Chinese mountain plant confirmed carnivorous, validating Darwin's 150-year-old prediction

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Scientists have confirmed that a flowering plant native to the mountains in southwest China is carnivorous, supporting a prediction made by Charles Darwin more than 150 years ago.

The species, Saxifraga candelabrum , grows at elevations above 8,200 feet (2,500 meters) in the Hengduan Mountains, which connect the southeast portions of the Tibetan Plateau to the Yunnan-Guizhou Plateau.

The plant has stems and branches covered in sticky hairs that can trap small insects, predominantly nonbiting midges. However, scientists were unsure if these insects were being ingested by the plant. In a new study, researchers showed that S. candelabrum can digest and absorb nutrients from insects after capturing them, proving that the plant is carnivorous. The scientists published their findings Tuesday (Aug. 4) in the journal Nature Communications .

This is the first demonstrated case of carnivory in the saxifrage plant group, said study co-author Douglas Soltis , a distinguished professor of biology at the Florida Museum of Natural History.

Other carnivorous plants typically grow in moist, boggy soils, but S. candelabrum is found in dry alpine areas, Soltis told Live Science in an email. "These are all nitrate poor soils — that's why this carnivorous habit has evolved a number of times" to supplement the plants in vital nutrients, he said.

In 1875, Darwin theorized that two species closely related to S. candelabrum that also have sticky hairs — S. rotundifolia and S. umbrosa — may be carnivorous, based on their ability to catch insects. But the 19th-century naturalist couldn't show that the plants digested their prey or reused the nutrients in insect tissues — two conditions that are key to confirming carnivory in plants. Without these conditions, a species is considered simply "protocarnivorous," meaning it catches and kills insects or other small animals but doesn't digest or absorb them, according to the study.

"Although untested at the time, Darwin’s hypothesis provided an important inspiration for our study," study co-author Hang Sun , a researcher in the Chinese Academy of Sciences' Kunming Institute of Botany, told Live Science in an email.

The researchers conducted field observations and herbarium surveys of S. candelabrum , which revealed that mature plants with more numerous and longer branches had an average of 71 trapped insects on them. In the herbarium surveys, the team found that 43 out of 45 specimens had caught insects, suggesting that insect-trapping is a widespread phenomenon in this species.

Next, the study's authors showed that S. candelabrum possesses phosphatase, a digestive enzyme commonly found in carnivorous plants. They did this by labeling the phosphatase enzymes with fluorescent molecules to visualize them under a specialized microscope, a technique known as fluorescence tagging.

S. candelabrum secretes a viscous substance that coats its hairs, meaning insects get stuck on the stem and branches if they fly too close. (Image credit: Xin-Jian Zhang) To determine if S. candelabrum digests and absorbs nutrients from insects, the researchers labeled fruit flies with a stable form of nitrogen before feeding them to the plants. This led to a marked rise in stable nitrogen in S. candelabrum and in a different, known carnivorous species, which the team used as a control group. By contrast, a non-carnivorous plant that formed a second control group did not show an increase in stable nitrogen, indicating that S. candelabrum is carnivorous.

Finally, the researchers performed a genetic analysis, which revealed that S. candelabrum has genes in common with known carnivorous plants that are linked to prey attraction, digestion and nutrient absorption. Together, the results showed that S. candelabrum evolved carnivorous traits to cope with its nutrient-poor environment, adapting its hairs — which were initially used for herbivore defense — into insect traps, according to the study.

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Larger insects, which S. candelabrum requires for pollination, rarely get stuck to the hairs, the researchers noted in the study, suggesting that the species has perfectly balanced its needs through evolution .

Other Saxifraga species and plants belonging to the same lineage could turn out to be carnivorous, too.

"Many members of the Saxifragaceae, which includes genera other than Saxifraga , have sticky glandular hairs and are often covered with insects," Soltis said. "I would not be surprised if other members of the family are also carnivorous — as Darwin predicted."

The study broadens the scope of carnivorous plant research, Sun said. "Carnivory may therefore be far more widespread among flowering plants than previously recognized," he said.

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