Compartmentalization reduces conflict in multipartner plant-insect symbioses.

Many symbioses involve one host species having several mutualist partners, yet theory predicts that unrelated symbionts lead to destabilizing conflict through competition for host resources. We combined isotope labeling, computed-tomography three-dimensional models, behavioral field experiments, and mathematical models to show that Squamellaria plant hosts reduce conflict among their multiple ant symbiont species by offering nesting sites (domatia) divided into compartments with separate entrances. As long as compartmentalization is maintained, different symbiont species can peacefully coexist, but experimental removal of compartment walls leads to deadly conflicts. Modeling suggests that compartmentalization optimizes nutritional benefits by increasing the time during which domatia harbor large ant colonies. These results reveal a conflict-reduction mechanism that allows hosts to take advantage of unrelated symbionts, which may be widespread in multipartner mutualisms. Editor's summary: Mutualistic relationships between ants and plants have evolved repeatedly. In these relationships, the ants often supply the plants with nutrients, while the plants provide structures suitable for ant nests, generally referred to as "domatia." In some pairs, these relationships are exclusive, but in many others, a single plant species may have relationships with more than one ant species. However, such flexibility, although beneficial to the plant, poses a challenge because ants are often territorial and aggressive. Chomicki et al. studied such a promiscuous species of tropical vine and found that conflict among ant species was minimized or completely eradicated simply by physical separation within a single domatium. —Sacha Vignieri [ABSTRACT FROM AUTHOR]