Plant death traps become nurseries

Arisaema thunbergii

The plant Arisaema thunbergii has pitcher-like flowers that can become a death trap for fungus gnats. © Hiroaki Yamashita/ CC by 4.0

When insects pollinate flowers, both the plant and the pollinator usually benefit. But some flowers deceive their pollinators and trap them in the calyx, causing them to die inside. However, a study now shows that some insects still know how to use this death trap in a positive way. The researchers discovered that a type of fungus gnat, which regularly falls victim to flower prisons, lays its eggs in the calyx of flowers. As the flower rots over time, it serves as food for the larvae and thus becomes a nursery for a new generation.

Most plants worldwide rely on animal pollination. Many attract their pollinators with bright colors and unusual shapes and offer them tasty nectar in return. Plants of the genus Arisaema, also known as cobra lilies or fire pistons, pursue a very selfish strategy: they lure their pollinators with an intoxicating smell into their pitcher-shaped flowers, from which there is hardly any escape. While the insects can free themselves from male flowers with some effort - and come out laden with pollen - female flowers offer them no way out and become a death trap.

Classic view on the test bench

“This interaction is traditionally viewed as highly antagonistic, as pollination of the plant comes at the expense of the insect,” explains a team led by Kenji Suetsugu from Kobe University in Japan. “We put this traditional view to the test and examined whether there are more nuanced interactions that go beyond the seemingly antagonistic relationship.”

To do this, the team collected male and female flowers of the native species Arisaema thunbergii in the forests of the southern Japanese island of Yakushima. This prefers to grow in shady, moist regions and often relies on fungus gnats for pollination, which usually feed on mushrooms and lay their eggs on them. With the help of a mushroom-like smell, A. thunbergii attracts the fungus gnats and imprisons them in its calyx. In the laboratory, Suetsugu and his team examined which insect species were trapped in the flowers and what happened after pollination.

Death trap and nursery
Although some fungus gnats fall victim to the pitcher-like flowers of the Arisaema, others lay their eggs in them, making the flowers a nursery for the next generation. © Hiroki Nishigaki and Kenji Suetsugu/ CC by 4.0

Intermediate stage of evolution

The researchers made an astonishing discovery: young fungus gnats emerged from some of the flowers after a few weeks. “Our studies show that one of the most important pollinating species, the fungus mosquito Leia ishitanii, uses the deadly flower trap of the inflorescences of A. thunbergii as a nursery,” report Suetsugu and his colleagues. Apparently, captured female mosquitoes lay their eggs in the calyx before they die. The nutrient-rich tissue of the flower, which rots after pollination, serves as food for the larvae. In individual cases, larvae also grew in flowers in which the researchers did not find any dead adults. This suggests that individual specimens managed to escape from the flower after pollination and egg laying.

The relationship between plant and fungus mosquito is still far removed from typical cases of symbiosis. The fungus mosquito L. ishitanii does not rely on flowers to reproduce, but usually reproduces on fungi. The captivity in the flowers of A. thunbergii is therefore still rather disadvantageous for the mosquitoes. “We suspect that this interaction likely represents an intermediate stage in the evolution of a mutualistic relationship,” says Suetsugu. “Our results highlight the need to refine existing models of pollination biology to gain a more nuanced understanding of plant and pollinator dynamics.”

Source: Kenji Suetsugu (Kobe University, Japan) et al., Plants, People, Planet, doi: 10.1002/PPP3.10494

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