Early in the twentieth century, a fungus that attacked and nearly eradicated the American chestnut was accidentally imported in foreign shipments of trees. This man-made disaster became known as the chestnut blight. Only a few trees planted by settlers far outside the American chestnut's native range in the eastern United States have avoided infection and continue to thrive. Of the billions of trees infected, fewer than 50 survive as full-size trees. The vast majority either died completely or barely survive as understory shrubs by growing sprouts from damaged trees' root collars. Other infected trees are kept alive in research groves through management and treatment of the blight by scientists.

With the gene pool of mature, seed-producing American chestnuts so small and the genetic diversity stored in immature sprouts dwindling as the shrubs gradually succumb to re-infection and other pressures, extinction of this magnificent species is a serious threat. Several interventions to decrease that threat are currently in progress.

The intervention demonstrated in this video segment is a breeding program designed to produce a chestnut hybrid that blends the desirable lumber qualities, nut taste, and forest suitability of the American species with the blight resistance of the Chinese species. Chestnut blight resistance is an incompletely dominant trait coded on two genes at different locations and thus requires a homozygous genotype at both locations for full expression. Early attempts to breed homozygous resistance resulted in too few American traits in the hybrid, but this newer breeding program appears to be working.

The program begins with a straight cross of American and Chinese trees, followed by three backcrosses. Each backcross is a cross of the resulting hybrid with an American chestnut tree to increase the American character of the progeny. To build genetic diversity, different American parents are used at each step. Also at each step, progeny are selected for American morphology after several years of growing and then inoculated with the blight fungus to screen for resistance. Only those progeny with some blight resistance and an American chestnut appearance are grown to sexual maturity and continue in the program. After selecting and backcrossing the third time, the newest progeny are genetically 15/16, or almost 94%, American chestnut. To achieve full blight resistance, selected progeny of this third backcross will be crossed with each other, in a process known as an intercross. Of the intercross progeny, scientists hope 1/16 are homozygous and therefore demonstrate high levels of blight resistance while retaining the desired American chestnut traits. Each backcross generation requires a minimum of five years to complete. The beginning cross can be completed in three years.

Other research programs may also contribute to the restoration effort. One transgenic project involves inserting genes in the American chestnut genome to improve blight resistance. Another transgenic project is inserting genes in the blight fungus genome to reduce its virulence.

At least one effort must succeed if the American chestnut species is to ever again flourish in eastern North America’s forests.

To learn more about this species of tree, check out American Chestnut Tree.

To learn more about how plants pollinate, check out Floral Arrangements, Life Cycle of a Seed Plant, and The Reproductive Role of Flowers.

To learn more about principles of genetic dominance, check out Mendel's Laws of Genetic Inheritance and Some Genes Are Dominant.

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