Drew thanks for the comment. Plants tolerate changes of ploidy (chromosome number) better than animals. Most of the bearded irises we grow now are tetraploid, and a lot of daylilies. For those, changing from diploid (2n) to tetraploid (2n) is done artificially with a chemical (colchicine) that originates from a plant (Autumn crocus, what my grandfather called "squill") and is used in humans to treat gout. Colchicine causes a change in the chromosome separate in the plant, which makes the chromosomes stick together in reproduction, so the end result is 4N. There's an art and science to converting plants to tetrapoloid, and I don't know how to do it.
I would love to see someone cross more plums with peaches to make the peaches resist leaf curl disease that destroys the trees here in PNW. Or a late blooming plum with Apricots to create apricot trees that don't bloom early and get killed by the late frosts here. I am curious about whether Neils Hansen accomplished that in the early 1900s with crossing Asian plums or apricot-plums with American plum species to create Hanska, Ember, and other hybrid plums. Crossing a Prunus cerasifera plum with a peach could give a beautiful tree and a burgundy peach plum that has a zingy flavor. Unfortunately, the Dave Wilson / Zaiger PeachPlum "Tri-lite" was a susceptible to leaf curl as any of their peaches and I finally culled it.
Here is a nice brief article about ploidy in prunus species, from USDA. I would say that reading about plum ploidy pleases me, but I won't. The article states "Prunus is a large, diverse genus with a basic chromosome number x = 8" and relevant to this topic, " from the genetic improvement perspective, the subgenus Amygdalus,to which peaches and almonds belong, and the subgenus Prunus, which includes section Prunophora comprised of diploid Japanese plums and hexaploid European plums and section Armeniacacontaining apricots, are considered to be a single gene pool (Watkins, 1976). The subgenus Cerasus, comprising diploid sweet cherry and tetraploid tart cherry constitutes a distinct group distantly related to the other two subgenera... breeding barriers exist among taxa possessing different ploidy levels, even within the same section, but hybrids are generally successful when both parents have the same ploidy level (Okie and Weinberger, 1996). " So the best bet is to use species with the same number of chromosomes, and that appears to be the case for most, except Euro plums and sour cherries.
Weatherman, I hope I'm not hijacking your topic. It's interesting to me and maybe this info is helpful. You seem to be the new pioneer on creating prunus hybrids, very impressive.
Drew, I think the self fertile sweet cherries are still diploid, or they wouldn't pollinate the non-self fertile ones. I could be and often am wrong, however.
Zaigers remove the anthers from their flowers before pollinating them with pollen from other types, to ensure not self pollinating, but if you use changes in leaf color or shape to select progeny, I think that is reassuring the cross pollination occured, like Weatherman has with red leaf progeny on his baby trees. Especially your Indian Free that can't pollinate itself. I think you are right about peaches being diploid.
Speedster, as an amateur I would use a small paintbrush to collect pollen from one type, and brush the pollen onto the stigma of the other. Such as, brushing on pollen from a red leaf plum, onto a non-red leaf plum or other Prunus. Then you can plant the seeds, and keep only the ones with red leaves. Or, if your female fruit is a peach and you pollinize with a plum, you can select only the seedlings with a wide short leaf - plum - like, that came from the peach. I have genetic dwarf peaches that I used to pollinize "normal" peaches. One seedling looks like a genetic dwarf. We'll see how it grows out.