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Ant Garden in a Tree: Smells Help Explain Rainforest Relationship Between Ants and Plants

In the Amazon rainforest, ants live in trees and plant gardens, and now, research led by a graduate student at North Carolina State University has explained in part how this symbiotic relationship between ants and plants functions.

In a paper published this week in Proceedings of the National Academy of Sciences, Elsa Youngsteadt, a doctoral student in entomology, describes research designed to help understand the relationship between certain rainforest ants and plants.

Youngsteadt is the lead author of the paper. Her co-authors are Dr. Satoshi Nojima, senior researcher in entomology at NC State; Dr. Coby Schal, Blanton J. Whitmire Distinguished Professor of Entomology at NC State; and Christopher Häberlein and Dr. Stefan Schulz, both from the Institut fur Organische Chemie, Technische Universitat Braunschweig in Braunschweig, Germany.

Youngsteadt traveled to Peru in 2004, 2005 and 2006 to study what are known as ant gardens and the ants and plants that live in them.

The ant gardens Youngsteadt studied are made by an ant known as Camponotus femoratus. C. femoratus ants are found throughout the Amazon, Youngsteadt says, and these relatives of the carpenter ant are known for the nest gardens they build in trees. In these gardens, which Youngsteadt says range from the size of a fist to as much as 3 feet across, grows a plant called Peperomia macrostachya.

C. femoratus ants are gardeners, Youngsteadt explains. They collect P. macrostachya seeds and carry them to their nests, where the seeds grow and form ant gardens.

That the gardening ants carry seeds, Youngsteadt says, is not unusual. A number of different types of ants around the world practice what is known as myrmecochory, collecting and dispersing seeds. Yet other ants eat a part of the seed, and have no lasting association with the mature plants. The ant gets nourishment from this arrangement, while the plant gets its seeds dispersed.

The symbiotic relationship between C. femoratus ants and P. macrostachya is unusual, Youngsteadt says, in that the ants do not derive nourishment from the seeds. Rather, what both ant and plant get is a home. As the plants grow, Youngsteadt explains, they provide structure for the ant nest. Youngsteadt adds that P. macrostachya grows well only in ant nests. Seeds left on the forest floor may germinate, but the plants that grow from the seeds quickly die. The plants flourish only in ant gardens.

What Youngsteadt went to the Amazon to find out was: How do ants know which seeds among the millions in a tropical rainforest to pick up and take back to their nests?

She started with an extract. She soaked P. macrostachya seeds in a solvent to pull the seed chemistry out of the seeds. She put the extract on seeds from other plants, and ants carried the treated seeds, so she knew something in the extract attracted the ants and cued them to carry the seeds.

But, Youngsteadt says, the extract is a complicated mix of more than 100 different chemical components. So she broke down the extract into fractions, which she tested by exposing each to ants. She identified one fraction ants seemed to like the best, and working with Nojima, the senior researcher in entomology at NC State, was able to identify five chemical compounds that attract ants.

Nojima pinpointed the compounds by dissecting ant antenna, then putting the antenna in a saline solution and hooking it to electrodes and an amplifier. When exposed to certain compounds, the antenna responded by registering electrical activity through the electrodes. In this way, Nojima found out which compounds were of potential interest to ants.

Working with Häberlein and Schulz, the German chemists, the research group identified the chemical structures of the five compounds and put them together in a seed-like blend, which they tested to see if ants were attracted by the odor. Using an olfactometer, Youngsteadt exposed ants to the chemical blend and found that it attracted them. She says there are no other examples of which she is aware of ants being attracted to seeds by odor.

And indeed, the ant gardens of the Amazon are aromatic. Youngsteadt describes the odor as “a pleasant, citrus-vanilla-pine smell.”

Yet while Youngsteadt learned much about the interaction of ants and plants in the Amazon, her research did not fully explore the chemicals on P. macrostachya that cause C. femoratus ants to carry seeds.

Youngsteadt found that while the chemical cocktail she mixed attracted ants, the ants would not pick up and carry off seeds soaked in the chemicals. Yet when Youngsteadt put the extract she made from P. macrostachya seeds on seeds from other plants or even other objects such as twigs, ants would pick up the objects and carry them back to their nests.

So, says Youngsteadt, while the chemical compounds she identified certainly attract ants, there’s another set of chemical cues, yet to be discovered, that plays a role in telling an Amazonian ant which seeds it can use to plant and grow an ant garden.

– caldwell –

Note to editors: An abstract of the paper follows.

“Seed Odor Mediates an Obligate Ant-Plant Mutualism in Amazonian Rainforests”

Authors: Elsa Youngsteadt, Satoshi Nojima and Coby Schal, North Carolina State University; and Christopher Haberlein and Stefan Schulz, Institut fur Organische Chemie, Technische Universitat Braunschweig

Published: The week of Jan. 21, 2008, in the online version of Proceedings of the National Academy of Sciences

Abstract: Seed dispersal mutualisms are essential for the survival of diverse plant species and communities worldwide. Among invertebrates, only ants have a major role in seed dispersal, and thousands of plant species produce seeds specialized for ant dispersal in ”diffuse” multispecies interactions. An outstanding but poorly understood ant-seed mutualism occurs in the Amazonian rainforest, where arboreal ants collect seeds of several epiphyte species and cultivate them in nutrient-rich nests, forming abundant and conspicuous hanging gardens known as ant-gardens (AGs). AG ants and plants are dominant members of lowland Amazonian ecosystems, and their interaction is both specific and obligate, but the means by which ants locate, recognize, and accept their mutualist seeds while rejecting other seeds is unknown. Here we address the chemical and behavioral basis of the AG interaction. We show that workers of the AG ant Camponotus femoratus are attracted to odorants emanating from seeds of the AG plant Peperomia macrostachya, and that chemical cues also elicit seed-carrying behavior. We identify five compounds from P. macrostachya seeds that, as a blend, attract C. femoratus workers. This report of attractive odorants from ant-dispersed seeds illustrates the intimacy and complexity of the AG mutualism and begins to illuminate the chemical basis of this important and enigmatic interaction.