Plant lock and ant key: pairwise coevolution of an exclusion filter in an ant-plant mutualism

Although observations suggest pairwise coevolution in specific ant-plant sy mbioses, coevolutionary processes have rarely been demonstrated. We report on, what is to the authors' knowledged, the strongest evidence yet for reci procal adaptation of morphological characters in a species-specific ant-pla nt mutualism. The plant character is the prostoma, which is a small unligni fied organ at the apex of the domatia in which symbiotic ants excavate all entrance hole. Each myrmecophyte in the genus Leonardoxa has evolved a pros toma with a different shape. By performing precise measurements on the pros tomata of three related myrmecophytes, on their specific associated ants an d oil the entrance holes excavated by symbiotic ants at the prostomata, we showed that correspondence of the plant and ant traits forms a morphologica l and behavioural filter. We have strong evidence for coevolution between t he dimensions and shape of the symbiotic ants and the prostoma in one of th e three ant-Leonardoxa associations.