Visual ecology of sexually deceptive orchids

The eye-catching colored corollas of many angiosperm plants and their function for attracting insects to obtain pollination service is one of the most intriguing examples of communication between plants and animals. However, the selective pressures and constraints involved in the evolution of such visual signals are in many cases poorly understood. This proposal aims to understand the functional significance of visual signals in a genus of sexually deceptive orchids, Ophrys, in the context of pollinator attraction. Entomophilous orchids face a high risk of pollen loss since the entire pollen of a flower is packed in two packages and thus may be accidentally removed by an unspecific flower visitor at a single visit. To ensure efficient and specific pollen transfer orchids have developed amazing strategies to attract their specific pollinator. The sexually deceptive orchids, which mimic specific signals employed in insect mate interactions, constitute one of the most impressive examples of signal mimicry in plant-animal communication. Seemingly, the orchids perfectly imitate the sex-pheromone of a particular insect species` female, mostly hymenopterans, to attract the males which search for mates. During their mating attempts the deceived males come into contact with the orchid`s pollen packages, and pollen is transferred upon visitation of subsequent flowers. This private and highly specific odor communication channel is usually accompanied by the absence of any conspicuous color signal which may accidentally attract unspecific flower visitors to the flower. However, within the genus Ophrys a small group of species possess a conspicuously colored perianth and a bright labellum pattern. The function of these visual signals is not yet understood and difficult to explain in an evolutionary context since unspecific visual signals might attract unspecific pollinators and thus may cause pollen loss. Unraveling the functional significance of visual signals in this plant-pollinator communication system will contribute to a more comprehensive understanding of the evolution and potential constraints of these signals and their function for speciation in the orchid genus Ophrys. Furthermore, the outcome of our study will allow us to identify the circumstances which may have favored the evolution of unspecific visual signals like colored corollas for pollinator attraction in plant-insect communication in general, since our study system allows us to directly compare pollination attraction of both phenotypes, flowers with and without colored perianths, in the same genus. Finally, the amazing spatial complexity of flower pattern at the labellum of some Ophrys species constitute a unique system to address questions about sensory, learning, and memory limitations of pollinators in the context of flower detection and identification.

Many angiosperm plants possess eye-catching colored flower to attract pollinators, but the selective pressures and constraints involved in the evolution of such visual signals are in many cases poorly understood. This project aimed to understand the functional significance of such color signals in a genus of sexually deceptive orchids, Ophrys, in the context of pollinator attraction, which are in most cases hymenopteran males. Our results show that some orchid species use color signals to mimic food plants of the pollinator, or imitate optical features of the females, to increase conspicuousness and attractiveness for the pollinating males. To ensure efficient and specific pollen transfer, orchids have developed amazing strategies to attract their specific pollinator. Sexually deceptive orchids, which mimic specific signals employed in insect mate interactions, imitate the sex-pheromone of a particular insect species female, mostly bees or wasps, to attract the males which search for mates. During their mating attempts the deceived males come into contact with the orchids pollen packages, and pollen is transferred upon visitation of subsequent flowers. This highly specific odor communication channel is usually accompanied by the absence of any conspicuous color signal. However, within the genus Ophrys some species possess a conspicuously colored perianth or a bright labellum pattern, and the function of these visual signals is not yet understood. Using two species, Ophrys heldreichii and O. speculum, we investigated the function of such signals in respect to pollinator attraction and reproductive success. Manipulation experiments of the pink perianth in various O. heldreichii populations in the field showed a reduced reproduction success of flowers with removed perianth compared to unmanipulated flowers or flowers, of which the original perianth was replaced by an artificial one. Ophrys heldreichii flowers also possess a conspicuous line pattern on their lip. Using choice experiments with honeybees as insect models for learning and memory, we could show that bees can discriminate between flower patterns from different plants, but not between patterns from flowers of the same plant. Thus, the pattern variation may constitute a strategy of the orchid to prevent male bees from associating the negative experience of an unsuccessful mating attempt with the flower appearance and thus ensuring further visits to conspecific plants. In the second investigated orchid species, O. speculum, the lip possesses a conspicuous bluish mirror, which was suggested to imitate the female of the pollinator, males of the scoliid wasp Dasyscolia ciliata. Using spectral reflectance analyses and behavioral experiments in the field, we showed that males are attracted to the blue mirror and also to the females wings, and that in a bee specific perceptual color space the female wings significantly overlap with the appearance of the blue mirror, suggesting the presence of a visual mimicry signal.