Ants as pollination vectors of ant-adapted Euphorbia hirta L. plants

A wide range of ant-plant mutualistic associations are known to occur in terrestrial ecosystems. Although many species of plants recruit ants to obtain protection from herbivores, myrmecophily is rare, since only few plant species are known to be adapted for pollination predominantly by ants. The annual herb, Euphorbia hirta, is characterized by the ‘ant-pollination syndrome’ including traits such as unisexual flowers, inconspicuous cyathia clustered in globose cymes, inflorescence-associated tiny nectaries and short, semi-erect stem. In present study, we aimed to examine the pollination ecology of Euphorbia hirta and the role of ants in its pollination. Field and laboratory experiments conducted to understand the association between E. hirta plants and ants revealed that seven ant species (belonging to five genera and three subfamilies) visited E. hirta plants. The visitation patterns of ants revealed that in a single foraging trip the ants visited a number of plants indicating their potential role in cross-pollination. Field-based exclusion experiments revealed production of significantly higher number of fruits in E. hirta plants under open pollination treatment as compared to other treatments. Scanning electron microscopy revealed that the adherence of the pollen grains to the bristles located on ant mouth-parts was facilitated by the three vertical, semi-circular grooves present on the grain surface. The number of fruits produced per plant was higher in ant-included plants as compared to only winged insect-pollinated and only self-pollinated plants suggesting that the plant-visiting ants are successful pollination vectors of E. hirta and while promoting self-pollination also contribute towards cross-pollination services.