(Castanea dentata) in Ontario - Ministry of Natural Resources ...

Recovery Strategy for the American Chestnut in Ontario
Supporting Narrative
The approaches described in Table 3 primarily address chestnut blight, the most
important threat to the species. The recommendations focus primarily on the need to
develop and evaluate blight control methods. If the blight is controlled, the small
immature stump-sprout saplings which currently account for a large portion of extant
populations may grow to maturity, increasing numbers of fruit-bearing individuals to
levels where healthy breeding and seed production can occur.
Until blight can be controlled, the greatest promise for recovery of the species lies in
developing and deploying a blight-resistant locally-adapted American Chestnut
genotype. As outlined in section 1.6, natural hybridization is considered a potential
threat to the recovery of American Chestnut. However since chestnut blight is still the
primary cause of endangerment for this species, controlled breeding with close relatives
that exhibit a higher degree of resistance such as Chinese and Japanese Chestnut,
may be needed to accelerate the evolution of resistance. This research is being
pursued by the Canadian Chestnut Council breeding program. The intent of this
program involves introducing the disease resistance of Chinese Chestnut into Ontario
genotypes of American Chestnut through an initial hybrid cross, backcrossing the
hybrids with Ontario genotypes over multiple generations to reduce the Chinese
Chestnut genetic contribution to the target level of less than six percent and then
selecting crosses that perform well in disease resistance screening tests. It is
anticipated that blight-resistant trees that meet these genetic criteria and are
phenotypically indistinguishable from naturally occurring genotypes could be produced
within 15 years. In addition, naturally occurring individuals with lower blight
susceptibility are also being assessed (intraspecific resistance). These efforts would be
followed by diligent out-planting efforts to get the resistant genotype established in the
network of priority populations. However, there is no information on the durability of
resistance as trees mature.
Utilizing this method for recovery comes at the cost of introducing genes from
interspecific crosses with other Castanea species and thus may cause some ambiguity
between hybrids developed specifically for the recovery strategy versus those that occur
naturally (Jacobs 2007). As indicated in section 1.6, hybridization that occurs naturally
is a potential threat to American Chestnut as the genetic component of offspring that are
produced is likely to be 50 percent or less American Chestnut. In contrast, the genetic
component of hybrids that are produced using the rigorous methods of backcrossing for
the recovery strategy will be close to 94 percent or more American Chestnut (Hebard
2005). Therefore, to avoid any ambiguity, reintroduction programs that involve
backcrossed American Chestnut should clearly articulate how they differ from naturally
occurring hybrids that are a potential threat.
Over time, it is anticipated that selection will favour genotypes with a combination of
resistance and local adaptation. Ultimately, the survival of American Chestnut which is
affected so severely by blight, may depend on this infusion of genetic variation.
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