The ecology of the attine ant–fungus symbiosis

Cooperation is at the core of the major transitions in evolution, including the origins of cells, multicellular individuals, and societies. It also occurs between different species, as in mutualisms. However, explaining the maintenance of cooperation in these systems is an evolutionary problem, for non-cooperators are predicted to exploit such systems and eliminate cooperation. One mechanism proposed to stabilize cooperation in mutualisms is partner choice, in which mutualists control whom to interact with, preferring to associate with cooperative partners and avoid cheaters. We aim to test partner choice in a classic example of mutualism: the attine ant-fungus symbiosis. These ants obligately depend on fungus as their major food source while the fungus receives nourishment from the ants and protection from pathogens. Although propagation of fungal cultivars occurs vertically from parent to offspring nest, horizontal transmission of cultivars does occur occasionally between ant colonies, and why ants choose certain cultivars as replacements is unknown. We present phylogenies of ants and cultivars of Cyphomyrmex longiscapus, C. muelleri, and related species from across their ranges. The data indicate that a single ant species is associated with a diversity of cultivar haplotypes and a given fungal haplotype can be cultivated by (a) a single ant species collected from a variety of geographical locations, (b) more than one ant species from the same geographical location, and/or (c) multiple ant species collected from different localities. Information about “preferred” and “non-preferred” cultivars can be used to construct tests to determine whether preferred cultivars enhance fitness of the ants relative to non-preferred cultivars, as evolutionary theory predicts. Such work will reveal how partner choice operates and evolves within this mutualism.