Sunday School as a child, perhaps), but this comes to mind, from Job 12: 7-8:

"But ask now the beasts, and they shall teach thee; and the fowls of the air, and they shall tell thee:
Or speak to the earth, and it shall teach thee: and the fishes of the sea shall declare unto thee."

How much more eloquent are the myriad and ancient microbes all around (and upon and within) us! Evolution has tuned them to such intricate and interrelated perfection! Symbiotic associations like you describe may provide us with new medicines and approaches to therapies---as well as their intrinsically interesting and informative natures.

Wonderful post, Elio. I hope to teach a seminar course in microbial symbioses next year; we'll see if it is approved. Yet more grist for the mill.

I was fortunate enough to visit Professor Currie at Madison a few years ago. I will not soon forget his showing me a room filled with plastic tanks with various species of ants scurrying around leaf litter. The very soft spoken and quite nice Professor Currie reached into one tank, and pulled out a huge queen to show me...while defending ants swarmed up his arm, visibly biting him. Without reacting, he continuing telling me with great enthusiasm about the queen in his one hand, while gently pulling off the defenders and tossing them back into the tank with his other hand.

I paid very, very close attention to where those ants went, and to Professor Currie's animated description of his research!

Again, what a lovely post to see after lecture today!