Some may find my speculative thoughts annoying, but I'd like to present a personal theory I have about the genetic ancestry origins of Green Gage Plum.
This is just based on some very general knowledge I have about plums and plum morphology.
I think Green Gage plum most likely originated from a cross between a Mirabelle type plum and sour green Persian plum (Prunus vachuschtii).
Prunus vachuschtii is native to the countries of Georgia and Azerbaijan, and is a very popular plum in Persia, and is also commonly grown in other parts of the Middle East. In Lebanon and Syria they are called Janerik. It's not a sweet plum, but is nevertheless enjoyed as a refreshing snack (often with salt, or a flavorful sauce, or as the main ingredient in a sauce). I have tasted them.
Prunus vachuschtii is more of a tentative species name for this plum variety and is not universally recognized.
I think this cross most likely happened in Italy or another country across from the Adriatic Sea from Italy, which has a lush forested climate similar to the Caucasus region, where plums grow very well. (This might make sense because these countries were at a cultural border zone between Europe and the Middle East region, under the Ottoman Empire at the time)
Mirabelle plums most likely derive most of their ancestry from Prunus brigantina. But likely also have some ancestry from "Prunus domestica" (I mean what is known as the "Italian prune plum"). I believe Prunus brigantina may have resulted from hybridization between Prunus cerasifera ("cherry plum") and apricot, or represents a species intermediate between these two other species.
In fact I believe there are many "subtypes" of Prunus cerasifera that probably have some genetic introgression from Prunus brigantina, because some varieties of Prunus cerasifera have a very orange color (orange-yellow, but almost more orange than yellow) and have a strong aroma kind of reminiscent of apricot. (I have tasted such fruits from a wild tree)
This might help explain the low chill requirement of Prunus cerasifera, which has even been able to become naturalized in the wild in southern Australia.
(Prunus domestica would have come from a complex process of hybridization between Prunus cerasifera and Prunus spinosa)
Chemically, I believe the distinctive "green" aroma and flavor primarily comes from esters of 2-methylbutyrate. But also just a little bit from cis-3-hexenol (smell of grass).
Sour Green Persian plums do sort of have what could be described as an "exquisite" plum aroma or flavor, but it's specifically the very dry "textured" smell of plum (concentrated in the skin), rather than the "fruity" smell of plum. (The part of smell I'm attempting to describe, probably more of a terpene or terpenic, "woody", very important component in the smell of plum, clearly identifiable and recognizable as the unique characteristic and distinctive smell of "plum", despite lacking any fruity or "sweet" qualities)
Very quickly, usually how number of chromosome sets work is an offspring gets half its number from each parent. So, for example, if one parent had 2 (diploid) and the other had 4 (tetraploid), the offspring would have 3. But if the offspring has an odd (uneven) number of chromosome sets, its seeds will (in most cases) be sterile (unable to grow). In less common cases, sometimes a parent with an odd number can sometimes contribute its chromosomes in an "unreduced" form to the gamete (sperm of egg cell). So one parent with 3 sets of chromosomes might in rare cases contribute all 3 to the offspring, while 1 other chromosome set comes from a diploid parent (which is normal), resulting in a tetraploid (4) offspring. In rarer cases than that, it's sometime possible for the chromosome number in a species to spontaneously double. (This can be intentionally induced in plants with the use of colchicine)
