Thank you for pointing me to this! I currently have specimens of black sapote and texana with the intention of eventually attempting controlled crosses. More than half of the tiny one year-old texana seedlings survived outdoors here, low of 14.7°F in January during an extended freeze. So definitely at least hardier than black sapote!
I do have some questions about the methodology in the paper you link, because the various different charts in that paper seem to move texana around quite a bit depending on which method was used, and nigra is only listed on one of the charts at all as far as I can tell (it’s possible I missed it?). They had two nigra accessions, so it’s kind of weird it wasn’t even listed in most of the charts.
Here are my annotated versions of the relevant parts of the charts, where green is nigra, red is texana, and blue are kaki/lotus/virginiana:
I know he’s stepped away for a bit, but the question of how trustworthy that paper is would be a good one for @Richard , who wrote his recent paper on this topic, but I was never able to get my head around Richard’s paper well enough to apply its findings when I look at something like that study.
In any case, nothing in the paper has scared me off from my texana/nigra hybridization plans. I don’t actually expect to succeed, but if they really are diploid from the same clade, it’s at least possible!