The dual role of insects in parasitic plant reproduction solves a long-standing botanical mystery
In the dark and moist understories of the subtropical forests of Shimoshima Island in Japan grow parasitic plants that feed on the roots of other plants. They are called Balanophora, and for over a century, the mechanism ...
It has been suggested that the tiny seeds are carried away by the wind, but there is little to no wind in the understories of (sub)tropical forests. Some plant species among the Balanophora are bright red, resembling strawberries, and attract birds and rabbits to eat and thus disperse the seeds. However, others, like Balanophora subcupularis, have dull colors and a yeasty smell, making it unlikely that vertebrates are involved. The mechanism of the plant's pollination has similarly remained unclear.
The Kobe University botanist Suetsugu Kenji specializes in understanding the interactions between these parasitic plants and their surrounding ecosystems, including the often overlooked but crucial role of invertebrate pollinators and seed dispersers. To find out who aids B. subcupularis in its reproduction, he and his team watched the plants for more than 100 hours and took tens of thousands of automated night photographs while the flower was in bloom or bore fruit, identifying the visitors.
In addition, they conducted both animal exclusion experiments and seed feeding experiments to ascertain whether the animals on the photographs are actually effective pollinators and seed dispersers.
In two back-to-back papers published in the journal Ecology, the Kobe University team published that B. subcupularis is pollinated by ants and camel crickets, which visit the plants for their pollen and nectar. Remarkably, these same species also later feed on the fleshy leaves carrying the seeds, aiding in seed dispersal.
A Balanophora subcupularis inflorescence (a cluster of flowers) is pollinated by an ant seeking the plant's pollen and nectar. Ants have not typically been considered effective pollinators because they are small and tend to remain on a single inflorescence for extended periods, allowing only small amounts of pollen to attach to their bodies. Credit: Hashiwaki Hiromu
Ants carry the bracts with the attached seeds to their nest, dropping some of the seeds on the way and disposing of others in "waste piles" outside their nests. Usually, seeds dispersed by ants are surrounded by nutrient-rich bodies called "elaiosomes" that serve as food for the ants. However, in the case of Balanophora subcupularis, the bracts seem to take over this function. Credit: Hashiwaki Hiromu
Because camel crickets are small, it is very unusual that they can swallow the seeds together with their food. However, like for other parasitic plants, Balanophora subcupularis seeds are tiny and robust, and many survive the passage through the cricket's digestive tract unharmed. Pictured is a camel cricket's dropping containing Balanophora subcupularis seeds. Credit: Suetsugu Kenji
A Balanophora subcupularis inflorescence (a cluster of flowers) is visited by a camel cricket seeking the plant's pollen and nectar. Similar to bee pollination, the cricket unwittingly transfers some pollen to female flowers on the same plant or on others. Pollination by camel crickets has so far been confirmed only in one other plant species, Mitrastemon yamamotoi, which is also a non-photosynthetic parasite. Credit: Hashiwaki Hiromu
After pollination, the flowers of Balanophora subcupularis develop into seeds attached to fleshy leaves, called "bracts." Camel crickets and ants return to feed on the bracts but also take the seeds with them. Credit: Hashiwaki Hiromu