A flower in Asia bucks the rules of evolution, puzzling scientists

By Taylor Nicioli, CNN

(CNN) — When plant biologist Jing-Yi Lu was earning his bachelor’s degree in Taiwan, he began to take note of the local flora. He paid particular attention to the lipstick vines, which, in Taiwan, have short, tubelike flowers with a yellowish-green hue.

However, in other parts of Asia, lipstick vines are known for their long, bright red, tubular flowers, which are primarily pollinated by sunbirds. The green-flowered species are also found across Asia alongside their red counterparts. But in Taiwan, where there are no sunbirds, the red-flowered species does not grow.

Lu wanted to know why the species he was observing in Taiwan looked different from its red-flowered relatives, and what was pollinating it if there were no sunbirds around. But figuring it out was easier said than done.

Typically, when a plant population spreads to an area that lacks its usual pollinators, the plant will strive to adapt, evolving so that it can be pollinated by local wildlife instead. This process is described in the Grant-Stebbins evolutionary model, which explains how pollinators can drive the formation of a new species.

Lu hypothesized that if the plants had come from mainland Asia and spread to Taiwan, they would have evolved shorter flowers so that they could be pollinated by wildlife on the island. But when he and three other biologists looked into the flowers’ DNA, they found something odd: The plants evolved before they spread into Taiwan, contradicting the Grant-Stebbins model.

“It was really surprising,” said Lu, who led the research as a doctoral student at the University of Chicago. Lu is now a research associate at the Field Museum of Natural History in Chicago.

“Because the result did not follow the simple prediction by the classic model, we need to find some alternative explanation for it, and it’s kind of exciting and also kind of puzzling,” Lu added.

Lu’s discovery, which was detailed in a paper published in January in the journal New Phytologist, highlights an exception to the long-standing evolution model.

But a mystery remains: Why would the flowers evolve before spreading into a new area, instead of the other way around? The researchers have some theories.

Oddball flower evolution

The lipstick vine with green flowers, known as Aeschynanthus acuminatus, is found across Southeast Asia, including northern India, the Himalayas, Vietnam, Thailand, southern China and Taiwan, according to a news release from the Field Museum.

To find out which birds were pollinating these flowers in Taiwan, researchers set up camera traps and found that various birds with shorter beaks than the mainland pollinators were visiting the lipstick vines, a result of the plants having a shorter and wider flower. On the mainland, they found shorter-beaked birds were also pollinating the green flowers in addition to the sunbirds that were capable of feeding from the shorter tubes as well as the longer, red flowers.

But when researchers looked into the lipstick vines’ family tree by extracting and analyzing the plant’s DNA, they found that the green flower species descended from plants on the mainland. Despite having long-beaked sunbirds to pollinate their flowers, the mainland plants had split and evolved to have shorter green flowers that supported a wider range of pollinators.

“It turned out to be that the data did not support the simplest explanation. It required a more complicated explanation. Ultimately, it still remains kind of mysterious, how plants switch from one pollinator to another and evolve into new species when the ancestral pollinator is still around,” said the study’s senior author, Richard Ree, a plant biologist and a curator at the Field Museum’s department of collections, conservation and research.

The researchers hypothesize that the sunbirds on the mainland might have dwindled at some point in the last few million years, and such a decline may have prompted the lipstick vines to evolve and branch out to other pollinators. However, there is no direct evidence of any period of absence of the sunbirds, Ree added.

“The Grant-Stebbins model is a simple verbal model, a caricature really, of how pollinator-driven speciation can happen. But we actually have very few real-world examples that support such a simple process, and scientists have identified many reasons why evolution would deviate from the Grant-Stebbins model,” said Kathleen Kay, a professor of ecology and evolutionary biology at the University of California, Santa Cruz, who was not involved with the study. “So it makes sense that when researchers look in detail, they find surprising results like this.”

The study authors are still studying the lipstick vine species while also examining close relatives to observe further the role of pollinators in the evolution and speciation of the plants, Lu said. While the question of the plant’s origin on the mainland is solved, the mystery of why it evolved there instead of Taiwan remains.

“What makes this study so impactful is that it tests the explicit hypothesis set forward by the Grant-Stebbins model and finds that things are more complex,” said Chelsea Specht, the Barbara McClintock Professor of Plant Biology at Cornell University in Ithaca, New York, who was not involved with the study. “While local adaptation to pollinators can certainly lead to speciation, the local environment itself can be highly variable — and shifts in pollinator density and efficacy over time might in fact benefit plants that have the variation necessary to respond to environmental change.”

“I think in most cases species studied don’t exactly fit this model, but the model gives us a place to start looking and the inspiration to find all the ways in which biodiversity breaks the mold,” Specht added in an email.

Beyond finding the origin of the lipstick vine species, Ree said the study emphasizes the importance of scientific fieldwork and observing ecological interactions. During the over 4,000 hours of pollinator observations, the researchers also took note of rodents visiting the vines in Vietnam, the first documented case of rodent pollination for plants in this genus, Lu said. The study authors look to put out research papers on their additional findings in the future, Ree said.

“We rely on plants for so much — food, clothing, shelter, medicine, cultural practices, decoration, even our mental well-being. So I really believe there is basic value in people understanding more about plant diversity, especially as we are altering the earth at such speed and scale,” Kay said in an email.

“Studies like this can show important areas to protect to preserve unique lineages and unique interactions and can tell us about how reliant certain animals are on particular plants.”

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