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Fungi-eating orchids assist fruit flies with reproduction

Scientists have long known that orchids deceive their pollinators to visit their flowers by imitating food sources, breeding grounds, or even mates, without in fact offering anything in return. This strategy is also used by the fungi-eating, non-photosynthetic orchid genus Gastrodia which, in order to attract fruit flies, emits a smell resembling the flies’ typical diet of fermented fruits or decaying mushrooms. 

The fruit flies are lured into the orchid’s flowers, where they get trapped for a short period of time and get pollen attached to their backs, which they afterwards transport to other orchids of the same species – a deceptive relationship benefitting only one partner.

Nursery pollination 

However, as a new study conducted by Kobe University in Japan has recently showed, a certain species of this genus, Gastrodia foetida, engage in a rather different relationship with the flies: they offer their flowers to fungi-eating fruit flies in exchange for pollination.

This is the first evidence for nursery pollination – i.e. a plant offering a breeding ground to its pollinator – in orchids. This peculiar plant-animal relationship hints at an evolutionary transition towards mutualistic symbiosis.

Focus of the study

G. foetida has particularly fleshy petals which decompose and fall off a few days after pollination. Before this though, fruit flies frequently lay their eggs into the plant’s flowers, offering their larvae the opportunity to fully develop into adult flies in this environment.

“The most intriguing aspect is that, contrary to its common name as the ‘fruit’ fly, Drosophila bizonata, a species specialized in mushroom-feeding, predominantly utilizes decaying Gastrodia foetida flowers as brood sites,” said study author Kenji Suetsugu, a biologist at Kobe.

A prime target

“A possible explanation is the fact that Gastrodia foetida is a non-photosynthetic orchid that feeds on fungi. These non-photosynthetic orchids often exhibit chemical resemblance to the fungi they assimilate, underlining the age-old adage ‘You are what you eat.’ As a plant that feeds on mushrooms, G. foetida likely tastes similar to a mushroom, making it a prime target for the mushroom-specialized fruit fly.”

These findings reveal a new type of nursery pollination system that goes beyond the mere deceptive strategies commonly found in this genus. 

Mutualistic symbiosis

As Suetsugu argues, this relationship is neither obligatory nor specific since the fruit flies also lay fully developing eggs on fungi. This suggests an important transition from a deceptive relationship towards mutualistic symbiosis, most likely driven by the low cost to the plant (since its petals are not needed anymore after pollination).

“This study represents the first evidence of nursery pollination in orchids, comprising nearly 30,000 species, and being the most diverse plant group in the world,” said Suetsugu.

“In addition, it contributes vital understanding to the intricate and mutually beneficial relationships that can develop in nature. The understanding of how plants can offer genuine benefits rather than merely deceiving pollinators could impact the broader study of plant-animal interactions and their evolutionary dynamics.”

The study is published in the journal Ecology.

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