Hanging With the Chestnuts

by Bill Chameides | March 25th, 2010
posted by Erica Rowell (Editor)

Permalink | 2 comments

Stand of seedlings grown from nuts of selected third-generation backcrossed trees. One of this group may be selected to produce nuts that will be planted for propagation. Fred Hebard, Steve Hopp, Bonnie Chameides, and golden retriever Mattie in picture.

The seeds for returning a grand tree to American forests may be growing at the American Chestnut Foundation Research Farm.

About a year ago TheGreenGrok honored the American chestnut tree on Arbor Day. The once-dominant sylvan presence from Maine to Florida was essentially wiped out by the chestnut blight, an exotic disease that was likely imported on a Chinese or Japanese chestnut around the beginning of the 20th century.

But the saga of the American chestnut may not be over. Dedicated environmentalists and foresters have been working hard to develop a new hybrid of the American chestnut that is resistant to the blight. If successful, our native chestnut could return to the American landscape.

The American Chestnut: Turning Over a New Leaf?

Yesterday, I had an opportunity to visit ground zero in that quest: The American Chestnut Foundation Research Farm in Meadowview, Virginia. I was in the area visiting Steven Hopp of Emory and Henry College. Steven drove me over to the farm where I got a personalized tour from one of the head honchos there — Dr. Fred Hebard.

Fred, a tall, lanky sort, speaks with a slow drawl and uncorks his dry wit at regular intervals. Yesterday, his blue cap, blue shirt, and blue jeans — just like the outfit he sports on the farm’s Web site — made me wonder if that was standard attire. But no mind.

Fred, accompanied by Mattie, a golden retriever, showed me plot after plot of chestnuts in various stages of development —  from seedlings to mature trees. The hope, he explained, is that some of those trees contain the right set of genes that will render them resistant to the chestnut blight.

Why We Want the American Chestnut Back

The different genetic mixes found on the farm come from crossing a blight-susceptible native chestnut with a blight-resistant Chinese chestnut.

Why not simply plant Chinese chestnuts and call it a day?

Not to state the obvious, but Chinese chestnuts are not American chestnuts, and thus do not carry the full panoply of ecosystem benefits that the native tree has, such as supplying food to a wide range of animals from bears to birds to humans.

For centuries, we ate and sold its nuts. We also fortified rural economies with a robust chestnut lumber industry, and built fence posts, railroad ties, barns and homes from the tree’s straight-grained, lightweight, easily worked, and highly rot-resistant wood. The loss of the majestic tree hit its home soil hard.

Mixing It Up: How to Get an American Chestnut From a Chinese Chestnut

The American Chestnut Foundation is dedicated to re-introducing a tree as close to the American chestnut as possible, There are basically two ways to achieve this.

One is through genetic engineering, where the Chinese chestnut’s blight-resistant genes are spliced into an American chestnut, which is then propagated. The problem here is that it produces a single cultivar for each genetic modification, not a good recipe for developing a robust population.

The other approach is through the more difficult backcross method. Here’s how it works:

Step 1: Crossbreed Chinese chestnuts with American chestnuts.
Some of you might be wondering how you crossbreed a tree? I wondered about that too, and so Fred gave me a quick primer on “the birds and bees” of trees.

Crossbreeding trees is nothing more than human-assisted tree-insemination. Chestnut trees have two types of flowers: one male, one female. To crossbreed, you take the male flower from one tree (say, the American chestnut) and rub it on the female flower of the other (in this case, the Chinese chestnut), then wait till the end of the season and harvest the nut. That nut’s genetic makeup is equal parts American and Chinese chestnuts.

Step 2: Plant the nuts from the cross-bred trees, and then select the nuts from the resulting subset of blight-resistant trees to breed further.
Any nuts showing susceptibility to the blight are destroyed. Fred told me that typically only one tree out of about 150 makes the cut.

Step 3: Back-cross the cross-bred nuts with an American chestnut.
The nuts from these first-generation back-crossed trees that show resistance to the blight are then harvested for the next round. This produces a hybrid whose genes are three-quarters American and one-quarter Chinese.

Steps 4 and 5: Back-cross again the first-generation back-crossed trees. Repeat.
Again, the nuts from trees that show resistance are harvested. This process produces a hybrid that is seven-eighths American and one-eighth Chinese. When the blight-resistant nuts are selected from this new batch and then back-crossed again, the result is a hybrid that is 15/16ths American and 1/16th Chinese.

Step 6: Plant these third-generation back-crossed chestnuts.
These chestnuts are genetically quite close to actual American chestnuts. Nuts harvested from individuals that still show resistance to the blight are then planted in the wild. The hope is that they will propagate and return the American chestnut to its rightful place in the American forest.

The American Chestnut Foundation began planting these hybrid nuts last year, and plantings will continue as new variations of the nuts are developed on the research farm. Who would ever believe that rubbing a male chestnut flower on a female chestnut flower could lead to a forest of grand American trees. That’s what I call real flower power.

Slide Show: American Chestnut Trees En Route to a Comeback?

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  1. Brad Stanback
    Apr 2, 2010

    The excerpt:encoded of the backcross breeding above left out the final intercross step. The third backcross trees (BC3) are crossed with each other to produce offspring (B3F2) that are homozygous for blight resistance. After selection for blight resistance at the B3F2 level, the selected trees will produce B3F3 nuts which will breed true for the resistance trait. It is hoped that the B3F3 trees will be suitable for widespread restoration.

    • Bill Chameides
      Apr 5, 2010

      Brad – You are correct, of course; I neglected to note the last step that produces the B3F2. Thanks for keeping me honest.

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