Modern Tree Breeding
TACF was founded in 1983 by a group of prominent plant scientists, including Nobel Prize-winning plant breeder Dr. Norman Borlaug; Dr. Peter Raven, Director of the Missouri Botanical Garden; independent chestnut researcher Philip Rutter; and the late Dr. Charles Burnham (at left in photo below), the eminent Minnesota corn geneticist, and his former student Dr. Larry Inman. In 1989 TACF established the Wagner Research Farm, a breeding station in Meadowview, Virginia, to execute the backcross breeding program. A second research farm in Meadowview was donated to TACF in 1995, and a third Meadowview farm was purchased in 2002. Dr. Burnham is credited with having developed the backcross breeding program for which TACF was developed to implement. Dr. Burnham wrote the following article published in 1989 which describes the breeding process.
Author: Dr. Charles Burnham – 1989 – TACF Journal Vol 4 Issue 1
The current American chestnut breeding program is using the Chinese chestnut as the best source of resistance. The blight fungus fails to grow at the point of infection. The hybrid between the two species is moderately resistant, more resistant than the American, but less resistant than the Chinese chestnut. Resistance is incompletely dominant. By crossing that hybrid back to the American chestnut and following with successive back-crosses to the American chestnut, using blight-resistant selections each time for the next back-cross, the American chestnut is recovered automatically, and at the same time resistance to the blight is being added by selection. The third back-crosses are, on the average, 15/16 American chestnut and some will have the gene(s) for resistance derived from the Chinese chestnut, but only from one parent and, consequently, are only moderately resistant. Progeny from crosses between those moderately-resistant selections will include some that have received the gene(s) for resistance from both parents. They are homozygous for those genes are expected to be as resistant as the Chinese chestnut. They will “breed true” for resistance.
Genetics and Incorporating Diversity
Based on the work of the early researchers at the CT Agricultural Experiment Station, it was determined that blight resistance was controlled by two or possibly three genes acting in an incompletely dominant fashion. This means that when a fully-resistant Chinese tree (RRRRRR) is crossed with a fully susceptible American chestnut (rrrrrr) the resulting F1 progeny would be moderately resistant to the blight (RrRrRr(. With the exception of the intercross (BC3F2 and BC3F3) generations, the most resistance possible is moderate resistance.
As the program was being developed, regional and geographic diversity was identified as a major input and Regional Adaptability became a cornerstone of the program. To capture and include the adaptations that might be expressed genetically, TACF proposed and developed the state chapter system where state volunteers would find surviving American chestnut throughout the range, and capture their genes through backcross breeding.
Author: Dr. Larry Inman – 1987 – TACF Journal Vol 2 Issue 1
The proposed strategy has two parts. One is the preservation of the American chestnut germplasm adapted to different geographic zones. The other is the introduction of blight-resistance into each. Begin by using American chestnut survivors in each zone for crosses with Chinese-American chestnut hybrids. First, divide the natural range of the American chestnut into zones, primarily on the basis of climate, latitude, and altitude. Preserving a representative sample of germplasm from each zone is essential. It must be available for the later generations of breeding. All or most of the trees within each zone must be included, since the collections must be large enough to include the adapted ecotypes in each zone.