SYRACUSE — Brian Caldwell wrote a Guest Appearance last Sunday stating that the blight-tolerant American chestnut trees developed at ESF should be “cloistered” solely based on the fact that we used the tools of genetic engineering — or GE. He argues for continuing the traditional hybrid breeding between the different chestnut species, a path that has been tried essentially without success for the past century.
Even the promising backcross breeding program, as employed by The American Chestnut Foundation, has fallen short of its goals, producing trees that have less than the expected 95% American genome and less blight resistance than Chinese parent trees (See Reference No. 1). Hybridization is a useful technique for agriculture but isn’t the best way to make a restoration tree that has to survive in the wild. Unlike an agricultural tree grown in a managed orchard, trees returned to the wild must retain all their original characteristics that enable them to adapt to their natural environment. Therefore, one must use tools that can enhance blight resistance or blight tolerance in a way that retains the integrity of the original tree. Genetic engineering accomplishes this.
We are living in an exciting age of genomics, where thousands of species genomes (i.e. all the DNA that make up all their genes) have already been sequenced and more are being completed almost weekly. Over the past decade, old beliefs that traditional breeding methods cause fewer changes to the plant’s genome than genetic engineering are falling by the wayside.
For example, soybean cultivar genomes were sequenced specifically looking for structural variations in the DNA (i.e. mutations). What researchers found was, "On average, the number of genes affected by structural variations in transgenic plants (i.e. GE plants) was one order of magnitude less than that of fast neutron mutants (mutational breeding) and two orders of magnitude less than the rates observed between cultivars (traditional selection breeding)." (1)
In this example GE plants had 100 fold fewer genomic changes than traditionally breed cultivars. This study didn’t compare hybrid breeding, which causes even more DNA changes.
In another recent research article, the authors wrote, “During hybridization, hybrid offspring receive a genomic shock due to mixing of distant parental genomes, which triggers a myriad of genomic rearrangements, e.g., transpositions, genome size changes, chromosomal rearrangements, and other effects on the chromatin (i.e. DNA structures). Recently, it has been reported that, besides genomic rearrangements, hybridization can also alter the somatic mutation rates in plants.” (2)
"Somatic mutation rates" are the mutations that can happen in cells of the plant while it grows. In chestnut hybridization, these types of genomic mutations can result in intermediate traits, male sterile offspring, or conditions like Internal Kernel Breakdown (3) where approximately 40% of the nuts decay in their shell.
So, published studies that have actually compared these different breeding tools show just the opposite of what people have long assumed: Hybridization actually causes vastly more changes to the plant’s DNA than genetic engineering. Not just the mixing of genes, but actual structural mutations and changes to the chromosomes. Also, because of our ability to sequence our GE American chestnut tree's genome, we know exactly where our "packets of genetic material" inserted and only selected lines where the inserted DNA doesn't disrupt any of the chestnut's natural complement of genes.
I am not saying that hybrids should be “cloistered” because of these large number of unknown and unpredictable changes to the plant's genome. Hybridization has been used extensively in agriculture and is a very useful tool. But if our goal is to make the fewest number of changes to the tree so that it would be fully adapted to its wild environment, then GE is a genuinely a better tool.
Some people may not care that genetic engineering is more precise and causes fewer changes, if they are concerned that it just doesn’t seem natural. Several genomic studies have been published over the past five years that also address this question, culminating in a study that demonstrated that 7% of the common flowering plant species they examined were genetically engineered in the wild by the same bacterium we used to develop the blight-tolerant American chestnut trees. This included three species of walnut trees in their study, which are natural GMOs (4). So a GE American chestnut will not be the first GE plant to enter the natural environment, because there are many natural GE plants already there.
Even though we used the most precise tools available, the GE American chestnut will be the most studied of any chestnut variety ever developed. Some examples (6) include environmental interactions with mycorrhizal fungi, bees, tadpoles, and insects, leaf litter decomposition and native seed germination in the leaf litter, nut nutrition including fatty acids and tannins, and consistently found no significant differences between the GE American chestnut and wild-types. We did find tadpoles had better growth development on both American and GE American chestnuts leaves compared to American Beech, Sugar Maple, hybrid chestnut, and Chinese chestnut leaves, suggesting amphibians may benefit from reestablishment of the American Chestnut. All these studies will be evaluated by the USDA, EPA, and FDA and will publicly available before the trees become available for public use.