The leafcutter ants truly highlight the process of evolution, the complexity of life beneath the forest floor and the benefits of eusociality. For more than 50 million years, these lineages of ants have been collecting and processing plant material to feed their subterraneous fungal farms. Today, these ants are amongst the most organized, social organisms in the animal kingdom. Some species have up to 12 different casts, represented by morphologically distinct individuals that carry out different roles. I’m ecstatic to announce that I’ll be seeing these ants in just a few weeks during my January stay in the cloud forests of Monteverde Costa Rica, but another reason I’m revisiting this topic has to do with the current research I’m working on.
Leafcutter ants tending to their fungal gardens. Photographed by Alex Wild.
In Dr. Robert Warren’s lab at Buffalo State, we are looking into the antimicrobial properties of native and non-native ants. Working with living colonies of ants, researchers in the lab noticed that native ants became infested with fungal parasites at a higher frequency compared to other invasive ants. Our leading hypothesis is that these non-native ants produce a suite of chemical compounds that reduce fungal growth. These novel weapons may help non-native ants outcompete other wood inhabiting and parasitic fungi, ultimately aiding in their success in their newly invaded territory.
The leafcutter ant Atta colombica.
In hopes of finding proper methods, I explored the work of the undisputed king of this quadripartite symbiosis; Dr. Cameron R. Currie. In these gigantic leafcutter colonies, ants, bacteria, beneficial and parasitic fungi have all interacted in a 50 million-year-old crescendo of evolution. By observing these ants, Currie first realized that the specific worker ants tending to the fungal gardens would rub parts of their head, thorax, and abdomen on portions of the fungal garden that were colonized by fungal parasites. He later swabbed these specific ants and cultured bacteria that produced chemicals that knockback these parasites.
Later work showed that these ants actually have specific appendages on their bodies that house these beneficial bacteria. Spaces in their exoskeleton called cuticular crypts give the bacteria a place to stay and divide. This same study identified exocrine glands at the base of these crypts that supply the bacteria with nutrients and carbohydrates. There are several species of filamentous bacteria that engage in these leafcutter mutualisms, but in this specific study, these bacteria were identified to be within the genus Pseudonocardia, a group of bacteria known for its ability to produce antibiotics.
Morphological adaptions arise in species that engage in fungal cultivation. Species with greater dependency on fungal cultivation have more specific physical adaptations. Leafcutter ants in blue are solely dependent on there fungal gardens for their colony’s success. Leafcutters have the most sophisticated physical adaptations, including cuticular crypts and exocrine glands. Currie et al. 2006.
Evolutionarily, it makes sense that bacteria with antimicrobial properties would pair up with these eusocial insects. Over long enough time scales, selection pressures filter out species with little or minimal benefit to the colonies, leaving microbes that provide these insects with imperative ecological services. The physical adaptations of the cuticular crypts and the exocrine glands highlight how ancient this quadripartite symbiosis has been ensuing. Closely related ant species that don’t cultivate fungi do not have these specific appendages, again revealing the temporal scale and success of these incredible organisms. I honestly can’t wait to visit my fungal farming friends come January and to possibly describe another reason why some non-native ants succeed in my own neck of the woods.
[Link 1]