A recent study by Cardoso and colleagues in Annals of Botany reveals an intricate pollination strategy employed by the lady’s slipper orchid, Phragmipedium vittatum. This remarkable plant mimics aphids, tiny insects that serve as food for hoverflies’ larvae, to attract pollinators and guide them through a specific path that ensures effective pollen transfer.
The study found that Phragmipedium vittatum is pollinated by two species of hoverflies belonging to the Syrphidae family. The orchid uses a unique set of micro-morphological traits to manipulate the movements of the hoverflies in a way that culminates in precise pollen transfer. The trap flowers of the orchid, which are the parts of the plant that contain reproductive organs, are the key to this process.
Trap flowers, like those of Phragmipedium vittatum, are fascinating examples of adaptation often associated with oviposition-site mimicry systems. When a flower uses oviposition-site mimicry, it attempts to draw in insects looking to lay their eggs. Instead of imprisoning pollinators for a set period, these flowers manipulate the insects’ movements, forcing them through a predetermined route that maximizes pollen transfer.
The researchers found that the trap flowers of the orchid have dark, elevated aphid-like spots that attract the attention of the hoverflies to a slipping zone. This region has downward projecting papillate cells, cells with protuberances that make it difficult to climb up, and mucilage secretion that promote slipperiness, causing potential pollinators to fall into the labellum, a specialized petal that forms a slipper-like structure.
Once inside the slipper, the hoverflies follow a specific upward route towards inner aphid-like spots by holding onto upward-oriented hairs that aid their grip. The lateral constriction of the labellum then funnels the hoverflies towards the stigma, the female part of the plant. If they have any pollen from another orchid, they’ll deposit it here. The hoverflies then squeeze under one of the articulated anthers, which places pollen smears onto their upper thorax. Finally, they depart through one of the narrow lateral holes by holding onto hairs projecting from the petals.
What is remarkable about this system is that the orchid mimics aphids, which are the food source of the hoverflies’ larvae. More common forms of deception are looking like a food-rewarding plant or else looking like an insect of the opposite sex. Eggs laid by the flies on or near the raised black spots on the flowers indicate this mimicry system. As the aphids are a mirage, any eggs the fly lays are doomed. Cardoso and colleagues note that the flies get nothing from the interaction. In their article, they write:
We found that P. vittatum is a rewardless, self-compatible, non-apomictic and pollinator-dependent species. Both artificial self- and cross-pollination successes led to high fruit set, demonstrating that there is no self-incompatibility mechanism. However, we found lower levels of seed viability in artificially selfed fruits when compared to both artificial cross- and open-pollination, suggesting some inbreeding depression. Furthermore, total seed numbers after selfing were lower when compared to cross-pollination. Thus, self-fertilization apparently brings deleterious effects, and our results highlight the importance of pollinator-mediated crossing in seed quality and quantity.Cardoso et al. 2023
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João C F Cardoso, Steven D Johnson, Uiara C Rezende, Paulo E Oliveira, The lady’s ‘slippery’ orchid: functions of the floral trap and aphid mimicry in a hoverfly-pollinated Phragmipedium species in Brazil, Annals of Botany, Volume 131, Issue 2, February 2023, Pages 275–286, https://doi.org/10.1093/aob/mcac140