Mechanisms and evolution of deceptivepollination in orchids
Orchids are renowned for their diverse and deceptive pollination strategies, setting them apart from many other plant families. Unlike typical flowering plants that offer nectar or other rewards to attract pollinators, orchids have evolved various non-rewarding mechanisms to achieve pollination. This unique approach includes strategies such as generalized food deception, floral mimicry, brood-site imitation, shelter imitation, pseudoantagonism, rendezvous attraction, and sexual deception. These strategies present a fascinating evolutionary puzzle, prompting extensive research into their development and ecological implications.
Generalized Food Deception
Generalized food deception is the most common strategy among orchids. In this mechanism, orchids mimic rewarding flowers without actually providing nectar. This deception relies on the visual and olfactory cues that suggest the presence of a reward, thus luring pollinators to visit the flowers. Despite not receiving any reward, pollinators continue to visit these flowers due to their resemblance to nectar-producing species. This strategy is particularly effective in environments where pollinators have a high diversity and abundance, as it reduces the likelihood of any single pollinator learning to avoid the deceptive flowers.
Case Study: The Bee Orchid (Ophrys apifera)
The Bee Orchid, Ophrys apifera, is a classic example of generalized food deception. Its flowers closely resemble those of nectar-producing plants, attracting various bee species. Research has shown that this form of deception is effective in ensuring pollination without the need to produce nectar, thus saving the plant’s resources for other reproductive processes.
Sexual Deception
Sexual deception is another intriguing pollination strategy employed by orchids. In this mechanism, orchids mimic the appearance and scent of female insects to attract male pollinators, leading to pseudocopulation. Male insects, deceived by the orchid’s mimicry, attempt to mate with the flower, thereby facilitating pollination. This strategy is highly specialized and relies on the precise mimicry of the female insect’s pheromones and appearance.
Example: The Spider Orchid (Ophrys sphegodes)
The Spider Orchid, Ophrys sphegodes, exemplifies sexual deception. This orchid emits chemicals that mimic the sex pheromones of female bees, specifically targeting the solitary bee species Andrena nigroaenea. Male bees, attracted by the scent, attempt to copulate with the flower, inadvertently transferring pollen in the process.
Brood-Site Imitation
Brood-site imitation involves orchids mimicking locations where insects lay their eggs. These orchids produce flowers that resemble the visual and chemical cues of such sites, tricking insects into visiting them for oviposition. Once the insects land on the flowers, they become covered in pollen, which they then transport to other flowers, facilitating cross-pollination.
Example: The Bucket Orchid (Coryanthes)
The Bucket Orchid, Coryanthes, utilizes brood-site imitation by producing flowers that mimic the brood sites of certain fly species. The orchid’s flower structure traps the flies, ensuring that they come into contact with the pollen. Once the flies escape, they carry the pollen to the next flower they visit, completing the pollination process.
Shelter Imitation and Pseudoantagonism
Shelter imitation involves orchids providing false shelters or mimicking territorial behaviors to attract pollinators. This strategy often involves producing structures that resemble safe resting places or territorial markers. Pseudoantagonism, on the other hand, involves mimicking aggressive or competitive interactions to attract pollinators.
Example: The Fairy Slipper Orchid (Calypso bulbosa)
The Fairy Slipper Orchid, Calypso bulbosa, uses shelter imitation by producing flowers that resemble small shelters. Bumblebees, in search of resting places, enter the flowers and come into contact with the pollen. Despite not finding a suitable shelter, the bees continue to visit these flowers, facilitating pollination.
Floral Mimicry
Floral mimicry involves orchids resembling specific rewarding flowers to deceive pollinators. This strategy relies on the visual similarity between the deceptive orchid and the rewarding flower, making it difficult for pollinators to distinguish between the two. This form of mimicry can be quite complex, involving not just visual cues but also olfactory and tactile cues.
Example: The Lady’s Slipper Orchid (Cypripedium)
The Lady’s Slipper Orchid, Cypripedium, employs floral mimicry by producing flowers that closely resemble those of nectar-producing plants. This resemblance attracts various pollinators, including bees and butterflies, which visit the flowers in search of nectar. Despite not finding any reward, the pollinators inadvertently transfer pollen from one flower to another.
Evolutionary Puzzle of Non-Rewarding Flowers
The evolution of non-rewarding flowers presents a significant puzzle in biology. Two main hypotheses have been proposed to explain how such deceptive strategies could increase the fitness of orchids:
Resource Reallocation Hypothesis
The resource reallocation hypothesis suggests that the resources saved from not producing rewards can be reallocated to other aspects of reproduction, such as flowering and seed production. By not investing in nectar production, orchids can produce more flowers or invest more in seed development, thus enhancing their reproductive success.
Cross-Pollination Promotion Hypothesis
The cross-pollination promotion hypothesis posits that deception leads to higher cross-pollination rates. Because pollinators visit fewer flowers per plant when they do not receive a reward, they are more likely to move between different plants. This behavior promotes outcrossing, resulting in more genetically diverse progeny and more efficient pollen export.
Why Deception is Prevalent in Orchids
Several hypotheses explain why deceptive pollination strategies are overrepresented in orchids:
Efficient Pollinaria Removal
One reason for the prevalence of deceptive pollination in orchids is the efficient removal of pollinaria. Orchids package their pollen in discrete units called pollinia, which can be removed in a single visit by a pollinator. This reduces the need for multiple visits and allows orchids to rely on fewer pollinator interactions to achieve pollination.
Low-Density Populations
Many orchids exist in low-density populations, where the likelihood of a pollinator associating a particular floral phenotype with a reward is low. In such environments, the deceptive strategy is more effective, as pollinators are less likely to learn to avoid the non-rewarding flowers.
Pollen Packaging
Orchids package their pollen in pollinaria, which have limited carry-over between flowers. This increases the risk of self-pollination in rewarding plants. By adopting a deceptive strategy, orchids can reduce the likelihood of self-pollination and promote cross-pollination.
Challenges and Stability of Deceptive Strategies
Despite the evolutionary advantages, the deceptive strategy often results in low pollinator visitation and fruit production. Recent studies have shown that deception promotes cross-pollination, but it remains unclear if outcrossing rates are higher in deceptive orchids compared to rewarding ones. Several factors influence the evolution and maintenance of deceptive strategies, including genetic load, pollinator abundance, and plant density.
Genetic Load
The genetic load of a population refers to the presence of deleterious alleles that can affect the fitness of individuals. In populations with a high genetic load, promoting cross-pollination through deception can help reduce the expression of these deleterious alleles by increasing genetic diversity.
Pollinator Abundance
The abundance and behavior of pollinators play a crucial role in the effectiveness of deceptive strategies. In environments with high pollinator diversity and abundance, deceptive strategies can be more successful. However, in areas with limited pollinator availability, the low visitation rates associated with deception can hinder reproductive success.
Plant Density
Plant density also affects the success of deceptive strategies. In low-density populations, the likelihood of a pollinator encountering multiple flowers of the same species is reduced, making deception more effective. In contrast, in high-density populations, pollinators are more likely to learn to avoid non-rewarding flowers, reducing the effectiveness of the deceptive strategy.
Conclusion
Deception in orchids provides significant evolutionary advantages by promoting outcrossing and efficient pollen transport. However, the success of these strategies is highly context-dependent, influenced by factors such as pollinator behavior, genetic load, and plant density. Further research is needed to fully understand the conditions under which deception constitutes an evolutionarily stable strategy. The intricate mechanisms of orchid pollination continue to fascinate scientists and highlight the complexity of evolutionary processes in the plant kingdom.
References
Schiestl, F. P. (2005). On the success of a swindle: pollination by deception in orchids. Naturwissenschaften, 92(6), 255-264.
Jersáková, J., Johnson, S. D., & Kindlmann, P. (2006). Mechanisms and evolution of deceptive pollination in orchids. Biological Reviews, 81(2), 219-235.
Paulus, H. F. (2006). Deception and mimicry as the evolutionary basis of orchid diversity. Acta Biotheoretica, 54(3), 159-177.
Tremblay, R. L., & Ackerman, J. D. (2007). The distribution and maintenance of sexual reproduction in orchids. New Phytologist, 176(2), 431-435.
Dafni, A. (1984). Mimicry and deception in pollination. Annual Review of Ecology and Systematics, 15, 259-278.
