Wednesday, November 26, 2008

Better Know an Insect: Fungus Farmers

Happy Thanksgiving!

A few weeks ago, I read some interesting papers about mutualisms in ecological networks.

Here is a thought-provoking opening sentence from one of them:
About 40 – 60 million years before the advent of human agriculture, three insect lineages, termites, ants, and beetles, independently evolved the ability to grow fungi for food (Mueller & Gerardo, 2002).

As you digest your delicious Thanksgiving meal, think of all the things we farm, and the complex relationship we have with agriculture, with farmers, and with the organisms we farm. Then contemplate that insects did it all way before us with their tiny little nervous systems.

Leafcutter ants are one of the lineages that have developed the ability to farm. You might have seen leafcutters in a museum -- they're popular exhibits because they will build their entire colonies readily inside an enclosure and only require a steady supply of leaves. They are the champion farmers of the insects; in some areas, they are considered major agricultural pests because they will strip all vegetation near their colonies to feed their fungus colonies.

The life cycle of leafcutter ants is something like this: when a new queen hatches, she takes a small amount of her home colony fungus along with her on a specialized structure on her thorax when she makes her nuptial flight. After mating, she founds a new colony with the fungus she carried with her. The fungus grows on decomposing vegetation -- leaves -- which the queen initially supplies. After her first offspring have developed, they take over the role of leaf-gathering so that the queen can focus entirely on laying eggs.

Humans go to great lengths to maintain their crops. We spray insecticides, fungicides, and other chemicals to keep our crops disease-free. Ants have developed a system to do this as well -- on each worker is a small patch containing beneficial bacterial colonies. These bacteria secrete antibiotics that help protect the fungal crop from disease. The ants can also adjust the kind of vegetation brought into the colony, and will not bring leaves that have previously been rejected by or toxic to the fungal crop.

Two amazing, related facts about leafcutter ants: 1) they are native to the Americas (especially Central and South America) and are not found on other continents; therefore, when you see a line of leafcutter ants in The Lion King (sorry, I'm having a hard time finding a screenshot) that is an error.

2) While most leafcutters make their homes in the tropics, one species actually lives as far north as southern New Jersey.

There's loads more to say about leafcutter ants, but I mainly wanted to talk about farming in this post, since it sort of relates to Thanksgiving. Perhaps I'll write about them again soon. Eusocial insects are my favorite insects to talk about since they do so many amazing things. Look for more on ants in the future!

Mueller, Ulrich G. and Nicole Gerardo. Proc Natl Acad Sci U S A. 2002 November 26; 99(24): 15247–15249.

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