Are you growing any long fruited mulbs? I’m crossing M. macroura selections with rubras & hybrids in hope of creating a tree with both cold hardiness & jumbo fruits. By “cold hardy” I was only thinking zone 7 or 6, but you have to start somewhere.
Here’s one of the crosses I performed last Spring, & have a few seedlings growing now from:
I also would advise NOT using M. nigra for breeding purposes. It has well over 10x as many sets of chromosomes as any other species you’ll be using, AND it’s already been crossed with the long fruited species M. macroura before. The many sets of genes for tiny fruit always seem to overwhelm any of the inherited “long fruit genes”. Mark Travis (Livinginawe) wrote a snippet about it one attempt on his website: Hybrids | growingmulberry