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May 25, 2005

Dream takes root

I grew up a 10-15 minute walk from the Hoar Bird Sanctuary, where I spent many a happy childhood hour wandering. In that sanctuary, grew a stunted, blighted American Chestnut tree:

American chestnut was once the most important tree of the Eastern North American hardwood forest. One fourth of this forest was composed of native chestnut. According to a historical publication "many of the dry ridge tops of the central Appalachians were so thoroughly crowded with chestnut that, in early summer, when their canopies were filled with creamy-white flowers, the mountains appeared snow-capped."

The nut was a central part of eastern rural economies. Communities enjoyed eating chestnuts and their livestock was fattened by the nut. And what wasn't consumed was sold. Chestnut was an important cash crop for many Appalachian families. Holiday nuts were railed to New York and Philadelphia and other big cities where street vendors sold them fresh-roasted.

What happened?

A chestnut disease was first introduced to North America through New York City in 1904. This chestnut blight, caused by a fungus and presumably brought in from eastern Asia, was first found in only a few trees in the New York Zoological Garden. The blight spread with a vengeance and in its wake left only dead and dying stems.

By 1950, Castanea dentata had disappeared except for shrubby root sprouts the species continually produces (and which also quickly become infected). Like many other pest introductions, blight had quickly spread into its new - and defenseless - host causing wholesale destruction throughout the entire range of the chestnut.

Believe it or not, as a child I dreamed of breeding a blight-resistant American Chestnut. Though I didn't pursue it, others did. Now, it seems, their efforts have paid off (via Instapundit):

The tree planted Friday came from a research farm in Virginia, where blight resistance was bred into the native chestnut with the help of the Chinese chestnut.

The American chestnut, prized for its timber and its crop of glossy dark nuts, once dominated Eastern forests from Maine to Georgia. The graceful trees were virtually wiped out by blight starting at the turn of the 20th century.

That loss, Case said, "was the greatest environmental disaster in the Western Hemisphere since the Ice Age."

Now, after years of breeding, cloning and crossbreeding, the U.S. Department of Agriculture is ready to reintroduce disease-resistant chestnuts to Eastern forests next year.

How did they do it?

For decades, plant pathologists and breeders tried to create a blight-resistant tree by crossing our own species with the resistant Chinese chestnut and other chestnut species from Asia, but always with unsatisfactory results. Now, advances in our understanding of genetics have shown us where those early researchers went wrong.

Old science told us that resistance is controlled by numerous genes running a very complex system. Scientists simply flooded chestnut progeny with Chinese chestnut genes by crossing their Chinese-American hybrids with other promising Chinese-American hybrids. The result was consistently a blight-resistant but very Chinese chestnut-like chestnut tree.

New techniques are now being used. By an elaborate and time consuming system of backcrossing and intercrossing, TACF's breeding program is attempting to develop a chestnut that will exhibit virtually every American characteristic. The desired tree is one that is fully resistant and when crossed, the resistant parents will breed true for resistance.

The method of breeding entails crossing the Chinese and American trees to obtain a hybrid which is one-half American and one-half Chinese. The hybrid is backcrossed to another American chestnut to obtain a tree which is three-fourths American and one-fourth Chinese, on average. Each further cycle of backcrossing reduces the Chinese fraction by a factor of one-half.

The idea is to dilute out all of the Chinese characteristics except for blight resistance down to where trees are fifteen-sixteenths American, one-sixteenth Chinese. At that point of dilution, most trees will be indistinguishable by experts from pure American chestnut trees.

Once a significant number of blight-free trees are produced, new crosses could potentially restore the full genetic diversity of the American Chestnut tree, with blight-resistance. Maybe we'll get our American Chestnut forests back. I can't wait!

(Cross-posted at Rishon Rishon.)

Posted by David Boxenhorn at 07:24 AM