Ming-Fai Liu and Richard Saunders, from the University of Hong Kong’s Division of Ecology & Biodiversity, School of Biological Sciences, discuss their research article: Aerial litter mimicry: a novel form of floral deception mediated by a monoterpene synthase
Mutualism is more of a rule than an exception when it comes to pollination—although a few plants that have independently evolved means to cheat insects for pollination by mimicking something they are not. This adaptation is known as floral mimicry. To hijack the communication channels between an insect and a third-party organism or substrate, these plants have undergone selection to develop peculiar phenotypes. These flowers can be very surprising, and often captivate public interest and imagination.
We discovered a novel floral mimicry system in the understorey treelet Meiogyne heteropetala. This plant, which bears flowers with dark petals, grows in coastal forests in Whitsundays, Queensland, Australia. While black flowers are often associated with foul smells, the flower of M. heteropetala instead produces a minty odour. This curious combination of scent and colour caught our attention. Further observation revealed that the flower imitates aerial litter to fool an aerial litter specialist beetle Loberus sharpi for pollination.
Aerial litter, also referred by some as canopy litter or arboreal litter, is a collection of dead leaves and twigs that remains on trees. These dead plant materials are retained either because they are withered but not shed by the plant, or because they are intercepted by understorey plants, fungal hyphae, or arthropod silks. This substrate has a unique arboreal microclimate and nourishes many microbes, arthropods, and even birds. Similar to these organisms, the pollinator beetles use aerial litter as a breeding ground. When they visit and pollinate M. heteropetala flowers, the beetles also lay eggs there. However, most of the petals eventually drop onto the forest floor, where the eggs and larvae perish. Although they pollinate the flowers, the beetles therefore suffer reproductive loss.
To investigate how the flowers convincingly achieve this trickery, we assessed the similarity between M. heteropetala flowers and aerial litter in the vicinity based on size, colour, and odour. By measuring the dimension and assessing how they reflect light at different wavelengths, we discovered that the flower matches with a subset of aerial litter in size and colour. We furthermore assessed the floral volatile components, and found that the flower emits a blend of terpenoid volatiles highly similar to aerial litter.
Terpene synthases can produce multiple products from a single substrate. This interesting property means that it is possible that floral odour can be produced by just a few enzymes. Characterisation of the functions and expression patterns of floral terpene synthase indeed revealed that the flower produces its odour primarily by a single cineole synthase, MhCINS.
This simple biochemical basis for odour mimicry suggests that aerial litter mimicry might be more prevalent than we thought. Aerial litter is a common substrate in woodland habitats, starkly contrasting with how few aerial litter mimics are known. The ubiquity of aerial litter highlights just how little we know about what substrates floral mimics can exploit.