ANN ARBOR—A team of researchers including a University of Michigan evolutionary biologist has assembled the largest dated evolutionary tree of flowering plants. They used the tree to show the order in which those plants evolved specific strategies, such as the seasonal shedding of leaves, to move into areas with cold winters.
Early flowering plants are thought to have been woody—which maintain a prominent stem above ground across years and changing weather conditions, such as maple trees—and restricted to warm, wet tropical environments. But they have since put down roots in chillier climates, dominating large swaths of the globe where freezing occurs. How they managed this expansion has long vexed researchers searching for plants' equivalent to the winter parka.
"Freezing is a challenge for plants. Their living tissues can be damaged. It's like a plant's equivalent to frostbite. Their water-conducting pipes can also be blocked by air bubbles as water freezes and thaws," said Amy Zanne, lead author of a study scheduled for online publication in the journal Nature on Dec. 22 and an assistant professor of biology at George Washington University. "So over time, if plants moved into colder climates, they've had to figure out how to get around these problems."
U-M computational evolutionary biologist Stephen Smith is a co-author of the paper and led the effort to combine a database on flowering-plant traits with a dated evolutionary tree to create the most comprehensive view to date of the evolutionary history of flowering plants.
"We constructed an enormous evolutionary tree to specifically address some of the most important and still unanswered questions in plant evolution and distribution," said Smith, an assistant professor in the Department of Ecology and Evolutionary Biology and a faculty affiliate at the U-M Center for Computational Medicine and Bioinformatics.
Team members identified three repeated evolutionary shifts they believe flowering plants made to fight the cold. Plants either:
- dropped their leaves seasonally, shutting down the pathways that would normally carry water between roots and leaves;
- made skinnier water-conducting pathways, allowing them to keep their leaves while reducing the risk of air bubbles developing during freezing and thawing, which would shut down those pathways (the fatter the pathways, the higher the risk); or
- avoided the cold seasons altogether as herbs, losing above-ground stems and leaves and retreating as seeds or storage organs underground, such as tulips or tomatoes.
The researchers also identified the order of evolutionary events. Most often, woody plants became herbs or developed skinnier pathways before moving into freezing climates. In contrast, plants usually began dropping their leaves after moving into freezing climates.
Identifying these evolutionary adaptations and likely paths to them required the team to build two robust sets of data. First, the researchers created a database of 49,064 species, detailing whether each species maintains a stem above ground over time, whether it loses or keeps its leaves and the width of its water-carrying pathways. To these they added whether it is ever exposed to freezing, using resources from the Global Biodiversity Information Facility and a global climate database.
Smith and his colleagues then combined the database with a dated evolutionary tree containing 32,223 species of plants, allowing them to model the evolution of species' traits and climate surroundings.
"Recent advances in computational methods and resources have made this kind of research possible like never before," Smith said.
The "timetree" created by the team is the most comprehensive view yet into the evolutionary history of flowering plants.
"Until now, we haven't had a compelling narrative about how leaf and stem traits have evolved to tolerate cold temperatures," Zanne said. "Our research gives us this insight, showing us the whens, hows and whys behind plant species' trait evolution and movements around the globe."
Researchers will use the massive tree to explore other aspects of the evolutionary history of plants, especially to examine how plants respond to additional environmental pressures, in addition to freezing. The data and tools developed for this study will be made available at the Dryad digital repository.
The National Evolutionary Synthesis Center, National Science Foundation (grant number EF-0905606) and Australia-based Macquarie University's Genes to Geoscience Research Centre funded this study.