How to act like a mushroom: Olfactory and visual cues in the attraction of drosophilid flies by neotropical Dracula orchids

Background/Question/Methods

Orchids in the genus Dracula look and smell like mushrooms. Through a unique system of mushroom-mimicry they are pollinated by mushroom associated flies. As this suite of morphological and olfactory phenotypes is novel within the genus, we wanted to know to what extent each aspect is responsible for attracting the drosophilid pollinators. To tease apart the visual and olfactory cues we performed a series of experiments where we both deconstructed and then reconstructed the signaling motif of Draculaflowers.        

In the first set of experiments we selectively removed aspects of the signaling phenotype by covering the flowers with either green cotton bags (eliminating visual cues) or with odor-proof plastic bags (eliminating olfactory cues). During each observation period an unmanipulated flower was observed as a positive control. Each treatment also had its own negative control, a bag (cloth or plastic respectively) that was not covering a flower, to rule out any attractive or repellent properties of the treatments themselves (n=15 half hour periods). In the second set of experiments we used life-like replicas of the flowers made out of weather-proof, sugical grade silicone and presented either the visual model alone or the model with added fragrance (hexane extracts of real flowers diluted 2:1 with mineral oil) to the model to recreate both aspects of attraction (n=18 half hour periods). All analyses were done by ANOVA on log-transformed data with similarities and differences between treatments assessed by Tukey HSD.

Results/Conclusions

In the deconstruction of cues we show that both aspects of signaling are important, with the visual and olfactory treatments alone eliciting a number of approaches intermediate between the real flower and the negative controls. However when it came to actual landings, the real flower received the most visits, suggesting that while each signal independently may attract a fly, both must be present to elicit an actual landing. When we reconstructed the Dracula flowers with the models and fragrance extracts we found a similar pattern. Flies approached both the visual-only and the fragrant model at similar rates as to the real flower, but only the fragrant model received as many landings as the real flower.

These experiments suggest that both the morphological and olfactory aspects of the Dracula phenotype are selected for in the evolution of mushroom mimicry, and that they act in concert (potentially over different spatial scales) to attract dipteran pollinators in a fungal-rich cloud forest environment.