Earlier this year, we embarked on a process of DNA fingerprinting each of the chestnut cultivars we're working on propagating. The original intent was to use the DNA fingerprints for internal quality control. If there was ever uncertainty in the future about which cultivar a given tree is, we could compare it back to the fingerprints. We're not experts in genetic analysis and want to eliminate any potential bias in the results, so we engaged a 3rd party plant lab whose whole business is plant genetic ID and pathogen testing.
The method they used to fingerprint the DNA of each cultivar is called "Single Sequence Repeats," or SSR. In layman's terms, SSR fingerprinting is a process of analyzing areas of a plants DNA for markers, or elements of the DNA that are different from others in the same species.
For example, the vast majority of the DNA in Hypothetical Chestnut Cultivar #1 is the same as Hypothetical Chestnut Cultivar #2 and Hypothetical Chestnut Cultivar #3. There are, however, a few places where their DNA is different. Those areas of difference are identified for the species and then focused on as "markers" that can be used to see if Hypothetical Chestnut Cultivar #4 is actually the same as Hypothetical Chestnut Cultivar #3, or if it's truly a unique cultivar.
What we learned
As we started getting the fingerprints back on the cultivars we were working on, we began looking for others who'd done similar analyses. With the help of some current and former researchers across the U.S., we were able to get access to the largest database of chestnut SSR fingerprints that's ever been done in the U.S. You can read the paper that was published based on these data here.
We were thrilled to access this treasure trove of data! Less thrilled to see that many of the most commonly grown chestnut trees that people referred to as "cultivars" were actually not cultivars at all. The researches tested multiples of named trees that were believed to be graft propagated cultivars and found a large degree of genetic variation. A couple examples below:
- Only two of the seven tested Colossal trees were genetically identical
- There were at least four different Peach trees of the seven tested
Other cultivars, especially the ones that have been developed more recently and have been under study at research institutions shown through as more consistent cultivars. For example, all eight Qing trees that were tested had the same genetic fingerprint.
On one hand, this is not that surprising. Growers, researchers, and nursery folk have been growing chestnuts from seed, grafting from their best trees, and selling seedlings & grafted trees in the U.S. for more than 100 hundred years. Only recently has genetic analysis become affordable. Moreover, it isn't that important to the average chestnut grower to be 100% sure if the "Colossal" (or Gillet, or Gideon, etc.) tree they purchase from their neighbor or local nursery is genetic match to the original Colossal tree in Nevada City, CA, or the same "Colossal" that is grown in another state. What they care about most is that the tree they're planting thrives in their climate and produces good nuts. There are also few, if any, patented chestnut cultivars that are widely grown, so there's no one keeping an eye out to make sure that a specific name is only being used for a specific cultivar.
However, this does create a challenge for new growers seeking to make an informed decision about what trees to plant to meet their goals. If they're neighbor with great producing trees tells them that their trees are "Cultivar X" how can that new grower be sure that's the same "Cultivar X" that they'll get if they purchase the trees from a nursery?
An alternative path the new grower could take is to ask their neighbor for seed nuts, germinate and plant the nuts, and trust that a significant number of those seedling trees will produce similar to the parent trees. This is how the vast majority of chestnuts are planted in the U.S. today and it is does tend to work. The downside is that as those seedling trees mature, their nut quality, harvest timing, nutrient needs, cold tolerance, blight susceptibility, etc. of each tree will be a little different. That's not a deal breaker for most operations, but does present operational challenges for larger growers.
Where do we go from here
We are but a tiny sliver of a piece of the U.S. chestnut industry pie. It is pure hubris to assume that any action our little nursery takes could move the needle in a meaningful way. What we can do however, is make sure that the trees we're producing are genetically identified and communicate what we've learned with the industry as a whole. We're also taking a few specific actions internally:
- Planting a "mother tree" area of each of the cultivars we produce to collect data on them going forward and have as a backstop for re-propagation if needed in the future.
- Tissue culture propagating cuttings directly from the original Colossal tree which is still alive in Northern California.
- Continuing to work with breeders and researchers to genetically identify and propagate true cultivars suited for different regions of the U.S.
- Engaging with these same researchers a breeders to establish clear names, fingerprints, and phenotypical identifications for cultivars to minimize the chance of mis-identification in the future.