The American Chestnut Foundation’s Breeding Program for Blight Resistance Frederick V. Hebard, William Y. C. White & Shawn Yarnes Meadowview Research Farms 14005 Glenbrook Ave. Meadowview, VA 24361 USA http://www.acffarms.org TACF was created in 1983 as a private, non-profit organization. Mission: to restore the American chestnut to American forests through a scientific program of breeding
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The American Chestnut Foundation’s Breeding Program for Blight Resistance Frederick V. Hebard, William Y. C. White & Shawn Yarnes Meadowview Research Farms.
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The American Chestnut Foundation’s Breeding Program for Blight Resistance
Frederick V. Hebard, William Y. C. White & Shawn YarnesMeadowview Research Farms14005 Glenbrook Ave.Meadowview, VA 24361 USAhttp://www.acffarms.org
TACF was created in 1983 as a private, non-profit organization.
Mission: to restore the American chestnut to American forests through a scientific program of breeding and cooperative research.
C x A F1 x A B1 x A B2 x A B3 x B3
B3-F2 x B3-F2
B3-F3
Expected ProportionChinese
1/2
1/4
1/8
1/16
1/16
1/16
Year Cross Made
1930s
1940s
1989-1995
1994-2002
1999-2009(estimate
2005-
Backcrossing• First, American and Chinese chestnut trees are crossed with each other.• The progeny from this first cross are then backcrossed to American chestnut.• Each cycle of backcrossing reduces the fraction of Chinese genes by a factor of one
half.• Finally, backcrosses are intercrossed to yield progeny that will breed true.• Blight resistance is retained by inoculating progeny and selecting resistant ones.
More than one American parent is used at each backcross to
avoid inbreeding. One set of American parents constitutes a
line, the different Americans are denoted A1 to A4 below.
C x A1F1 x A2B1 x A3B2 x A4B3
BackcrossingMore than one American chestnut line also is needed, also to avoid inbreeding, when B3s are intercrossed with each other.
C x A13F14 x A14B14 x A15B24 x A16B34
C x A9F13 x A10B13 x A11B23 x A12B33 x
C x A5F12 x A6B12 x A7B22 x A8B32
C x A1F11 x A2B11 x A3B21 x A4B31 x
B3-F212
B3-F234
Twenty B3 lines (families) eliminates most inbreeding
after intercrossing, enhancing the chance that a population
will not collapse.Effect of Partial Diallel Size on Inbreeding
Chapters add genetic diversity
The effect of adding sets of 20 B3-F2 progeny from our chapters on inbreeding effective population size, for one source of blight resistance. Restoration ecologists estimate that effective population size needs to be 50 to avoid immediate collapse from inbreeding depression and 500 for mutation to offset long-term erosion of genetic diversity by drift. Chapters also add adaptation to their local environment in addition to general genetic diversity.
Number of
Chapters
Inbreeding
Coefficient
Inbreeding Effective
Population Size
1 0.0207 72
2 0.0115 130
3 0.0085 176
4 0.0070 214
5 0.0060 248
Source Number Currently Used
C. mollissima 23
C. crenata 2
C. sativa 1
C. dentata 19
C. henryi & sequinii ?????
Our primary source of blight resistance is Chinese chestnut, because that was identified as the most resistant species by the USDA in the 1930s. No resistance was known in American chestnut until the 1980s. We also have a few crosses with Japanese and European chestnut. C. henryi and C. sequinii also may have individuals with high resistance, based on recent observations in China.
B3-F2 family size needs to be 9 or 10 selected trees to capture most alleles from each B3. Obtaining fewer selections per parent would result in loss of genetic diversity.
0.00
0.25
0.50
0.75
1.00
0 5 10 15
Family Size
Pro
ba
bil
ity
of
Ca
ptu
rin
g A
ll A
lle
les
Effect of Family Size on the Probability of Capturing Both Alleles from 20 Independently Assorting Loci
B3-F2 Selection
B3-F2 Seed Orchard
Selection for Recurrent Type
• Trichome density and type
• Stipule size and shape
• Leaf shape
• Tooth shape
• Lenticel density, size and color
• Stem color
• Form
• Time of leaf emergence
• Vigor
Each backcross generation will vary in the remaining proportion of Chinese heritage. By picking trees with the fewest Chinese traits (except blight resistance), we accelerate the recovery of American type. At the present time, this selection is done using the following morphological traits:
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Price Farm viewed from Bryan Farm
Cutting disks of inoculum Inserting disk in treeTaping inocu-lation point
Elder Hostel Inoculation Crew Crew in Action
Selection for resistance We determine the blight resistance of chestnut trees by giving them the disease, inoculating them with the blight fungus, and observing the resultant cankers.
Highly blight-resistant and blight-susceptibleClapper x Graves B1-F2 progeny, characterizedby small and large cankers, respectively.
Table 1. Number of trees in canker size classes in 1993.