Lipstick vines get their name from their bright red, tube-shaped flowers. But one member of this group of plants has lost its lipstick-like appearance— its flowers are shorter, wider, and yellowish green in color. It also attracts shorter-beaked birds than its crimson cousins do, and it's found in different places. Scientists wanted to know how this plant evolved from its lipstick-like relatives. After observing birds visiting hundreds of plants and examining the plants' DNA, the researchers found that the story of the green flower's evolution contradicts a long-standing scientific "rule" about how plants evolve into new species.
The green flower at the center of the discovery (and the subject of a paper detailing said discovery in the journal New Phytologist) is called Aeschynanthus acuminatus. Its range extends across Southeast Asia from northern India and the Himalayas, across Indo-Chinese countries like Vietnam and Thailand, and into southern China. It's also found off the coast of mainland China on the island of Taiwan.
"Compared to the rest of its genus, this species has weird, unique flowers," says Jing-Yi Lu, the study's lead author and a research associate at the Field Museum who recently graduated with his PhD at the University of Chicago. Lu wanted to know how these flowers got to be so odd.
The red flowers of the other lipstick vines help attract sunbirds, whose long, slender beaks fit perfectly into the skinny, tube-shaped blooms. By going from flower to flower and drinking nectar, the sunbirds spread the plants' pollen. But while sunbirds are found throughout the mainland where the other lipstick vines live, there are no sunbirds on Taiwan.
When Lu was an undergraduate student in Taiwan, he began wondering about the short, green-flowered A. acuminatus growing on the island. "It made me curious about how this species could occur in Taiwan. Since we don't have sunbirds, there must be something else to pollinate it," says Lu.
Figuring out which species of birds pollinate a type of plant can be a tricky task. "Most of the time, you don't really see the birds, because they don't come too often, and they are shy," says Lu. To solve this problem, he set up camera traps that recorded birds coming and going from the flowers, and determined that a variety of birds with shorter beaks than sunbirds were visiting the short, wide, green flowers of A. acuminatus.
However, A. acuminatus doesn't only live in Taiwan— it's also found throughout mainland Southeast Asia, where there are plenty of long-beaked sunbirds. This presented an evolutionary riddle to Lu and his fellow scientists, something like the question of what came first, the chicken or the egg. Did A. acuminatus's ancestors find themselves transported to sunbird-less Taiwan, where they then had to evolve shorter, wider flowers to accommodate the shorter-beaked birds that lived there? Or did they split off from their tube-shaped cousins while they were still on the mainland, even though there were sunbirds around, and arrive in Taiwan later on? And, what kinds of birds pollinate the shorter, wider A. acuminatus flowers on the mainland?
"At the heart of our study is a question of where species originate," says Rick Ree, a curator at the Field Museum's Negaunee Integrative Research Center and the study's senior author. "There must have been a switch when this species evolved, when it went from having narrow flowers for sunbirds to wider flowers for more generalist birds. Where and when did the switch occur?"
For more than 50 years, botanists have turned to a sort of rule called the Grant-Stebbins model that aims to answer questions like these. "The Grant-Stebbins model says that when a plant species extends its range into an area with new pollinators or without its ancestral pollinator, that switch to a new pollinator will drive speciation," says Ree. "This model basically predicted that A. acuminatus should have evolved in Taiwan— when its ancestor colonized Taiwan, it left behind its old pollinators and evolved into a new species that could accommodate the new pollinators in its new home."
To test whether the Grant-Stebbins model was accurate for A. acuminatus, Ree and Lu analyzed the DNA of A. acuminatus specimens growing in Taiwan and in mainland Asia, as well as the DNA of other species of lipstick vines. The researchers used software that compared the similarities and differences in these plants' DNA and built family trees showing the likeliest explanations for how the plants were related to each other. The results were surprising.
"The branching patterns on the family trees we made revealed that the A. acuminatus plants on Taiwan descended from other A. acuminatus plants from the mainland— the species originated on the mainland," says Ree. Even though the lipstick vines on the mainland lived alongside sunbirds that could pollinate their skinny flowers, some of them split off into a new species that accommodated birds with shorter beaks (in addition to sunbirds, which were still capable of feeding from the shorter flowers). The Grant-Stebbins model, for this particular example of plant evolution, didn't fit.
"It was really exciting to get these results, because they don't follow the classic ideas of how we would have imagined the species evolved," says Lu.
While the mystery of A. acuminatus's origin has been solved, that discovery opens up new questions. "Why did this pollinator switch happen, when the original pollinator, the sunbird, is still there?" says Ree. "Our hypothesis is that at some point in the past, sunbirds stopped being optimal or sufficient pollinators for some of the plants on the mainland. There must have been circumstances under which natural selection favored this transition toward generalist passerine birds with shorter beaks as pollinators."
Beyond answering questions about the evolution of one particular species of flowering vine, Ree says that the project offers a look at the work that goes into scientific discoveries.
"This study shows the importance of natural history, of actually going out into nature and observing ecological interactions," says Ree. "It takes a lot of human effort that cannot be replicated by AI, it can't be sped up by computers— there's no substitute for getting out there like Jing-Yi did and spending months traveling to different field sites to see where these plants grow and what kinds of birds pollinate them. There's no substitute for that."
