Pollination Biology of the Mushroom-Mimicking Orchid Genus Dracula

Abstract

Dracula orchids are hypothesized to rely on mushroom mimicry for pollination. These orchids look and smell like mushrooms and are pollinated by mushroom-associated flies in the family Drosophilidae. Dracula includes over 130 species, representing a significant radiation, yet there has never been a systematic study of their pollination biology. Elucidating the processes and mechanisms of pollination in these flowers will broaden our understanding of mimicry within the Orchidaceae, a family well known for its diverse pollination strategies, as well as add to the growing literature on the evolution and maintenance of communication signals. In this study we demonstrate the co-occurrence of the mimics and the putative mushroom models, which is important for evolution by natural selection. We also showed that the resemblance to mushrooms is in fact adaptive, a requisite for floral mimicry. We did this by determining that insect visitors are required for pollination and subsequent fruit set with a hand pollination experiment. We also measured increased visitation rates to the orchids when adjacent to mushrooms.

The mechanisms whereby plants attract pollinators can be diverse and often multi-modal, particularly in deceptive systems. Dracula orchids are no exception, with both visual and olfactory signals contributing to the overall success in attracting visitors. We used a series of experiments, first selectively masking the visual and olfactory cues successively, and then using 3D-printed artificial flowers to further disentangle these cues and determine their effect in combination. Upon confirmation that both play a role, we dissected each aspect further. We utilized the artificial flowers to determine the roles of color, contrast, and pattern and employed gas chromatography-mass spectroscopy to identify the volatile signals. The results show that fine-scale contrast is critical to the visual component and that these flowers produce the volatile `mushroom-alcohol' (1-octen-3-ol) in their labella.

Finally, we specifically address the hypothesis of brood-site mimicry by using a combination of field observations, insect collections, and rearing studies. The flies gain shelter, a rendezvous location, and food from the flowers. However, no mushroom visiting flies hatched from the flowers, suggesting this may be a brood-site mimicry.

This dissertation includes previously unpublished co-authored material.