New Pollination Mutualism Discovered in Tropical Asia

A team of scientists has discovered a new type of oil-flower/oil-bee pollination mutualism involving male-bee-pollinated orchids in tropical Asia. This is the first report of oil-offering orchid flowers in Asia and the first observation of male bees collecting floral oil from orchids. The study was published in the journal National Science Review.

Pollination is a vital process for the reproduction of flowering plants and the maintenance of biodiversity. Many plants rely on animals, especially insects, to transfer pollen from one flower to another. In return, these pollinators receive rewards such as nectar, pollen or oil.

Oil-flower/oil-bee mutualisms are interactions between plants that offer oil as a reward and bees that collect oil with specialized hairs or setae. These mutualisms are mostly known from South America and South Africa, where hundreds of plant genera and bee species are involved. However, in Asia, oil-flower/oil-bee mutualisms have been poorly studied, despite the presence of oil-offering flowers in Cucurbitaceae (cucumber family) and Ctenoplectra bees (oil-collecting bees) since the Early Eocene (about 50 million years ago).

The researchers found that 33 of 41 species of Dendrobium and Galeola orchids, two orchid genera, have fatty oil in the minute hairs on the labella, the lower lip of the corolla. These orchids are mainly visited by male bees of Ctenoplectra cornuta, not females, over a six-year field study in Xishuangbanna and Malipo, southern Yunnan Province.

Dendrobium is a large genus of mostly epiphytic and lithophytic orchids with more than 1,800 species that are found in diverse habitats throughout much of south, east and southeast Asia, including China, Japan, India, the Philippines, Indonesia, Australia, New Guinea, Vietnam and many of the islands of the Pacific. The center of distribution lies in the area from India to Indochina including China and Thailand.

The male bees collect floral oil from the orchids with their hind legs and thorax, and transfer pollen between flowers as they move around. The researchers observed that these male bees are the sole effective pollinators for 12 Dendrobium and one Galeola orchid species. They also found that these orchids have evolved several adaptations to attract male bees, such as bright colors, strong fragrance and long-lasting flowers.

The discovery that many orchid species offer oil and are pollinated by male Ctenoplectra cornuta, the females of which depend on Cucurbitaceae oil and pollen, illustrates multipartite oil-flower/oil-bee pollination mutualisms. This means that there are multiple interactions between different plant and bee species that share oil as a common resource.

The researchers suggest that this novel mutualism may have evolved through shifts in floral traits and pollinator preferences in response to environmental changes and competition. They also highlight the importance of conserving these rare and endangered orchids and bees that play a key role in maintaining the diversity and functioning of tropical ecosystems.

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