Sex lives of orchids reads like science fiction

Newswise — New research, featured in the Botanical Journal of the Linnean Society, employed the database to uncover that orchids exhibit notable variation in extensively specialized pollination tactics that vary among worldwide areas.

The recently issued database encompasses more than 2900 orchid species, providing intricate data on their pollinators' identities and the methods they employ to entice them. Significantly, the database unveils reproductive biology patterns based on habitat, geography, and taxonomy.

“From these data, we identify general patterns and knowledge gaps limiting our understanding of orchid biology at the global level,” Dr Phillips said.

Charles Darwin utilized orchids as a subject of study to explore evolution, asserting that their intricate flowers evolved as an adaptation to improve the likelihood of pollen transfer between plants, subsequently enhancing the fitness of their offspring.

"Due to their distinctive floral characteristics and frequently unconventional methods of attracting pollinators, orchids have played a prominent role in comprehending the adaptations of flowers to their pollinators," remarked Dr. Phillips.

Undoubtedly, Darwin made a well-known prediction regarding the Madagascan orchid Angraecum sesquipedale, which possesses an extraordinary 40 cm long nectar spur. He postulated that this orchid would be pollinated by a moth equipped with an equally remarkable and elongated proboscis.

Leveraging the newly available database, the research paper, spearheaded by Dr. James Ackerman from the University of Puerto Rico, discovered that more than 75% of orchid species rely on pollinators for their reproductive success. Intriguingly, nearly half of the studied orchids did not offer any form of reward to visiting animals; instead, they employed deceitful strategies to entice their pollinators.

Orchids exhibited a tendency to specialize in a single dominant pollinator species, regardless of whether they resided in the rainforests of Costa Rica or the montane grasslands of South Africa. However, this trend was even more pronounced among orchids that employed deceptive tactics to attract pollinators.

Dr. Noushka Reiter, a co-author of the study, emphasized that the reliance of many orchids on a single pollinator species makes them highly susceptible to anthropogenic threats such as climate change. The potential loss of pollinators would not only endanger the pollinators themselves but also result in the extinction of orchid species dependent on them.

The pollination strategies devised by orchids present a captivating narrative akin to a crime thriller. Notably, Australia serves as the global hub for pollination through sexual mimicry, where a diverse array of insect groups, including wasps, bees, and gnats, are deceived by these intricate and elaborate deceptions orchestrated by orchids.

In South Africa, orchids employ the strategy of mimicking carrion, while on Reunion Island they mimic rainforest fruits, and in Brazil, they mimic the scent of aphids. These deceptive tactics are utilized by orchids with the specific goal of misleading pollinators.

In the enchanting realm of the American tropics, hundreds of orchid species emit fragrances that specifically attract certain bees. These bees collect these aromatic offerings and incorporate them into their courtship bouquet, adding a touch of romance to the intricate relationship between orchids and their pollinators.

Science fiction? 

In Australia, a fascinating orchid known as Caladenia barbarella exists, earning its name from the Latin term for "little beard" that references the flower's appearance. Interestingly, the name also alludes to the notorious comic book character who was renowned for her adventurous sexual escapades. This sexually deceptive orchid adds an intriguing twist to the diversity of orchid species found in Australia.

Dr. Phillips highlighted a surprising revelation from the database, stating that "a defining characteristic of the orchid family is the significant proportion of species that utilize deceptive tactics to attract pollinators. They achieve this by capitalizing on the sensory capabilities of pollinators through chemical, visual, or tactile stimuli, often in combination." This observation underscores the remarkable adaptability and ingenuity of orchids in their pollination strategies.

Orchids demonstrate two primary forms of deceitful pollination. The first form revolves around food deception, wherein the orchid adopts visual or olfactory characteristics resembling a specific type of food to allure pollinators. The second form is sexual deception, in which male pollinators are enticed to visit flowers that exhibit visual, tactile, and/or olfactory signals mimicking a female insect. These fascinating strategies highlight the remarkable ability of orchids to manipulate their surroundings for successful pollination.

“The floral signals can be so persuasive that insects attempt copulation and may even ejaculate,” Dr Phillips said. 

“I’ve even had the wasps fly in through the car window at the traffic lights and start making love to the orchids specimens on the front seat”. 

Far from being a freak occurrence, this strategy is now known from 20 genera around the world, including 100s of orchid species.

Until now, a third form of deception known as brood-site deception, commonly associated with mimicking larval food sources like mushrooms, dung, or carrion to attract female flies seeking suitable egg-laying sites, was believed to be more prevalent in other plant families and rarely observed in orchids.

According to the database:

• In scientific studies, the orchid diversity of Australasia and Africa has been relatively well-covered, with approximately 15% and 20% coverage, respectively. However, the orchid floras of Temperate Asia, Tropical Asia, and South America are considerably underrepresented in research, indicating a need for further exploration and documentation of orchid species in these regions.
•  Approximately 76% of orchid species are entirely dependent on pollinators for reproduction. 
• Highly specialised pollination systems are frequent, with approximately 55% of orchids studied having just a single known pollinator species.
• Among orchid species, approximately 54% provide rewards to their pollinators. Out of those rewarding species, about half (51%) produce nectar as a reward. Orchids that rely on insects attracted to floral fragrances for pollination account for around 24% of the rewarding species, while approximately 15% produce floral oils. The remaining 10% consists of species that offer various rewards such as trichomes (food hairs or pseudopollen), resins, pollen, or sleep sites to entice their pollinators.
• Deception, encompassing food, brood-site, and sexual deception, was documented in 46% of the orchid species within the database. Among the deceptive species, food deception was the most prevalent, accounting for 60% of the deceptive records. Sexual deception, on the other hand, constituted 38% of the instances of pollination through deceit and was observed in 20 different orchid genera.
• Wasps and bees are the group that make up the most common type of pollinator with flies and mosquitoes coming in a close second
• The authors caution that there is much data collecting yet to be done.

Dr. Phillips highlighted that despite the inclusion of more than 2900 orchid species, the database represents less than 10% of the entire orchid family. Furthermore, there is a significant underrepresentation of tropical regions in Africa, South America, and Asia, which are known as centers of orchid diversity. This gap in research is particularly noticeable among epiphytic orchids, indicating the need for further studies and exploration in these regions to better understand orchid pollination.

The study of orchid pollination presents a remarkable opportunity to uncover new and unconventional pollination strategies while unraveling the adaptations that flowering plants have developed to attract pollinators. Although the tropics remain largely unexplored in orchid biology, it is worth noting that even some of the well-known Australian orchids have not been thoroughly studied in terms of their pollination mechanisms. There is vast potential for exciting discoveries and a deeper understanding of orchid biology awaiting further research in these areas.

“Aside from scientific interest, this has important practical implications for conservation, given that many orchid species are reliant on one primary pollinator species for their persistence,” Dr Phillips said.