Researchers at Harvard and the University of California, Santa Barbara have discovered the driving force behind the rapid radiation of columbine flowers—the explosive speciation from a single recent floral ancestor.

According to findings published in “Proceedings of the Royal Society B: Biological Sciences,” cell elongation—rather than cell proliferation—is responsible for the varying lengths of floral spurs in these species. These nectar-containing spurs, which protrude from the bases of the flower petals, have evolved to attract and fit the tongue of specific pollinator insects.

“In the end, we found that the traditional ideas of how the spurs develop were wrong,” said co-principal investigator Elena M. Kramer.

Kramer, a professor of organismic and evolutionary biology, began studying the Aquilegia genus over 10 years ago.

The young genus has undergone rapid radiation into about 70 columbine flower species in the past 3 million years.

“The petal spurs play an important role in allowing proper evolution,” said Joshua R. Puzey, a graduate student in the Department of Organismic and Evolutionary Biology and co-lead author of the study.

For this work, Kramer partnered with co-principal investigator L. Mahadevan, a professor of applied mathematics, organismic and evolutionary biology, and physics at Harvard. While Kramer and Puzey analyzed the problem from an evolutionary standpoint, Mahadevan and co-lead author Sharon Gerbode, a postdoctoral fellow at the School of Engineering and Applied Sciences, approached the problem through quantitative morphology, the statistical analysis of organism shape.

“We just came at it from completely different angles,” said Puzey.

Until now, most biologists assumed that cells in the spurs divide and proliferate to produce spurs of different lengths. However, the investigators discovered that the spur cells stop dividing when the organ is only 2-20 percent of its final length. From there, the cells change shape and elongate, yielding the final size of the spur.

“You can explain 99 percent of the difference in spur length with difference in cell length,” Kramer said. “It made me appreciate that there were a lot of questions we could answer with simple experiments.”

The scientists plan to further explore the process of diversification in these flowers by examining cell division and expansion in older plant species.

“The best thing about the project was the really rewarding collaboration,” Kramer said. “I think it was a good combination of people willing to speak the same language.”