A century after scientists first noted that the environment contributes to the evolution of adaptive differences among plant populations, scientists are on the verge of figuring out how that adaptation happens — by combining results from huge “common garden” experiments with genomic sequencing.
That’s the contention of Penn State molecular geneticist Jill Hamilton, a leader in a nationwide study of climate adaptation and hybridization in poplars, called PopUp Poplars. This 18-arboreta and university partnership, funded for $2.5 million over five years by the National Science Foundation, is spearheaded by Penn State, Virginia Tech, the University of Vermont and the University of Maryland.
The objective of the research is to characterize adaptive variation across natural hybrid zones between black cottonwood (Populus trichocarpa) and balsam poplar (Populus balsamifera), explained Hamilton, director of the Schatz Center for Tree Molecular Genetics in the College of Agricultural Sciences.
The research will explore the important role hybridization plays in adaptation to climate, Hamilton explained, with a goal to predict adaptation to changing climatic conditions by understanding how genomic ancestry, genomic variation and environmental variation interact to produce traits important to forest tree fitness.
“The genetic basis for local adaptation has been studied in smaller, reciprocal transplant experiments, but now we are pairing whole-genome-sequence data with large-scale networks of replicated common garden experiments within model systems such as perennial switchgrass and poplar trees,” she said, adding that one of the poplar common garden experiments is located at The Arboretum at Penn State.
Hamilton recently wrote an article about advances in understanding plant adaptation in the journal Current Opinions in Plant Biology, referencing a historical context.
“A hundred years after Swedish evolutionary botanist Gote Turesson first clearly described how locally adaptive variation is distributed within species, today scientists are close to making major breakthroughs in understanding mechanisms underlying adaptation from local populations to the scale of continents,” she wrote.
Turesson’s work stood in sharp contrast to most researchers at the time, who believed that differences within species reflected acquired characters, noted Hamilton, who is the Ibberson Chair of Silviculture Research in the Department of Ecosystem Science and Management. She pointed out that Turreson concluded that differentiation of plant populations was largely driven by natural selection, resulting from the evolution of locally adapted plant populations.
“His work on adaptation led him to coin the term ‘ecotype’ in 1922, a term that describes a genetically distinct geographic variety within a species, which is genotypically adapted to specific environmental conditions,” Hamilton said. “Typically, ecotypes are capable of interbreeding with other geographically adjacent ecotypes without loss of fertility or vigor. So, 100 years later — pairing contemporary genomic approaches with traditional forestry provenance trials — that’s what we are testing in the PopUp Poplars project.”
To get that study off the ground, researchers — led by Hamilton, who worked for North Dakota State University at the time — collected poplar cuttings in transects across the northwestern U.S. and southern Canada during the winter of 2020. They shipped those cuttings to Virginia Tech, where they were chopped up to create a bank of living poplar seedlings for future distribution across North America.
“At Virginia Tech, they had greenhouses full of poplar seedlings,” she said. “And when they got large enough that we could expect them to survive in the field, we sent 100 of those clones to each of our 18 common garden sites around the U.S., where they were planted. Now we will allow them to grow for years and assess which ones do best.”
At Penn State, Hamilton intends to get graduate and undergraduate students involved in the PopUp Poplars project, using genetic and environmental variation to predict tree health across complex environments. Students will learn about tree genetics, evolution, ecology and climate adaptation.
David Lowry, associate professor of plant biology, and Acer VanWallendael, postdoctoral scholar, both at Michigan State University Department of Plant Biology, contributed to the article.
The National Science Foundation Plant Genome Research Program partially supported this work.