A large group of flowering plants, commonly known as Orchids, often have flowers whose shape coincides with that of their insect pollinators. Recent research has shown how this uncanny flower morphology is guided by evolutionary selection.
Orchids have long been recognized for the bizarre structure of their flowers. This is especially true of Ophrys, a genus commonly referred to as the “bee orchids” because of their furry and colorful floral patterns.
Despite the physical resemblance of their flowers to pollinators, Ophrys species primarily use chemical pheromone mimicry to attract insects that facilitate their sexual reproduction, tricking males into picking up pollen during attempted copulation.
It has been hypothesized that, rather than an attractant, an insect-like flower shape simply provides a “landing zone” for a particular pollinator, since each Ophrys species generally attracts a single insect species and relies on this sole pollinator for its survival.
Whether the specific shape or behavior of the insect pollinator influences the shape of Ophrys flowers, however, is unknown.
Researchers are interested in this question because if an interaction between an Ophrys flower and a pollinator leads to efficient pollination, that particular flower shape may be naturally selected for over time. This could be a strong but hitherto overlooked process driving the evolution and diversification of an abundant group of plants.
To test this evolutionary process, researchers from the University of Vienna and the University of Ulm teamed up to perform experiments on flower-pollinator interactions in the natural environment.
The group chose to examine Ophrys leochroma (a European orchid) and its pollinator Eucera kullenbergi (a long-horned bee) as a model system.
They manipulated the shape of wild orchid flowers, changing the shape of areas of the flower where wild bees physically land and move (active areas), as well as areas with which the insects do not interact (inactive areas).
The results of these experiments showed that “normal” flower shape was essential for guiding bees to the correct location for effective plant pollination. Even slightly changing flower shape, for example to resemble closely-related orchids, significantly reduced pollination efficiency.
Furthermore, the shape of active flower areas was much more important for pollination efficiency than those of inactive areas. Changing the shape of the landing zone, or “lip”, was much more detrimental for plant pollination than changing the shape of a non-active area, such as the “appendix”.
From these results, the authors first concluded that flower shape is important for orchid-pollinator pairs by physically guiding the insect to the correct location after landing.
Second, the authors hypothesize that active areas of the flower, such as the lip, are under strong selection to remain the same shape. For example, a change in lip shape may lead to less effective pollination, causing that shape to be inherited less often.
Conversely, the shape of the non-active areas, such as the appendix, may be under less evolutionary pressure. Changing their shape did not affect pollination in the experiments: thus, the shape of these flower parts could vary widely without the plant being worse at passing on its genes.
This is the first experimental evidence for natural selection acting in different ways on different parts of the same flower.
And even more importantly, these evolutionary forces likely play a role in the large diversity of Orchid flower shapes around the world.