Assembling a mutualism: ant symbionts locate their host plants by detecting volatile chemicals

In most mutualisms, partners disperse independently of each other. For instance, in ant-plant symbioses, plants disperse as seeds, and ants disperse as winged queens. For an ant-plant mutualism to persist, therefore, queens must be able to locate and colonise host plant saplings. It has been suggested that host plants emit volatile chemical cues that attract dispersing queens, but this has never been demonstrated experimentally. We used a Y-tube olfactometry protocol to test this hypothesis in the tropical understorey ant-plant Cordia nodosa Lam. (Boraginaceae), which associates with two genera of ants, Azteca (Dolichoderinae) and Allomerus (Myrmicinae). Both genera show significant attraction to the volatiles of C. nodosa over control understorey plant species that do not associate with ants. These results support the hypothesis that ants are attracted to volatiles emitted by their host plant and suggest a key preadaptation that promoted the evolution of ant-plant symbioses.