Orchids and their obligatory fungal symbiotes represent an incredible mutualism that has allowed the rapid diversification of both kingdoms involved. Last August, I introduced orchids and their mycorrhizal fungi. In that blog post, I describe how these fungi promote seed germination and nourish the plant even before the plant is able to send sugars to its fungal mutualist. I also explained how a specific orchid can actually switch its mycorrhizal partner during a drought. When water becomes scarce, the orchid Goodyera pubescenscan switch partners, selecting a fungal species that could access water more readably. Today, I learned yet another ecological service that these fungi provide; services these plants have tapped into.
Usually, flowering plants attract potential pollinators with visual and olfactory cues. Bright colors and scents entice insects to land on their reproductive parts and transfer genes from other individuals. Unlike most other flowering plants, about one third of them achieve pollination through deception, not even offering up nectar reward. There is even a group of orchids that mimic female bees that seduce male bees to ‘mate’ with them! With so many ecologically diverse orchids around the world, there’s almost nothing atop the forest floor this family of plants doesn’t mimic. Scents orchids mimic include carrion, prey pheromones, opposite sex pheromones, fermenting fruit and fungi. Regardless of how they achieve pollination, scent is usually a key mechanism to attract the pollinator.
Many orchids utilize specialized insects that they’ve coevolved with for millennia. Over time their anatomy and physiology has become fine-tuned. In most orchids, pollen is condensed in little packets called pollinia. Much of the time, sticky patches of pollinia hitch a ride to another individual’s stigma. The insects involved tend to fit just perfectly in the flower, as they come into contact with both stigma and pollinia. The scents orchids produce can be detected by their insect specialist easily, as their olfactory system picks up these scent molecules, even when the abundance of them is incredibly miniscule, (in the parts per billion).
Earlier this year, Kenji Suetsugu published an article in Ecology that describes this novel service orchid fungi provide. Kenji studied the non-photosensitizing, (achlorophyllous) fungal parasite Gastrodia pubilabiata. Already, this orchid steals carbon from saprotrophic fungi mainly from the genus Mycena. This new research also shows that pollination is enhanced when Mycena fruit near the flower. The closer G.pubilabiata is to a patch of these forest floor decomposers, the better chances it has to reproduce.
Drosophila flies tend to lay their eggs in saprotrophic fungi like Mycena species. These insect larvae can easily digest these fungal tissues, eventually maturing into flying adults. These nearby orchids which also attract other fly pollinators too (Family: Muscidae), show that maybe the interaction isn’t as specialized as it first seems. The flowering of the plant and the fruiting of these Mycena mushrooms is not synchronous either, further hinting that the interaction is less of a long-lasting evolutionary byproduct. Instead, Drosophila flies become attracted to generic scents, resembling fermented or decaying substrates that have evolved to attract the flowers main fly pollinator.
Although Gastrodia pubilabiata takes full advantage of these Mycena species by stealing the saprotroph’s sugars, these mushrooms facilitate pollination. Mycena species don’t really benefit at all. Because these mushrooms attract more Drosophila flies, more of them mistake Gastrodia pubilabiata as just another fruiting body. Already, this orchid occurs near patches of saprotrophic fungi because it needs to steal another species carbohydrates. Overtime, we may see more and more selection towards fungal mimicry. Already, the orchid parasitizes fungi. Since Drosophila larvae fail to mature in the flowers tissue, the plant also parasitizes these insects. Every time I read about this incredibly diverse plant family, I am left shook. Today I learned that G. pubilabiata is a double parasite, taking advantage of fungi and the Drosophila flies associated with them.