1. Chestnut Blight A Case in Introduced Disease,
Genetic Manipulation, and
Disease Biology

2. The American chestnut once comprised 25% of the eastern deciduous forest.
The nuts were once an
important economic
resource in the U.S.
The wood is straight-grained,
strong, and easy to saw and
split.
Being rich in tannins, the
wood was highly resistant to
decay and therefore used for
a variety of purposes,
including furniture.

5. Chestnut blight, or chestnut bark disease, is caused by an introduced fungus, Cryphonectria parasitica
The fungus enters wounds,
grows in and under the
bark, and eventually kills
the cambium all the way
around the twig, branch,
or trunk.

6. Cankers were first reported in the United
States in 1904 on American chestnut trees
Castanea dentata in New York City.
An American chestnut tree growing in
Connecticut in The tree was 83 feet
tall, 27 inches in diameter, and 103 years
old.

7. By 1926 the fungus was reported throughout
the native range of American chestnut, and a
major forest tree had been reduced to a
multiple-stemmed shrub.
In 1912 the Plant Quarantine
Act was passed to reduce the
chances of such a catastrophe
happening again.

8. The blight fungus moves from tree to tree as
spores on the feet, fur, and feathers of the
many animals and insects that walk across
the cankers.
Sprouts develop from a burl-like tissue at the
base of the tree called the ‘root collar,’ which
contains dormant embryos. Sprouts grow,
become wounded and infected, and die, and
the process starts all over again.

9. There has been little chance for disease
resistance to evolve in American chestnut,
since the sprouts that come up after trees
are killed by chestnut blight disease are
often killed before they become sexually
mature.
Since two flowering chestnut trees are needed
for seed formation (they must be cross-
pollinated), sexual reproduction has been
drastically reduced.

10. Breeding plans have been developed
for making hybrids of resistant Asian trees
with susceptible American trees.
The hybrid trees would have the qualities
of the American chestnut, along with the
disease resistant qualities of the Asian tree.

11. Researchers have
determined that the
disease resistance is
determined by two
independent genes.
By selecting certain types
for breeding, the desired
genetic characteristics
can be achieved.
It is now known that the
heterozygous condition
only renders partial
resistance to the disease.

12. AABB x aabb 100% AaBb Parental Cross: One parent homozygous
Dominant for both genes (PD), one parent
homozygous recessive (Pd) for both genes.
AABB x aabb
100% AaBb
The F1 generation is now heterozygous for both genes.
The heterozygous individuals are only partially resistant

13. Parental Cross
AABB x aabb

14. A back cross is used to verify the assumptions
of genetic identity of the P generation, and to
produce individuals with different combinations
of the genes (different genotypes).
A backcross of an F1 individual and a parental
genotype should result in a population in which
25% of the individuals will have partial resistance.
AaBb x aabb

15. Now do the Punnett Square!
Remember, the genes are independent, so during meiosis
the gene alleles can sort independently.
What are the possible gametes for each parent?
aabb: ab is the only possibility
AaBb: The possibilities are: AB Ab aB ab
Now do the Punnett Square!

16. Back cross: F1 x Pd AB Ab aB ab ab
AaBb
Aabb
aaBb
aabb
25% of the offspring will have a partial resistance
to the fungus, 75% will be totally susceptible to it.

17. F1 x Pd Back cross:
AaBb x aabb

18. Crossing two of the F1 progeny should produce
1/16 of the F2 generation with the homozygous
dominant condition, giving them full resistance
to the fungus.
AaBb x AaBb
Start by listing the possible gametes for each.
Both are AaBb: The possibilities are: AB Ab aB ab
Now do the Punnett Square!

19. Progeny cross: F1 x F1 AB Ab aB ab AB AABB AABb AaBB AaBb AaBb Ab AABb

20. F1 x F1Progeny cross:
AaBb x AaBb