Ants represent some of the most successful organisms ever to have evolved on Earth. Many ant species form gigantic organized societies, comprised of different social casts, which has ultimately lead to their dominance in terrestrial habitats. Already, here at Forest Floor Narrative, I have described leafcutter ants and the fungal agriculture of the higher attine ants. We are just beginning to scratch the ecological surface of ants, but recent studies are opening brand new frontiers. For millennia, several ant species have paired with plants, while many other ant species have paired with fungi. This new study by Céline Leroy and her team discovered and described an ant-plant-fungal interaction that epitomizes a three-way mutualism. This tripartite relationship signifies that these types of interactions are considerably more common than we previously thought.
Allomerus decemarticulatus and their brood.
Two summers ago, I worked on an ant project in the Southern Appalachia. We wanted to better understand myrmecochory; plant seed dispersal by ants. Plants specialized in myrmecochory produce seeds with structures called elaiosomes on them. These structures entice ants into bringing the seeds back to their nests. Ant nests maintain relatively stable moisture content and temperature which increases seed germination. Some believe the ants acquire energy from the elaiosome. Other unconventional researchers have an inkling that the plants are actually tricking the ant into dispersing the seeds. If plants don’t provide the ants with nutrients, this interaction can be labeled as a plant parasitism. This is but one ant-plant interaction.
Some plants have evolved structures called domatia to house ant mutualists. Here, the ants get a structurally sound place to live, and the plants acquire protection from herbivory. Hungry, browsing animals that begin munching on these myrmecophytic plants are soon ambushed by ants protecting their homes. Some of these myrmecophytes provide their ant symbionts not only with a place to live, but with food too. Plants with extrafloral nectaries release sugars not to lure pollinators, but to enhance its invertebrate defense.
The understory Neotropical plant Hirtella physophora forms a known mutualism with the ant species Allomerus decemarticulatus. H. physophora produces domatia for the ants to nest permanently. Initially, researchers thought the ants were exploiting the plants, using domatia and consuming sugars from extrafloral nectaries without providing any services to the plant. A form of parasitism if you will. Upon closer investigation, researchers found unique trap structures the ants creates. Ants manipulate plant trichomes and utilize the hyphae from a particular species of sooty mold fungus as a scaffold to create these traps. The hollow, porous tubes a filled with ants’ ready to ambush. Unsuspecting insects that land on the trap, quickly become overwhelmed by the voracious invertebrates. It was once thought that these ants protect the plant from other insects, but this recent study uncovers a novel, nutritional function.
Cross-section of domatia showing hyphae (arrows) inside the domatia. Leroy et al. 2012.
As it turns out, the plants receive nutrients via insect waste. The more insects trapped and consumed by the ants, the more nutrients the plant receives. These plants can directly absorb nutrients in the aboveground structures occupied by the ants. Even more interestingly, the fungi don’t just act as a structural component of the ant traps. Their hyphae spreads within the structures and covers plant cells. Like mycorrhizae, these above-ground fungi send nutrients to the plant. This was indicated by the significant reduction of plant Nitrogen in plants treated with fungicide on their aboveground structures. Whether or not the plants send sugars back is still unknown, but the plant gives the fungus a place to inhabit so we can confidently call this a mutualism.
When symbiotic ants were fed more insects, leaf tissue nitrogen increased. Leroy et al. 2012.
This tripartite relationship is fascinating. The plants receive nutrients and protection from the ants, while the ants get a place to live. The ants help disperse and propagate the fungus, while the fungus helps hold the trapping structure together. The fungus too gets a place to live provided by the plants, and in turn, efficiently transfers nutrients from ant waste into the plant. The dynamism of the forest floor is exemplified by this three-way ecological relationship. Neotropical understory plants growing in the light limited understory environments provide an outlet for diverse evolutionary adaptations that ants and fungi took full advantage of. Interactions like this should make us look closer and to dig deeper into ecological inquiry.
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