The story of the decline of American chestnuts (Castanea dentata) is the stuff of conservationists’ nightmares. First, a magnificent and abundant species provides habitat and an annually stable mast crop (full of protein and fat) for overwintering forest dwellers. The species also serves as a timber product for people. But then the import of a related species (Castanea mollissima), bred for horticultural beauty, secretly brings along a pathogen (Chestnut blight) that decimates the native tree. Within 50 years (1904 ~ 1950s), the fungal pathogen killed approximately 4 billion American chestnut trees. See, the stuff of conservationists’ nightmares!
But what is strange about Chestnut blight is that it only kills the tree aboveground, the roots remain and can grow aboveground for a time until the blight infects it again. By continually killing the trees before sexual maturity, no new fruits are made which prevents natural selection from working to evolve resistance to the blight.
But a biotech solution from the American Chestnut Research and Restoration Project offers some promise for restoring the tree. Specifically, the group has transformed the chestnut by adding a wheat gene (oxalate oxidase) into trees. Why this gene? Well, the fungus uses oxalic acid in the virulence pathway that kills the tree; oxalate oxidase takes oxalic acid and catalyzes a reaction that turns it into less harmful products (carbon dioxide and hydrogen peroxide), thereby taking away the substrates needed to kill the plant. But why use a wheat gene? Oxalate oxidases are found in monocots but not dicots (like the chestnut), since wheat is a monocot (with a lot of information about its genome as a well-studied crop) it makes perfect sense! The researchers also show that the trans-gene was stably inserted into the tree genome as trees grown from the fruit of the transgenes also expressed the gene and had less cankers from the blight.
I think the potential for restoration of the American chestnut is near and will continue to be an interesting story to follow, especially from an evolutionary perspective. Specifically, it will be fascinating to see both how the Chestnut blight genome evolves in response to the transgene and how fast the genome evolves. Additionally, since the blight does not kill the chestnut root systems, there is probably a wealth of genetic diversity for this species already in the ground. Given advances in culturing trees in the lab, breeders can harness this diversity thereby aiding restoration and preserving locally adapted genomes.
Does this approach translate to other species?
Despite the promise of this biotech approach to restore American chestnuts, I wonder about the applicability to other species. Specifically, how unique is the chestnut story in that a pathogen with an understood virulence mechanism and insertion of a single gene may result in the ability to produce sexually mature individuals? So often we see stories where erosion of genetic diversity across the genome (e.g.- Florida panthers or black footed ferrets) either alone or in combination with disease threatens a species with extinction. Fortunately, there are whole genome solutions for genetic rescue (e.g.- crossing to a closely related subspecies; or for a biotech approach: somatic cell nuclear transfer).
When we’re just talking genetics (not considering other threats to species such as habitat availability): what other species may benefit from targeted transgene approaches? Leave your ideas in the comments.
Chestnut leaves and fruit photo by John Hilty via EOL