A few other flowering plants have bellows-like systems to release pollen, but those are activated when a pollinator’s beak or paw puts unintended pressure against the stamens, or male reproductive parts. (Related: “Beyond Bees: 4 Surprising Facts About Pollination.”)
The new research details the first example in which birds, plucking at stamens one by one, are the only target, and the first known case in this plant family of a flower offering a food reward of this kind.
“It’s a great example of how intricate the relationships can be that have evolved between flowers and their pollinators,” said Jürg Schönenberger of Austria’s University of Vienna, who co-authored a study on the subject published July 3 in the journal Current Biology.
The Big Squeeze
The flowering plant, of the genus Axinaea, lives in the mountains of Central and South America.
Its pollination works like this: Bulbous appendages on its stamens attract hungry birds. When a bird’s beak nabs an appendage, it “functions like a paper bag full of air,” Schönenberger said.
“When the bag is squeezed, the air contained in it is forced through a narrow connection into the pollen chambers. The resulting air current in the pollen chambers [tubular structures with a porelike opening at the end, like a syringe] then flushes out the pollen grains through a pore at the opposite end.”
That blast of pollen to the face virtually ensures the bird will transfer it to a plant’s female parts as it continues to forage—thus allowing the plant to reproduce. (Read more about pollinators in National Geographic magazine.)
Reproducing isn’t easy for plants: They’re “under tremendous pressure to get their gametes [sex cells, in the pollen] to the female flower parts—a very tiny target, much smaller than a soccer goal to a player,” said Susanne Renner of the University of Munich in Germany, who studies the family of plants (Melastomataceae) to which Axinaea belongs.
“This has resulted in a seemingly unending diversity of pollination mechanisms,” including the bellows.
The bellows strategy was first described in 1899 in another genus of the same plant family, she said, “but the way Axinaea does it has never before been described.”
More commonly, she says, pollen is offered more or less openly on the stamens, or the prize is shaken loose by “buzz pollination,” in which bees apply vibrations using their flight muscles to extract the powdery stuff.
She says it’s likely that the newfound mechanism evolved from buzz pollination, which is found in about 98 percent of the 5,000 species in the Melastomataceae family. (See more pictures of pollinators.)
When study leader Agnes Dellinger and colleagues first started examining Axinaea, it wasn’t clear how the plant worked or what pollinator(s) it would attract.
“Our hypothesis was actually that beetles might be the legitimate pollinators of Axinaea,” Schönenberger said, since the insects ”are known to be interested in food bodies and other floral tissues in other plants.”
But field observations proved it was birds.
“This was completely unexpected, a real surprise for all of us!” he said. “We were even more surprised when we realized that the birds are actually consuming the entire stamens, the male reproductive organs,” which hadn’t been seen before. (See “Earliest Bird Pollinator Found in Germany.“)
To figure out how the complex system worked, the scientists used light microscopy, scanning electron microscopy, and high-resolution x-ray computed tomography to get a clear picture of the anatomy and function of the stamens.
“We were also very interested to find out what the birds are going for. Why are they feeding on the stamens?” The chemical composition of the stamens gave them the answer: ”They contain a lot of sugar, similar amounts as in berries,” Schönenberger said. Jackpot.
The newfound partnership is a win-win: “reliable reproduction for the plant, efficient foraging for the bird,” said Stacey Smith of the University of Colorado, Boulder, who studies floral evolution and genetics.
But the downside of such mutual reliance, built up over some 150 million years of co-evolution, is that both species are at higher risk if either’s population declines.
“This group of species is found in a unique part of the Andes on the border of Ecuador and Peru, a relatively low-elevation area compared with the rest of the range” that’s more susceptible to development than higher-altitude areas are, Smith said. Not all of it is protected.
In the case of the pollen-spitting Axinaea, if habitat fragmentation or loss were to hit key bird populations hard, “it might not be able to attract another animal to consume the bellows,” she said, “so the plant might go extinct as well.”
And what a huge blow to pollinator and floral biodiversity that would be.
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