The Way Forward is Back
The American Chestnut Foundation (TACF) breeders began planting the final generation of blight-resistant chestnut hybrids on their research farms in Virginia in 2005. Backcrossing Chinese and American chestnut trees towards this point has been a long and painstaking process built on the efforts of people who just wouldn’t give up on the idea of restoring American chestnut to eastern forests.
Arthur Graves, professor at the Yale School of Forestry at the turn of the 20th century, tried for years to find native resistance in the American chestnut trees that remained after the blight first swept through. In 1930, he decided that the path to resistance lay in breeding, in crossing American chestnut with resistant Japanese and Chinese stock. He started by pollinating mature Japanese chestnuts with pollen from American chestnuts; he crossed the resulting hybrids with Chinese chestnut trees and with one another. As breeding continued, crossing hybrids with Asian stock started to yield inconsistent and discouraging results. An apparently healthy tree might grow for decades, but top out at 40 feet because of a high percentage of Asian genes. Many others never made it through the resistance trials.
Around 1980, Charles Burnham, a geneticist and plant pathologist who had worked with food crops most of his career, turned his attention to the American chestnut. Burnham’s expertise was in backcrossing, a plant breeding method designed to transfer a particular trait among species that are able to interbreed. With others interested in producing a blight-resistant tree with the timber qualities of American chestnut, Burnham set out the backcross method that TACF has used to move towards producing blight-resistant, almost pure American chestnut seedlings.
How It’s Done
The backcross starts off by pollinating the female flowers of a blight-resistant Chinese chestnut with pollen from an American chestnut, producing a first-generation hybrid with half of its genes from each parent and resistance somewhere between that of the parents. This Chinese-American hybrid is pollinated with pollen from an American chestnut—a cross “back,” or backcross, to the American that increases the percentage of American chestnut genes in the second generation—again, with blight resistance somewhere between that of the two parents.
After the seedlings from the first backcross have grown enough to reveal their form—usually 5 years or until they’re around 1.5 inches in diameter—they’re inoculated with the blight to test for resistance. Only the most resistant seedlings with straight (rather than spreading) forms are selected to be crossed back again to American chestnut for the second generation of hybrids.
The process of testing hybrids and backcrossing them with pure American chestnut is continued for three more generations, using the pollen from a variety of American chestnut trees to prevent inbreeding and maintain genetic variation. Trees grown from the third backcross are then pollinated with one another (intercrossed) to produce the fifth generation; a small percentage of these have the blight resistance of the first generation Chinese chestnut and the desired characteristics of the American chestnut. These fifth-generation trees are then intercrossed to produce the grail—a blight-resistant, almost pure American chestnut tree.
At Meadowview, TACF has reached the sixth generation of the breeding program; the first seedlings to be tested in the field were produced in 2005, and TACF projects that the seedlings will be widely available by 2015 to 2020. —ZH
Adapted from: Lord, W. 2007. Burnham relights the torch. In: Bolgiano, Chris, ed. Mighty giants: an American chestnut anthology. Bennington, VT: The American Chestnut Foundation: 204–205.