Ant-repelling pollinators: Unique pollination strategy of the ant-plant Macaranga (Euphorbiaceae)

摘要

In the tropics, some plants known as ant-plants have tight mutualistic relationships with ants. These plants offer food, such as extrafloral nectar and food-bodies, for symbiotic ants. In turn, the ants protect the plants by excluding herbivores. On flowers, however, the plants often face a conflict when ants exclude not only herbivores but also pollinators. In this study, we investigated how this conflict is addressed in the genus Macaranga (Euphorbiaceae). Macaranga includes approximately 30 ant-plant species that are inhabited by species-specific Crematogaster ants. They are pollinated by thrips Dolichothrips spp., tiny insects that are 2-3 mm in length. The thrips feed on nectaries on bracteoles and breed on the inflorescences of Macaranga. To investigate whether the ants deter pollination, we excluded ants from inflorescences and compared the number of thrips to control inflorescences. The number of thrips did not differ between ant-excluded and control inflorescences. We observed pollinator thrips secreting liquid from their anus and conducted bioassays to determine if chemical substances secreted by pollinator thrips function as ant repellents. Ants were brought into contact with individual thrips or 5 mm Teflon rods on which hexane (control), thrip anal secretions, or decanoic acid (a chemical found in the secretions) were applied. Ants were deterred more often by thrips than from controls, especially when the thrips raised their abdomens. The ants also retreated more often from thrip secretions and decanoic acid than from the controls. These results suggest that the plants avoid pollination deterrence by ants by being pollinated by ant-repelling pollinators. This pollination system might contribute to the maintenance of obligate protective mutualism with ants in this genus.