Functions of heteranthery and enantiostyly for wing pollination by pollen-collecting bees in Dilatris ixioides (Haemodoraceae)

Background and Aims Evolutionary floral modifications that enable plants to exploit particular body parts of animals for pollen transfer are considered a key contributor to the angiosperm radiation. Species of Dilatris (Haemodoraceae) have an unusual combination of floral traits including a centrally-positioned stamen with a large anther flanked by two stamens with smaller anthers (heteranthery), and styles that are deflected either to the right or left on flowers of the same plant (monomorphic enantiostyly). We investigated the pollination functions of these traits in D. ixioides to better understand their evolution. Methods We quantified floral traits including morphology, pollen production by different anthers and spectral reflectance of various organs. We also assessed the identity and pollen loads of flower visitors and their behavioural responses to anther removal, as well as the levels of self-compatibility and pollinator dependence of D. ixioides. Results Left- and right-styled flowers of D. ixiodes alternate consistently along inflorescence branches (pendulum symmetry). They do not produce nectar and were found to be pollinated primarily by female carpenter bees that collect pollen from the large central anther. The central anther is yellow, has spectral properties that are similar to those of pollen, and produces more than twice as many pollen grains as do each of the smaller lateral anthers which are red. Flowers with the central anther experimentally removed were ignored by bees. Pollen is transferred via the bee’s wings as they beat against the deflected lateral anthers and stigma while the bees grasp the central anther. Plants are self-compatible but reliant on pollinator visits for seed production. Conclusions The absence of nectar production and development of a large central pollen-rewarding anther in combination with laterally deflected anthers and styles in D. ixioides are functional components of an effective and novel system of wing pollination by female bees.