Since its accidental
introduction in 1904, Cryphonectria
parasitica has
wreaked havoc on the American chestnut (Castanea dentata). After about
100 years, more than 3.5 billion trees have perished as a result of the
chestnut blight fungus. Molecular sequencing of chloroplast DNA has
resolved the evolutionary history of Castanea, and suggests that
resistance to Cr. parasitica was lost in Ca. dentata as it diverged
from the Chinese chestnut, Ca. mollissima. Although Ca. mollisima is
resistant to the chestnut blight fungus, it is not as well adapted to
the North American climate and is not suitable for hardwood timber.
Numerous crosses have been attempted in order to introgress genes for
blight resistance, while maintaining the forest competitiveness and
hardiness of the American chestnut. While this has proven to be
difficult, genes for blight resistance have been introgressed from
Chinese into American chestnut trees. Furthermore, quantitative trait
loci (QTL) mapping has narrowed the search down to roughly three
genomic regions that have a significant effect on blight resistance. As
the evolutionary arms race between parasite and host has proceeded,
considerable variation in blight resistance has been detected in Asian
chestnut species. Further studies on the population genetics of blight
resistance in Asian chestnut species could provide additional sources
to blight resistance, and maximize the efficiency of conservation
breeding programs. --- E. T.
Watson
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