< Select A Different Album Backcross Breeding Program of The American Chestnut Foundation April 2, 2006 The first cross produces a chestnut hybrid that is 1/2 Chinese and 1/2 American. Pollen from selected hybrids are crossed back to 100% American chestnut parents for three generations so that additional American chestnut characteristics are regained. Selected trees that are 15/16 American and 1/16 Chinese are intercrossed with each other a first time and then selected trees from that intercross are intercrossed again so that blight resistance equal to that of the Chinese parent is regained. Selection of trees from each generation for further pollination efforts involves innoculating trees with the chestnut bark fungus to test blight resistance, and among those trees displaying good resistance characteristics, selecting trees that have strong American chestnut characteristics as well. Click on any diagram to enlarge it. The chart explains graghically the steps The American Chestnut Foundation and its growers use in its Backcross Breeding Program. The chart appears in the Spring 2004 Volume of the "The Journal of the American Chestnut Foundation" on page 15 in an article by Dr. Paul Sisco. The average fraction of American genes in the trees produced each generation is shown on the trees in the diagram. At each step of the backcross, trees are tested for resistance by innoculating each tree with the chestnut bark fungus. F1 Cross: A Chinese chestnut crossed with an American chestnut produces F1 trees which have 1/2 American and 1/2 Chinese genes. BC1 First Backcross: Pollen from an F1 tree, selected for its blight resistance and American characteristics when crossed with an American chestnut produces BC1 tress which on average have 3/4 American and 1/4 Chinese genes. BC2 Second Backcross: Pollen from a BC1 tree, selected for its blight resistance and American characteristics when crossed with an American chestnut produces BC2 tress which on average have 7/8 American and 1/8 Chinese genes. BC3 Third Backcross: Pollen from a BC2 tree, selected for its blight resistance and American characteristics when crossed with an American chestnut produces BC3 trees which on average have 15/16 American and 1/16 Chinese genes. BC3F2 First Intercross: Pollen from selected BC3 trees is used to pollinate other selected BC3 trees (intercross pollination) to produce BC3F2 trees. The fraction of American chestnut genes in the BC3F2 trees remains the same as in the BC3 trees, 15/16. The level of resistance to the chestnut bark blight is increased, however. After testing resistance by inoculating the intercrossed trees with the chestnut bark blight, those most resistant are kept for a second intercross breeding. All other trees are eliminated from the orchard. The selection process ensures that a high percentage of the remaining BC3F2 trees will have resistance to the chestnut bark blight to pass on to the next generation of trees. BC3F3 Second Intercross: Selected BC3F2 trees crossed with other selected BC3F2 trees produce BC3F3 trees. This second intercross produces 15/16 American chestnut hybrid trees which should breed true for blight resistance equal to that of the original Chinese parent. Highly resistant, 15/16 American chestnuts which have been produced through the breeding program have been planted in a seed orchard to produce nuts for testing and reforestation. It is anticipated that the first test plots in the forest will be planted by 2008, and seed may be available for wider distribution by 2012. < Select A Different Album < Home