image: (A) Inflorescences of D. gibsonii. (B) Glandular hairs covering the labellum in D. chryseum. (C) A hand-section of the labellum, with the hairs stained with Sudan Ⅳ. (D) A male Ctenoplectra cornuta trying to enter a flower of D. denneanum that is occupied by another male bee (male symbol marked). (E-H) Male C. cornuta bees leaving the flowers of four Dendrobium species, each with pollinaria attached to their thorax (white arrowheads). Note the characterized hind legs of males (purple arrow). (I) A female C. cornuta collecting nectar, oil and pollen from Thladiantha subglobosa, noting a pollinarium left on the corolla (pink arrow). (J) A male C. cornuta collecting nectar from T. subglobosa. (K) Ctenoplectra mating on D. chryseum, the male with a pollinarium on its back is delivering floral secretion (pink arrow) which he collected from Dendrobium flowers to the female.
Credit: ©Science China Press
It is well known that animal-pollinated plants usually offer nectar or pollen to reward vectors for pollen transfer. While nectar is a kind of sugar water, pollen is protein-rich, must-need nutrient to bee babies. That honey human consumes is made by female bee workers. Fifty years ago, German scientist Stefan Vogel first identified that droplets are non-water miscible ‘nectar’ but lipids in some flowering plants, discovering the so called oil-flower which offers oil as a reward for female bees.
The oil-bees collect fat droplets with special setae or abdominal hairs and then mix it with pollen as larval food or use the oil for nest lining. Worldwide, oil rewards are offered by about 150 genera in 11 families whose flowers are exploited by 370 species of oil-collecting bees. To date, our understanding of the ecology and evolution of the oil-bee and oil-flower interactions are majorly from studies in South America and South Africa, although in Asia since the Early Eocene oil-offering flowers in Cucurbitaceae have coevolved with Ctenoplectra bees whose females have unique setae and behaviours for oil mopping.
The team found that the fatty oil in the minute hairs on the labella, the lower lip of corolla appeared in 33 of 41 species sampled in Dendrobium and Galeola, two orchid genera, providing first document of oil-offering orchid flowers in Asia. Surprisingly, these orchids are majorly exploited by male bees of Ctenoplectra, not females over 6-year field study in Xishuangbanna and Malipo, southern Yunnan Province. As predicted, female bees prefer visiting oil-flowers in Cucurbitaceae to collect oil, nectar and pollen. The observation of males collecting floral oil from orchids has not been reported before. In this report, male Ctenoplectra cornuta was observed as sole effective pollinator for 12 Dendrobium and one Galeola orchid species.
The discovery that many orchid species offer oil and are pollinated by male Ctenoplectra cornuta, and the females of which depend on Cucurbitaceae oil and pollen, illustrating multipartite oil-flower/oil-bee pollination mutualisms. When Dendrobium inflorescences were set out in the field with cucurbit oil-flowers nearby, oil-bee visit frequencies (visits per flower per hour) to the orchid flowers increased hundreds of times, suggesting that the orchids’ pollination success was greatly attributed to the co-flowering of Cucurbitaceae species. Dendrobium genus is species-rich (1500 species), but the most economic importance orchids have been declined in tropical Asia in last two decades. Given that their reproductive success extremely replies on the male oil-bees, this finding also highlights the challenge of maintaining or restoring natural orchid populations.
See the article:
Multipartite oil-flower/oil-bee mutualisms involving male-bee-pollinated orchids in tropical Asia. National Science Review, DOI: 10.1093/nsr/nwae072
Journal
National Science Review