Co-evolution and the superorganism: switching cultivars does not alter the performance of fungus-gardening ant colonies

1. The fungus-gardening ants and their fungi represent a highly co-evolved, vertically transmitted mutualism. Mutualisms such as these are thought to be reciprocal antagonisms, so that the ants and the fungus can be expected to have some conflicting interests. 2. This paper reports the results from a cultivar switch experiment that documented the effects of switching the native cultivar from the basal ‘higher-attine’ Trachymyrmex septentrionalis with a derived cultivar from the leaf-cutting ant, Atta texana . If the cultivars have been modified significantly during the adaptive radiation of this clade, then they should differ in their ability to produce ant and fungal biomass. If the ants can perceive differences in cultivar performance, then their food preference may also change as a result of the switch. Lastly, if conflict is present in this mutualism, then the sex ratio in the switched colonies should be male biased. 3. Our results showed that food preference was not altered by the new cultivar. The A. texana cultivar did not change the performance of T. septentrionalis colonies relative to colonies that were cultivating conspecific fungal cultivars. Foragers preferred insect faeces and oak staminate flowers over fresh leaves or flowers. Most variation in colony performance was attributable to substrate type. Sex ratio of offspring was not affected by fungal cultivar. With respect to fungal growth, the A. texana cultivar appeared to be more of a generalist, while the T. septentrionalis cultivar performed better on substrates preferred by the ants than those unpreferred. 4. The results of this study indicate that cooperation and not conflict has been more important in shaping the evolutionary ecology of this mutualism. Although the cultivars were certainly genetically and physiologically distinct, these differences did not account for variation in the production of ant biomass or ant behaviour. The emerging picture thus indicates that this mutualism should be viewed as a highly integrated superorganism that is more than the sum of its parts.