The American Chestnut FoundationÕs
Breeding Program


	F. V. Hebard
	Research Farms
	Meadowview, VA
	Fred@acf.org

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Requirements for breeding


	Breeding Method: the sequence of crosses to perform.
	Materials: sources of resistance and flowering American chestnut trees.
	Crossing Method: for making the next generation of progeny.
	Cultivation Method: for growing the trees.
	Screening Methods: for determining the blight resistance of progeny, and other traits.

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Materials


	American chestnut trees flower in clearcuts

Materials
Sources of blight resistance


	The old USDA breeding program determined that Chinese chestnut had the most blight resistance of all the chestnut species. In American forests, the Chinese chestnut itself cannot reach the overstory when competing with other tree species. We were able to jumpstart our program by using two first backcrosses from the USDA & Connecticut programs.
	The low levels of blight resistance in some of the large, surviving American chestnut trees might also be useful in a breeding program, which we are exploring.
	We also use Japanese chestnut as a source of resistance.

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2006 Harvest in Meadowview

Cultivation Method

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"Table 1."


	Table 1. Number of trees in canker size classes in 1993.
	Cross Type Size Class (cm) _________
	1.0-2.6 2.6-4.2 4.2-5.8 5.8-7.4 7.4-9.0 9.0-10.6 10.6-
	______________________________________________________________________________
	American 3 5 2
	F₁ Nanking 2 4 3
	Seedling Chinese 2 7 3
	Meiling Chinese 1 2 2
	Nanking Chinese 3 2
	F₂ Mahogany 0 5 23 48 48 29 15
	B₁-F₂ ClapperxGraves 4 25 84 116 112 54 4
	______________________________________________________________________________
	Table 2. Number of trees in canker size classes in 2003.
	Cross Type Size Class (cm) __________
	1.0-2.0 2.0-3.0 3.0-4.0 4.0-5.0 5.0-6.0 6.0-7.0 7.0-8.0
	________________________________________________________________________________
	American 4 2 2 2 1
	F₁ Nanking 1 2 3 1
	Chinese 3 3 3 6
	B₂-F₂ Clapper 3 11 15 37 16 12 3
	B₂-F₂ Mahogany 3 11 21 31 14 14 1
	________________________________________________________________________________

"Blight-resistant ÔClapperÕ x Ô..."


	Blight-resistant ÔClapperÕ x ÔGravesÕ ÒB₁-F₂,Ó 12 years old, inoculated 10 years ago.
	Obtaining trees like this indicates it should be possible to backcross the blight resistance of Chinese into American chestnut.

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Backcrossing


	To avoid inbreeding, we need to use a different American chestnut parent at each backcross step and have more than one set of parents for intercrossing.

"A persistent question has been"


	A persistent question has been, what is the effective population size, N_e, of our breeding stock?
	A famous rule in conservation genetics is the 50/500 rule, which states that an N_e of 50 is the minimum effective population size needed to avoid immediate collapse of a population from inbreeding depression while an N_e of 500 is needed to counterbalance the slow loss of alleles due to genetic drift by the addition of alleles from mutation.
	We are trying to use 20 American lines per source of resistance per locale. Twenty lines yields an inbreeding effective population size of about 70, so it satisfies the 50 portion of the 50/500 rule.
	We can only handle three sets of 20 American lines at Meadowview, or 60 lines total.
	The remaining lines to achieve the 500 portion of the 50/500 rule come from our state chapter breeding programs.
	The state chapter breeding programs also add adaptation to their local conditions: we do not expect trees from Virginia to grow well in Maine, for instance.