Penn State geneticists monitoring new chestnut genes

Penn State geneticists monitoring new chestnut genes

University Park, Pa. -- Now that a blight-resistant chestnut tree is almost a reality, geneticists and forest biologists in Penn State's College of Agricultural Sciences are monitoring the genetic makeup of the hybrid trees to select strains that have the most American chestnut traits and researching how best to reintroduce chestnuts to Pennsylvania forests.

A fungus from Asia accidentally introduced into American forests a century ago created a blight that wiped out American chestnut trees across the Northeast. The huge trees -- with their valuable lumber and bountiful nuts for both humans and wildlife -- once were the dominant species in Pennsylvania forests.

During the last 20 years, scientists working for The American Chestnut Foundation have crossed Chinese chestnut trees -- which resist the blight they long have been exposed to -- with American chestnuts and then repeatedly "backcrossed" those hybrids with additional American parents to develop a tree that essentially is an American chestnut with blight resistance.

"We're almost there," said Kim Steiner, professor of forest biology and director of Penn State's Arboretum, where a grove of chestnuts has been started that eventually will contain 30,000 trees. "We have third-generation backcrossed trees in the ground right now that we think will yield blight-resistant seeds. Theoretically, one out of every 64 trees in the new arboretum orchard should have the same high level of resistance as Chinese chestnut."

Chestnut trees also are growing on plots at Penn State's agricultural research center at Rock Springs and in the University's experimental forest at Stone Valley. Researchers also are watching over test plantings on a state game land at Fox Hollow and on Tuscarora State Forest land in Perry County. Not all are blight resistant; some were planted as part of ongoing research on how best to reintroduce chestnuts into Pennsylvania forests once blight-resistant seed becomes available.

"To greatly simplify what we have found," explained Steiner, "chestnuts can thrive in today's forest habitats. On the right kind of sites, they compete extremely well with other trees and vegetation. The only thing they will not withstand is deer browsing at typical deer population levels. And they need sunlight -- clearcutting or light shelterwood cuts will be needed to reintroduce chestnut into forests."

The goal, explained John Carlson, a geneticist who is director of Penn State's Schatz Center for Tree Molecular Genetics, is to end up with an American chestnut tree that has no traits from the Chinese chestnut except blight resistance. He and a graduate student spent two years developing a simple, inexpensive test using a proven, decades-old molecular technique, called a dot-blot, to examine the genomes of blight-resistant chestnut trees produced by The American Chestnut Foundation breeding program.

"We want to repopulate the forest with trees that are as much like the American chestnut trees as possible," said Carlson. "So it was important to create an index of how much American chestnut genetic material is in each of the new blight-resistant trees. There are at least 30,000 genes that make up a chestnut tree. The simple and quick dot-blot technique lets us make a very broad assessment of the genetic background of each tree and has proven to be highly accurate. It is almost embarrassing how widely known the dot-blot technique is, but apparently no one tried this particular technique before for this task."

The same question was addressed through a "morphometric" analysis of visual traits by an honors student at Penn State who conducted his thesis project with Steiner. "Botanists use a fixed set of plant traits to distinguish American chestnut from Chinese chestnut" said Steiner. "If the breeding program is successful, the blight-resistant generation that we are producing now with the Arboretum planting should look like American chestnut."

Using a composite index derived from measurements of 24 leaf and twig traits, the student found that 96 percent of third-generation backcrossed trees could not be distinguished from American chestnut, and none would be mistaken for Chinese chestnut.

"The DNA analysis and the study of visible plant characteristics tell us the same thing," according to Carlson. "The first generation of blight-resistant chestnut trees will be 'American' as far as most people could tell, but perhaps not quite the real deal."

Now that blight-resistant chestnut tree seed soon will be available, Penn State biologists are ready to guide reintroduction of the species back into the forest. Full ecological restoration of the American chestnut will take literally decades, according to Steiner. "We can begin when the Arboretum orchard starts yielding seed," he said. "In time, I predict that continued breeding and new molecular techniques will produce trees that are even better than the ones we have now, but the current generation of trees promises an excellent start."

For more news from Penn State's College of Agricultural Sciences, visit http://aginfo.psu.edu