Perhaps I should start asking Acacia trees for relationship advice?

Acacia trees and ants have an interesting mutualistic relationship which is frequently used as a textbook example of a mutualism. The trees offer the ants hollow thorns in which to live and food to eat, and the ants protect the tree from herbivores, both insects and large mammals as well as vines which would overcrowd the tree and kill it by hogging it’s sunlight. If you’ve never seen a video of this thing, see if you can’t find one on youtube (I couldn’t). If the tree is disturbed in the slightest, streams of ants come pouring out of the tree and attack anything they can wrap their mandibles around.

Image of Acacia collinsii courtesy of Wikipedia Commons, showing hollow thorns in foreground and yellow food bodies in the background.

However, the thing to realize is that these relationships are always tentative. If there’s a way to get the food without wasting your time (and colony workers, I might add) this becomes a more evolutionarily attractive option. Mutualists will cheat on you if they can.

The objective for the Acacia tree is to keep their mutualists faithful by discouraging them from taking up a less productive lifestyle. Many people have the misconception that mutualisms are these beautiful things…two different organisms sticking together in the wild to accomplish the simple goal of survival. It’s certainly an appealing way to think of this, but it’s a bit inaccurate. As soon as one partner figures out how to get the benefits of the relationship without having to do it’s share of work, that mutualism dissolves into parasitism. It’s really a system of mutual exploitation rather than the system of co-operation that most folks like to think it is.

Turns out, there’s actually a damn interesting way Acacia trees avoid this problem.

There are many different types of sugars. Many plants transport energy from one part to another in the form of sucrose, which we know as table sugar. It’s break-down is catalyzed by an enzyme called invertase, which seperates the fructose and glucose monomers which comprise sucrose. If you lack the enzyme, you can’t digest the sugar. It’s as simple as that…it’s why we can’t live off of cellulose. We don’t have the enzyme to digest it. Same concept applies here.

Acacia trees pay their bodyguards wages of sugar. Most plants contain sucrose in their nectar and sap and this is attractive to ants. Acacia trees excrete nectar to pay their bodyguards, and they grow edible protrusions which are brought to the larvae by the workers. The larvae and the workers appear to live off of two different food sources. However, there’s a clever catch: the nectar contains invertase…the enzyme used to cleave sucrose. The nectar the Acacia trees secrete for the ants doesn’t contain surcrose…it only contains glucose and fructose as a result. The tree is giving the ants partially digested sugar. How sweet, right?

Not so much. This gives the ants an evolutionary incentive to lose their invertase gene, or at least downregulate it so much that there’s negligible invertase activity. Carrying around spare enzymes is expensive and if the tree you’re babysitting can produce them for you, then what’s the point of wasting energy by producing enzymes you don’t need?

We can actually tie this into our own evolution. Over the course of our history, we have lost the ability to produce our own vitamin C. This gene, and most likely the invertase gene in these ants has been destroyed by mutations which deactivate it. There’s no selection against individuals who can’t produce vitamin C because we encounter it all the time in our diet. Similarly, the ants who can’t digest sucrose suffer no ill effects because it’s digested for them. As a bonus, not producing an un-needed enzyme saves energy which can be shunted to other systems.

The end result? The ants can’t digest sucrose anymore, which means the only food source utilized by the ants would be the nectar the Acacia trees produce. If the ants stop providing the service, the Acacia tree dies either from overcrowding or from herbivory and doesn’t produce the nectar anymore. The ants can’t exactly move on to another food source, and thus their ticket to a meal dies along with the tree.

Really cool hypothesis…right? But do the phylogenies of parasitic and mutualistic ants agree?

Below, the researchers traced the evolutionary history. The presence of invertase activity is on the right, and the life-history of the ants is color-coded on the left. Of the central American Acaias and their ant mutualists, it appears that there aren’t any parasites which have evolved from mutualists.

On the left: White are the mutualistic species, grey are generalists species, black are parasitic species. On the right: Black indicates a presence of invertase activity, whereas white indicates the absence of invertase activity.

Stefanie Kautz, H. Thorsten Lumbsch, Philip S. Ward, Martin Heil (2009). HOW TO PREVENT CHEATING: A DIGESTIVE SPECIALIZATION TIES MUTUALISTIC PLANT-ANTS TO THEIR ANT-PLANT PARTNERS Evolution DOI: 10.1111/j.1558-5646.2008.00594.x