Extremely Rare Orchid Tricks Horny Beetles Into Carrying Its Pollen

Science
پنجشنبه 5 فروردین 1400 ساعت 16:17
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Smith sonian mag
For decades, the southern African orchid Disa forficaria was known only by botanical illustrations and a fuzzy slide captured in 1966. Only 11 specimens of the white and magenta orchid had been recorded since the early 1800s, and by the 21st century, most experts assumed the species was extinct.
So, when a single Disa forficaria plant appeared in South Africa’s Fernkloof Nature Reserve in 2016 , it made a splash in the botanical world.
The reserve is located in the Cape Floristic Region , which is recognized as a Unesco World Heritage site for its stunning diversity of plant species. So little was known about the one-foot-tall orchid that it was carefully supervised both for its protection and to study its basic growth and blooming patterns. It also attracted visits from orchid enthusiasts like Callan Cohen, a biologist at the University of Cape Town’s FitzPatrick Institute of African Ornithology.
One afternoon in March of 2016, Cohen visited the plant just after the flower had bloomed. Its light-colored outer petals surrounded a dark purple center, which has a pair of small, wavey petals at one end that branch out to either side.
While Cohen admired the flower, an insect landed on it.
“I could see that it was mating. It was quite obvious,” says Cohen. “He puts his head down where these other two little antennae [petals] stick up, and the way he was sort of vigorously moving his abdomen made me realized what was going on and that it needed to be studied further.”
At first, Cohen thought the insect was a wasp. Several orchid species use sex pheromones to attract bees and wasps and fool them into performing pollination. But a closer look revealed that the insect on Disa forficaria was actually a male longhorn beetle. Beetles are the largest group of animals on Earth with over 350,000 species, but until that point, no orchid on record had tricked a beetle to pollinate it through innuendo alone.
When the beetle flew away, Cohen saw a yellow packet of pollen stuck to its underside—mission success for the orchid.
Callan Cohen examines the orchid, Disa forficaria , when it has one small flower in full bloom. (Steve Johnson)
Cohen contacted Steven Johnson , an evolutionary biologist at the University of KwaZulu-Natal who specializes in deceptive pollination in orchids. Along with an international research team, they studied that one specimen of Disa forficaria and discovered a wealth of knowledge about the relationship between the insect and the orchid. The team’s findings, published today in the journal Current Biology , show how a small but fragrant flower redirects a common beetle’s sex drive to fulfill its own reproductive needs .
Studying the orchid presented challenges to the scientists. The plant may have several buds at once, but only one flower blooms at a time, and that flower only stays open for one or two days. Then a few days pass without any flowers before the next bud opens. The flower also only blooms every other year. Altogether, the researchers could only observe pollination for eight days in March of 2016 and four days in March of 2018.
Once the researchers identified Disa forficaria as a sexually deceptive orchid, they zeroed in on its strategy: imitating the beetles’ sex pheromones. When many female insects are ready to mate, they emit a potent eau de bug . Males of the same species have antennae that detect that chemical so that they can partner up. About 400 orchid species have evolved to take advantage of the males’ single-mindedness by releasing their own versions of specific species’ sex pheromones to attract pollinators.
Scientists noticed beetles were most likely to land on the flower right after it opened, when researchers suspected the orchid’s fragrance was at its strongest. The insects were not looking for flowers, but for females, and they were totally fooled. A sample gathered from the orchid with very fine forceps after one beetle’s visit showed that it had ejaculated on the flower during its misinformed intercourse.
“The thing that stood out to me is that they found sperm on the orchid,” says Amy Brunton-Martin , an evolutionary biologist at Manaaki Whenua Landcare Research in Auckland, New Zealand, who was not involved in the study. “I've always suspected that, perhaps, we haven't been looking as closely as we could at sexually deceptive relationships and that we might be finding that these extreme examples of deceit are more common than we used to think.”
Brunton-Martin’s research focuses on the only other orchid documented to inspire ejaculation from its pollinator, a solitary wasp found in Australia and New Zealand.
When the longhorn beetle pollinates the orchid, the resemblance between the orchid’s purple petals and the beetle’s antennae become clear. (Callan Cohen / FitzPatrick Institute)
For longhorn beetles, Disa forficaria ’s deceit seems to be both physical and chemical. When a beetle lands on the orchid, the purple inner structure fits perfectly underneath it. The beetle bites and strokes the petals beneath it, which resembles mating behavior recorded in other longhorn beetles, while inserting its aedeagus—essentially a penis—into a cleft at the other end of the flower.
But the beetle gets drawn in by the flower’s best imitation of a female’s sex pheromone.
“Its whole life is devoted to finding that signal,” says Johnson. The beetle is so sensitive to the flower’s scent that “it probably is almost unaware of any other signal in the habitat other than this one.”
The researchers wanted to identify the specific chemical in the flower’s fragrance that catches the longhorn beetles’ attention to better understand exactly how the orchid is sexually deceptive. But they only had one plant to work with, which presented a hurdle. The earliest research on insect pheromones required about a half-million female silkmoths to extract a few milligrams of pheromone; the smallest recent study of plant pheromones used 20 flowers. The new study used the extract from just one flower.
The extract held the complicated mix of all of the flower’s fragrant chemicals. So the scientists ran the extract through a tool called a gas chromatograph to separate the chemicals. They then snipped the antennae off of three anaesthetized longhorn beetles, connected the antennae to an apparatus that measures their electrical response and exposed them to each chemical from the extract in turn.
One chemical caused a reliable reaction in every antenna. And because male beetles’ antennae have evolved to detect sex pheromones, that was the best bet to be the orchid’s intercepting signal.
The team had mere microliters of the mystery chemical—so little sample that it wasn’t even visible in its vial. Johnson ferried the vial from South Africa to a conference in Switzerland, where he handed it off to a friend, who brought it to Aleš Svatoš at the Max Planck Institute for Chemical Ecology in Germany. Svatoš used the sample to determine the precise molecular structure of the chemical, and another colleague created synthetic versions of it and slight variations.
Then they sent a vial of the synthetic chemicals to South Africa so that Johnson could see the beetles’ reaction in the wild.
It was “one of those amazing moments in your life where you just open this vial, and a few minutes later, these beetles started arriving,” says Johnson. “Just completely amazing.”
The researchers put the different varieties of the synthetic flower fragrance on to artificial flowers. The beetles’ preference was strikingly obvious. They repeatedly flocked to a molecule that the researchers have now named “disalactone.”
The paper has “opened up doors for whole lots of research,” says Brunton-Martin. If scientists can locate a female longhorn beetle, they may be able to confirm whether the female’s sex pheromones match disalactone. Other future research could tackle how the orchids may impact the beetle’s population and evolution and whether other orchids use sexual deception on beetles for pollination.
“It's like a launching platform,” says Brunton-Martin. “I think this really highlights how special orchids are in that, they're just able to kind of fill every niche, take advantage of every possibility.”
Experts suspect that sexually deceptive orchids’ pollination strategy is what lets them persist even when they are extremely rare. Humans may struggle to find them, but pollinators have exactly the right equipment to track them down.
For now, Disa forficaria has returned to obscurity. In 2019, the specimen used in the research study disappeared. A hole had been dug in the location where the orchid was, and scientists don’t know whether an animal or a person made the hole. Although the only known plant is gone, the orchid’s story has one final twist.
A male longhorn beetle with pollen from Disa forficaria on its underside (Steve Johnson)
While the researchers tested disalactone’s ability to attract beetles at Fernkloof Nature Reserve in early 2020, after the disappearance of the orchid, three beetles arrived with bright yellow packets of pollen stuck to their undersides. DNA analysis confirmed the pollen came from Disa forficaria .
“It gives us some hope that the species is still around,” says Johnson. “But well off the beaten path, as it were, in areas where humans just haven't looked.”
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