The orchid and the fruit fly – Scientists discover unique new relationship between plant and animal

The mutualistic relationship between Gastrodia foetida and its pollinators, both of which feed mainly on fungi. The flowers give off a mushroom-like smell that attracts the flies. When the flies feed on the flowers, pollen grains attach to their spines (1). When the fly leaves one flower and visits another to lay eggs, the attached pollen comes into contact with the stigma and completes pollination (2). After successful pollination, the flower begins to decay and internally hatched larvae begin to consume the petals (3). The larvae grow further by consuming the petals that have fallen to the ground (4). About a week later they emerge as adults. Credit: ANSAI Shun

For the first time, fungus-eating orchids have been observed offering their flowers to fungus-eating fruit flies in exchange for pollination services. This discovery represents the first evidence of nursery pollination in orchids. This unique new plant-animal relationship suggests an evolutionary transition towards mutualistic symbiosis.

Orchids are well known for tricking their pollinators into visiting the flowers by imitating food sources, nesting sites or even mates without actually giving anything in return. The fungivorous, non-photosynthetic orchid genus Gastrodia is no different: Attracting fruit flies (Drosophila spp.), the plants usually emit an odor similar to their usual diet of fermented fruits or rotting fungi.

A fruit fly inheritance that consumes decomposing flower tissue on the ground. Credit: Suetsugu Kenji

The fruit flies are attracted into the flowers, are trapped there for a short time and get pollen stuck to their backs which they then transport to other plants of the same

” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>species. Thus, this deceptive relationship offers benefits to only one partner.

Kobe University plant biologist Suetsugu Kenji, a specialist in these orchids, noticed that a certain species of this genus, Gastrodia foetida, has particularly fleshy petals that disintegrate and fall off a few days after pollination. He decided to examine these plants in search of the first example of orchids participating in “nursery pollination,” which is a plant that provides breeding grounds for its pollinator.

A fruit fly lays its eggs inside a flower Gastrodia foetida. Credit: Suetsugu Kenji

And indeed in the study now published in the journal Ecologyhe reports that fruit flies often lay their eggs in the plant’s flowers and that their larvae can fully develop into adult flies in this environment.

Suetsugu says, “The most exciting aspect is that, contrary to its common name of ‘fruit fly,’ Drosophila bizonataa species that specializes in mushroom feeding, using mainly rotting Gastrodia foetida flowers as breeding grounds. 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 similarity to the fungi they assimilate, underscoring the age-old adage “You are what you eat.” Like a plant that feeds on mushrooms, G. foetida likely tastes similar to a mushroom, making it a prime target for the mushroom-specialist fruit fly.” This discovery is significant because it reveals a new type of nursery pollination system, which goes beyond deceptive strategies commonly found in the genus.

Bloom off Gastrodia foetida. Credit: Suetsugu Kenji

The Kobe University researcher further explains that the relationship is neither obligatory nor specific, that is, the fruit flies also lay fully developed eggs on mushrooms. Thus, this finding may represent an example of the transition from a deceptive relationship to mutualistic symbiosis, which is suggested by two factors: the low cost of the plant, since the petals are no longer needed after pollination; and the closely related Gastrodia dominantly uses a deceptive strategy without providing a nursery.

A flower of Gastrodia foetida on the verge of decomposition. Credit: Suetsugu Kenji

Suetsugu concludes: “This study represents the first evidence of nursery pollination in orchids, which comprise nearly 30,000 species and are the most diverse plant group in the world. In addition, it contributes to an important understanding of the intricate and mutually beneficial relationships that can develop in nature.” Understanding how plants can offer real benefits rather than just tricking pollinators can influence the broader study of plant-animal interactions and their evolutionary dynamics.”

Reference: “A New Nursery Pollination System Between a Mycoheterotrophic Orchid and Fungus-feeding Flies” by Kenji Suetsugu, 23 Aug 2023, Ecology.
DOI: 10.1002/ecy.4152

The study was funded by the Japan Science and Technology Agency.