Orchids in a clearing in Brazil.

Orchid Mimics Aphids to Lure Pollinators in Intricate Trap Flower Scheme

What appears to be a place of safety becomes a maze, as an orchid manipulates its visitor.

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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.

A fly falls into a trap. Source: Cardoso et al. 2023.

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.

A fly attempts to escape a trap. Source: Cardoso et al. 2023.

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. 

A fly finally escapes the orchid, with pollen on her back. Source: Cardoso et al. 2023.

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

READ THE ARTICLE

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 BrazilAnnals of Botany, Volume 131, Issue 2, February 2023, Pages 275–286, https://doi.org/10.1093/aob/mcac140

Fi Gennu

Fi Gennu is a pen-name used for tracking certain posts on the blog. Often they're posts produced with the aid of Hemingway. It's almost certain that Alun Salt either wrote or edited this post.

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