The Backcross Method
Chestnut blight was first introduced to North America in 1904. Like many other pest introductions, it quickly spread into its new - and defenseless - host population. American chestnut trees had evolved in the absence of chestnut blight, and our native species lacked entirely the genetic material to protect it from the fungus.
In Asia, however, where the pathogen originated, most native chestnut species and particularly Chinese chestnut are well defended against the blight. Over the course of their millennia of coexistence with the fungus, Chinese chestnut trees acquired the genetic material that confers resistance. Blighted North American chestnut species die, while blighted Chinese chestnuts suffer only cosmetic damage. Since all chestnut species can be crossed with relative ease, Chinese chestnut offers a potential solution to the American tree's susceptibility to chestnut blight.
But Chinese chestnut lacks many of the characteristics of the American. Most obvious is stature: the Chinese species is low-growing and spreading, much like an old apple tree - an American chestnut can grow straight and strong to a hundred feet or more. This habit of growth combined with the quality of wood makes the American a dominant forest tree species.
Less obvious is the role the American chestnut played in its native forests. The blight is a very recent introduction to the chestnut ecosystem. In those thousands of years preceding the blight's arrival, an enormously complex set of relationships evolved which tied the chestnut to innumerable bird, mammal, and insect species and other organisms, as well as to rocks, water, soils and fires. Essentially, chestnut was tied to the very shape of the hills and mountains on which the trees were found. This history of co-evolution on the North American continent is carried in the genetic material only of the American, not the Chinese chestnut.
The goal of TACF's breeding program is therefore two-fold: to introduce into the American chestnut the genetic material responsible for the blight resistance of the Chinese tree, and at the same time, preserve in every other way the genetic heritage of the American species.
Although the Chinese genes for resistance are only incompletely dominant, they nonetheless usually express themselves clearly when present in seedlings purposely inoculated with a virulent form of the blight fungus. And that is how each backcross generation is tested - by inoculation with blight. Only those seedlings that show the greatest resistance are used for further backcrossing to an American parent.
But every backcross, although necessary to recover desirable American traits, also reintroduces the genes for blight susceptibility from the American parent. In order to remove those genes, the next steps at TACF are intercrosses. In the first intercross, the most blight-resistant 15/16ths American trees are crossed with other blight-resistant 15/16ths American trees. Again, only resistant seedlings are saved.
At the first intercross, it may prove difficult to select inoculated seedlings which have only inherited genes for blight resistance from their Chinese ancestor and no genes for blight susceptibility from their American ancestors. Testing in subsequent generations or a test cross back to an American parent will confirm that first intercross trees contain only the Chinese genes for resistance. Most or all the progeny of parents containing only genes for blight resistance should show blight resistance, whereas some progeny of parents with genes for susceptibility should show susceptibility to blight.
When crossed with each other, these highly blight-resistant parents will breed true for resistance, since they will have no American genes for susceptibility to blight. This second intercross will yield nuts for restoration.
Inbreeding We cannot reforest the Appalachian Mountains with progeny from only one or two chestnut trees, because this would result in a tremendous amount of inbreeding, which chestnut probably could not tolerate. To avoid inbreeding, we have been backcrossing into 20 different American chestnut trees from each of our sources of blight resistance. And we have been doing this at multiple locations throughout the native range of the chestnut tree.
Guaranteeing Regional Adaptability and Long-term Resistance
Across the range of the American chestnut, traits such as stature and timber quality probably differ little. But regional conditions such as temperature, day length, soils, moisture, elevation and others differ greatly from Maine to Mississippi, and there probably are unique combinations of traits for adaptation to different regions. In order to preserve that wide array of genetic diversity and adaptability, TACF's breeding program uses American parents from a number of populations of trees. Our Connecticut, Georgia, Maine, Maryland, Massachusetts, Indiana, Pennsylvania, Kentucky, Tennessee and Carolinas chapters have active breeding programs that are extending the range of regional adaptation in backcross trees.
Plant pathogens frequently evolve to overcome plant defenses. Although the blight fungus is not known to have overcome the defenses of the numerous Chinese chestnut trees planted in the U.S, a future "breakdown" of resistance in blight-resistant chestnut trees is possible. To minimize this possibility, TACF's breeding program uses genetic material from different Chinese chestnut trees. Our most advanced breeding lines, which are entering the first intercross of third backcross trees, are derived from two Chinese chestnut trees known as 'Mahogany' and P.I. #34517. The 'Nanking, 'Kuling' and 'Meiling' cultivated varieties are the parents of a set of trees entering the second backcross stage. Other Chinese chestnut trees are being used to a lesser extent. It is necessary to choose Chinese chestnut parents with care, as some are more resistant than others.
The process of producing seeds and testing those seeds for blight resistance now requires about six years for each backcross generation and five years for intercross generations. .We planted our first set of intercross progeny from the third backcross in 2002. We'll have progeny from the second intercross - and our first line of blight- resistant American chestnuts - ready for planting in less than five years!