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Ants farming fungus, fungus 'farming' ants

LAB & LIBRARY: Ants created farming long before humans. An evolutionary biologist takes us a little deeper

In Europe and especially in Africa, we are used to large amounts of herbivore mammals, such as deer, wildebeest and buffaloes, eating the available plant material.

However, when we take a look at South and Central America, the number of large herbivores is incredibly small. Instead, we find ants in long trails carrying pieces of leaves into their colonies, in which they grow a fungus to which they feed these leaves.

In their own peculiar way, these ants and their fungus take over the role of large herbivores and are responsible for a great part of the phosphorus and nitrogen recycling in the neotropics.

During my research I try to understand the role of the fungus in the degradation of the plant material the ants bring into the colony, and how both partners are able to maintain this intriguing mutualism (a symbiosis where both partners have a fitness benefit from working together). To support our research we maintain our own little ‘ant-zoo’ in climate-controlled chambers, in such a way we always have ant colonies with fungus ready to work with.

For my PhD research I combined many techniques ranging from genetic analysis to enzyme activity assays. Aside from the wide range of techniques, we have close connections with the Smithsonian Tropical Research Institute in Panama, where we conduct our field studies, and where we collect our ant colonies.

Fertilisers

You need to understand the basics of this mutualism before you can go deeper into the wondrous world of these colonies.

Ants have been cultivating fungus with leaves for millions of years, before humans were even able to think of agriculture. The fungus, closely related to our common mushroom that you can find in the supermarket, stopped producing mushrooms, but started making groups of swollen hyphal tips filled with sugars and fats for the ants. The ants eat these tips (called gongylidia) and feed the leaves, which they chew to pulp, mixed with their own feces (their excrement) to their fungus.

Generally, the ants are seen as the farmers in these systems because they give leaves to their fungus, and feed on the fungus. Even though, over the last decades, researchers have tried to understand what the exact role of the fungus is in the degradation of the plant material. Aside from that, it would be interesting to know what the exact fertilizing role is of the feces the ants mix with the leaf pulp before they give it to the fungus.

In our laboratory we analyzed the contents of the ‘ant poo’ and found over 30 proteins and the interesting thing is that most of them are enzymes designed to break down plant material. On top of that, we found that most of these enzymes actually have a fungal origin, which means they actually do not come from the ant, even though it is in their feces.

Who is exploiting who?

Now is having these enzymes in the feces not very interesting?

It turns out that many of these plant cell degrading enzymes are active after they have passed the ants’ gut system. This makes the mutualism more interesting, because this means that the fungus farmed by the ants is somehow transferring its enzymes through the ants into the ant feces, to help with the degradation of the plant cells.

Because we know that the ants eat very specific structures of the fungus, the gongylidia, we decided to measure the gene expression of the genes coding for these enzymes in these structures, and compare it to the normal mycelium of the fungus. And what we found was that the enzymes are produced in higher amounts in the gongylidia than they are in the normal mycelium of the fungus.

This, all wrapped up together, gave us a new but striking insight into the biology of this system. A small detail I have to add is that the fungus is a living and ‘moving’ structure, with new parts with few mycelium and fresh plant material on top, degraded plant material with a lot of mycelium in the bottom, and in the middle the fungus grows the gongylidia, the parts the ants eat.

Because the ants are only eating these special structures of the fungus, it is for the fungus to force feed enzymes, which it can only produce if there is enough mycelium, and make the ants transport these to the top where the enzymes are actually needed.

And that’s how the ant farmers, in fact, become like the farmed ‘cattle’ of their fungus.

See the gallery below, and a previous University Post theme on the social life of insects here.

universitypost@adm.ku.dk

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