I’m reading a very interesting article about pecan genetics and very specifically about introgression of traits from other Carya species. They found that modern pecans share between 1 and 3 percent DNA with C. Aquatica aka water hickory. The shared genes are chopped up and spread across several different chromosomes indicating a cross between the species perhaps up to a few million years ago.
They also found a large introgression from C. Cordiformis aka bitternut hickory present in Major and several descendants. This represents something unique because the introgression appears to all be on chromosome 8 and shows evidence that it is highly conserved, meaning that it conveys very useful survival characteristics to pecans. It is telling that other sections of bitternut DNA do NOT show up which means they are selected against by some combination of processes. Note that this selection precedes human directed pecan breeding efforts. We have discussed this in other threads on Growingfruit so it is not really new to say that Major is a hican. Grauke documented this at least 7 years ago. I noticed it within a few hours of looking at Major nuts in the shell and at Hickory Major nuts which have thick husks very similar to other hickory species.
There was a chunk of DNA from C. Myristiciformis aka Nutmeg Hickory on chromosome 5. This is interesting because it indicates that pecan has picked up adaptive genetics from at least 3 close relatives. Nutmeg hickory is a 16 chromosome species that happens to share multiple traits with Pecan. In some ways, we could describe Nutmeg hickory as intermediate between the Carya (true hickory) and Apocarya (pecan hickory) sub groups. It would be interesting to compare a group of Nutmeg hickory trees with the pecan genome to see if the gene sharing was two way, i.e. that Nutmeg hickory picked up some pecan genes along the way.
Among the results, one of the most interesting is that pecans often contain unique DNA meaning there are genes in one variety that do not show up in another. This is consistent with a species that grows in a wide range of edaphic and climatic environments. There is a LOT of diversity in the pecan genome that is unique and likely not represented in the roughly 30 cultivars that represent most pecan production.