Cooperation, conflict, and coevolution in the attine ant-fungus symbiosis

Fungus-growing ants in the tribe Attini represent a classic example of a mutualism. These ants obligately depend on fungus as their major food source, while the fungus receives both vegetative substrate (nourishment) from the ants and protection from pathogens. Here, we try to identify both benefits and costs of the association by using cultivar switch experiments. We assessed the benefits to each mutualistic partner by replacing the native fungus (cultivar) used by the primitive attine ant species Cyphomyrmex muelleri with a novel cultivar, that of the closely related ant species Cyphomyrmex longiscapus. We show that interspecific cultivar switches caused a significant decline in worker number, garden biomass, and the number of reproductives produced by colonies. In contrast, these effects were not seen in intraspecific switches. We also examined possible costs of the mutualistic association. We estimated colony sex ratios for C. longiscapus to determine whether cultivars can bias reproductive allocation toward females; such bias may evolve because only female reproductives can disperse the fungus, and males are therefore of no value to the fungus. However, intraspecific cultivar switches did not significantly affect ant sex ratios. Cultivar switch experiments represent a new tool for studying cooperation, conflict, and coevolution between mutualistic partners in the attine ant-fungus symbiosis.