Pollination ecology of Dracula erythrochaete (Orchidaceae): brood-site imitation or food deception?

Abstract

Pleurothallidinae is the most diverse Neotropical subtribe in Orchidaceae and is almost exclusively pollinated by insects of the order Diptera. Dracula, a genus of 138 species in the Pleurothallidinae, is known to attract Zygothrica (Drosophilidae) flies, common macrofungi visitors, by imitating fungal volatile compounds and lamellae. Interestingly, Dracula orchids do not appear to offer any rewards to their floral visitors. While brood-site imitation of macrofungi has been suggested as their pollination system, the exact behaviour of flies during their extended visits to the orchid flowers has yet to be confirmed. In this study, we document the pollination mechanism of Dracula erythrochaete. We characterize the floral structures involved in the mechanism using anatomical and morphological evidence. Additionally, through in situ observations and camera recordings, we describe the insect behaviour. We show that flowers of D. erythrochaete share the same group of visitors as nearby macrofungi, including different Zygothrica species, seven of which were determined as effective pollinators. Male and female flies were attracted to the flowers and displayed feeding behaviour. Accordingly, proteins were detected in high concentrations on the papillae at the base of the movable lip and in papillary trichomes of the sepals, near the column. The concept of brood-site imitation is debated, as no oviposition events were observed and no eggs were found on the flowers. Therefore, a mixed strategy of congregation/brood-site imitation and food reward is proposed for pollination.