UI Prof Works with International Team to Study How Plants Evolved to Weather the Cold
Sunday, December 22 2013
MOSCOW, Idaho – Dec. 22, 2013 – A team of plant researchers including University of Idaho biologist David Tank has assembled the largest dated evolutionary tree, using it to show the order in which flowering plants evolved specific strategies, such as the seasonal shedding of leaves, to move into areas with cold winters. The results will be published Dec. 22 in the journal Nature.
Early flowering plants are thought to have been woody — maintaining 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, the study’s lead author and an assistant professor of biology in the George Washington University’s Columbian College of Arts and Sciences. “So over time, if plants moved into colder climates, they’ve had to figure out how to get around these problems.”
The research team 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 aboveground 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 team 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.
Then, researchers took that information and combined it with an unprecedented dated evolutionary tree with 32,223 species of plants, allowing them to model the evolution of species’ traits and climate surroundings. This “timetree,” which can be viewed in the OneZoom Tree of Life Explorer web application, 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.”
To build on these findings, Zanne and others 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 besides just freezing.
“One of the great things that we’ve done here is that all of the resources that we compiled for these analyses, including the tree and taxonomic resources, are publically available,” Tank says. “The working group has put together a large plant trait database, so with the phylogenetic context that we’ve provided, there are really a lot of questions you could explore at this scale.”
The team will make available at Dryad the data and tools developed for this study for other researchers’ use. 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.
To view the tree in OneZoom, visit www.onezoom.org/vascularplants_tank2013nature.htm
Note to editors: Images to accompany this study are available upon request
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