Flower colour adaptation in a mimetic orchid

Abstract

Although the tremendous variability in floral colour among angiosperms is often attributed to divergent selection by pollinators, it is usually difficult to preclude the possibility that floral colour shifts were driven by non-pollinator processes. Here, we examine the adaptive significance of flower colour in Disa ferruginea, a non-rewarding orchid that is thought to attract its butterfly pollinator by mimicking the flowers of sympatric nectar-producing species. Disa ferruginea has red flowers in the western part of its range and orange flowers in the eastern part--a colour shift that we hypothesized to be the outcome of selection for resemblance to different local nectar-producing plants. Using reciprocal translocations of red and orange phenotypes as well as arrays of artificial flowers, we found that the butterfly Aeropetes tulbaghia, the only pollinator of the orchid, preferred both the red phenotype and red artificial flowers in the west where its main nectar plant also has red flowers, and both the orange phenotype and orange artificial flowers in the east, where its main nectar plant has orange flowers. This phenotype by environment interaction demonstrates that the flower colour shift in D. ferruginea is adaptive and driven by local colour preference in its pollinator.

Figure 1.

Butterfly responses to flower colour. (a) Reflectance spectra of deceptive red and orange Disa ferruginea mimics match those of their respective rewarding models. (b) Preference for red orchid phenotypes and paper flowers (over orange) in habitats with red (Table Mountain) and orange (Langeberg) rewarding flowers. Asterisks indicate means that differ significantly from 0.5 (no preference). (c) Numbers of visits to red and orange inflorescences and to (d) paper flowers at the Langeberg and Table Mountain sites.

Figure 2.

Herbarium records showing the geographical overlap of Disa ferruginea colour forms and rewarding model plant species in the Cape region of South Africa. Bold symbols indicate study sites.