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March 2, 2015   Columns Articles | Research News | UGA researchers unlock new way to clone pest-fighting…
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Scott Merkle, associate dean of research in the Warnell School of Forestry and Natural Resources, was lead author in an effort to successfully cryogenically freeze germplasm from hemlock trees being wiped out across the eastern U.S. Photo by Peter Frey

UGA researchers unlock new way to clone pest-fighting hemlock trees

Sandi Martin

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By Sandi Martin | March 2, 2015
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For the first time, UGA researchers successfully have cryogenically frozen germplasm from hemlock trees being wiped out across the eastern U.S. by an invasive insect, hemlock woolly adelgid. The researchers also have unlocked a new way to clone the few hemlock trees apparently fighting off the insect, which may lead to a solution for the pest.

In a new paper published in Trees-Structure and Function, researchers in the Warnell School of Forestry and Natural Resources outline how they were able to generate hemlock tissue cultures, cryogenically store them and then grow plants from the cultures after thawing them several months later-the first to successfully do so.

As part of their efforts to freeze the germplasm, they also developed a method that will allow them to clone hemlocks, particularly important as they seek to propagate trees naturally resistant to hemlock woolly adelgids, which have destroyed millions of hemlocks in 18 states since they accidentally were introduced into the eastern U.S.

There are only two native hemlock species in the eastern U.S.-eastern and Carolina-and both are in terrible danger from an insect that first appeared in Virginia in the 1950s. The hemlock woolly adelgid, a pest native to East Asia, kills hemlocks possibly by injecting a toxin into the trees while feeding on sap. It has spread from Virginia and virtually exploded in the Appalachians, said Warnell professor Scott Merkle.

"It looks like a bomb went off where there were once pure hemlocks," Merkle said. "It's just dead trees because there doesn't seem to be much natural resistance."

Merkle, who is also Warnell's associate dean of research, was lead author on the paper and worked with Warnell researchers Paul Montello, Hannah Reece and Lisheng Kong.

Woolly adelgids could kill most of the hemlocks in the Appalachian region in a matter of years if the insect remains unchecked, according to a 2009 study by the U.S. Forest Service. Unfortunately, researchers are not yet sure how the insect is killing hemlocks.

A number of researchers are studying ways to fight the infestation. Merkle is looking at ways to not only introduce natural resistance to newly planted hemlocks, but also to successfully store hemlock germplasm cryogenically to conserve it in case a solution isn't found before they are wiped out.

Merkle and his research team took a different approach to standard storage methods: Using seeds from surviving hemlocks collected by cooperators at the Alliance for Saving Threatened Forests and North Carolina State University, they created in vitro cultures of a number of eastern and Carolina hemlocks that they then froze in liquid nitrogen at minus 196 degrees Celsius-something no one else had done successfully. The researchers cryofroze several samples from different hemlock lines from around the Southeast for several months, then thawed them out, allowed them to re-grow and began to produce trees from them. Of the five hemlock lines they tested, all samples of three Carolina lines and one eastern hemlock line regrew after coming out of cryostorage.

Merkle said the ability to cryostore and recover hemlock cultures, followed by production of new trees from them, provides a practical approach for storing the germplasm of a large number of trees indefinitely, so that the species can be repopulated once a system to deal with the adelgid is in place.

Being able to grow them after they're thawed won't make them resistant to the woolly adelgid, Merkle said, but it does mean that if need be, researchers might be able to save samples of the hemlocks from extinction. These cultures also allow the researchers to assist with testing other methods of introducing resistance to the insect, including trying to create clones of hybrids with Asian hemlocks that do have a natural resistance to the woolly adelgids.

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