Scientists reveal how Serapias orchids ensure their cross-pollination through a special flower morphology interacting with pollinators in a particular manner, without the need for pollen masses changes as in many other orchids.
Among terrestrial Mediterranean orchids, the genus Serapias employs an unusual strategy known as “shelter imitation”, where the floral elements form a tubular structure that insects use during their resting phases. A new study published in the journal AoB PLANTS by Micaela Lanzino and colleagues, suggests that in these orchids, morphological interactions alone between pollinators and flowers can help achieve effective cross-pollination, without the need of internal pollen-masses modifications as occurs in many other orchid species.
Orchidaceae is one of the largest plant families renowned for its diversity of pollination strategies. With over 30,000 species worldwide, orchids have evolved ingenious ways to favor outcrossing over self-fertilization.
One particularly fascinating group of orchids are the shelter-mimic orchids, which trick pollinators into thinking that their flowers are a suitable place to rest. These orchids typically have tubular flowers that resemble the nests or burrows of insects. When a pollinator enters the flower to rest, it inadvertently picks up pollen on its body. When the pollinator visits another flower, the pollen is deposited on the stigma, resulting in pollination.
The researchers conducted a series of breeding system experiments and hand-pollination treatments to investigate how Serapias orchids are pollinated and whether pollinarium reconfiguration (a process in which the pollen-masses of an orchid are repositioned to facilitate cross-pollination) is necessary.
The results of the study showed that Serapias orchids are highly self-compatible, meaning that they can fertilize themselves without the need for a pollinator. However, the researchers also found that Serapias orchids have a low natural fruit set, suggesting that they are pollinator-limited.
Generally, orchids have evolved very specialized pollination mechanisms that involve the pollinarium changing shape or position after picking up or depositing pollen. This helps ensure the pollen is transferred effectively between flowers of different plants. However, time-lapse photos showed that Serapias orchids do not undergo refolding of the pollinarium after its removal from the flower, suggesting that pollinarium reconfiguration is not necessary for cross-pollination in these orchids.
The researchers also observed that the morphology of the Serapias flower determines the position of the pollinarium on the body of the pollinator. When the pollinator leaves the flower, the pollinarium is unable to encounter the stigma. However, when the pollinator visits another flower, the pollen-masses of the first pollinarium are deposited on the stigma, while the pollinator picks up a second pollinarium.
To exit the flower, crawling backwards, the insect dragged out the pollinaria, which never managed to come into contact with the stigmatic surface of the orchid. When the insect made a second visit, bringing with it the pollinaria taken from the previous visit the pollen masses of the first pollinaria met the stigmatic cavity, leaving pollen grains on the stigma due to their characteristic stickiness, and at the same time, the sticky part of the pollinarium of this second flower remained attached to the insect’s head. At this point, the insect left the flower, dragging both pollinaria, but those of the second flower once again had no chance of ending up on the stigma.
Lanzino et al. 2023
These findings challenge the conventional understanding that sophisticated pollinarium adaptations are necessary for orchids to overcome self-pollination. Serapias demonstrates that morphological interactions alone can achieve highly specific pollen transfer and effective cross-pollination.
Serapias presents a prime example of intimate co-evolution between plants and their pollinators and highlights the importance of morphological interactions between them in the evolution of pollination strategies. Conservationists can use the findings of this study to develop strategies to protect shelter-mimic orchids and their pollinators.
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Lanzino M., Palermo A.M. and Pellegrino G. (2023) “Pollination mechanism in Serapias with no pollinaria reconfiguration” AoB PLANTS. Available at: https://doi.org/10.1093/aobpla/plad054.
Cover image: Serapias orchids. Image: Canva.