Stable nutritional provisioning emerges from interactions between leafcutter ants and their mutualistic fungal cultivar

The optimized nutrient exchange between partnered species can govern the stability of mutualisms; however, the underlying regulatory mechanisms are often poorly understood. In this study, these mechanisms were explored in obligate mutualisms between leafcutter ants, Atta and Acromyrmex , and the fungus, Leucoagaricus gongylophorus , they provision with plant fragments. The fungus concentrates nutrients inside swollen hyphal cells called gongylidia, which are consumed by ants. Compared with the well-studied mechanisms of resilient cultivar production by ants, little is known about whether and how the fungal cultivar regulates stable nutritional yield. Here, we initially isolated the fungus from ant farmers and then showed that gongylidia had variable levels of carbon storage glycogen when cultured in vitro on different carbon sources. Next, we tested whether leafcutter ants regulate their cultivar's nutritional yield when actively farming their crop. Results showed that fungal glycogen remained stable but with a low total glycogen yield when farmed by lab-maintained leafcutter colonies confined to a nutritionally imbalanced forage material. In addition, similar fungal glycogen stability with a low total yield was observed across three leafcutter species with different farming systems when foraging for plant materials containing diverse carbohydrates. Although the cultivar's nutritional yield can vary with provisioned nutrients, ant farmers thus appear to suppress this variation through their farming behaviours. Conducting further experiments is recommended to explore the mechanisms by which symbiotic nutrient exchange can be fine-tuned by the ants in response to the specific nutritional needs of the colony and the mechanisms by which the fungus crop signals its nutritional needs to ant farmers. • In mutualisms with nutritional trade, regulatory mechanisms often remain unknown. • Leafcutter ants farm a fungal crop and depend on its nutritional yield for food. • We find the crop's nutritional yield can vary when isolated from ant farmers. • However, nutritional yield was stable when actively farmed by leafcutter ants. • Ant farmers can regulate cultivar nutritional yield, and thus mutualism stability.