imo, there's still a chance nigras could be hybrids. While it is true that nigra's could live >600 years, it does not necessarily rule out its predecessors(with shorter lifespans) to actually be even more ancient albas and/or macrouras or any other morus species. What am trying to say is that we've ascertained a mule to be a hybrid because we knew beforehand what its parents were. The parent existed before the mule hybrid was created. Even more interesting are that mules could outlive horses by >10 years and supposedly more intelligent than both parents.
unfortunately for plants, which took millions of years to evolve, we are all clueless. Unless phylogenetic/genome studies determine which is more ancient than which, anything is still possible.
per the website below, mulberry's polyploidy, was a recent occurrence, "or new wave of occcurrence", which imply that nigra's are a more recent species than others with smaller numbers of chromosomes.
"Mulberry is rapidly evolving at the nucleotide level. It’s fast evolving genes may have contributed to the flexibility of mulberry to adapt to environments outside of its native range, facilitating its spread to Europe, Africa and the United States. In contrast to its rapid nucleotide changes, Rosales ploidies have evolved conservatively. Mulberry, strawberry, cannabis, papaya and grape underwent the most recent pan-eudicot hexaploidization. Widespread neopolyploidy in mulberry with up to 308 (44 × ) chromosomes22 and strawberry with up to 70, suggest an intriguing scenario that these lineages may be receptive to the benefits of a new wave of polyploidization."
since nigras have the most number of chromosomes of all mulbs, if not all in the plant kingdom, it is likely to be one of the youngest divergence from the moraceae family tree
and speaking of jigsaw epidermal cells and 'possible nigra hybrid' the link below shows Morus laevigata exhibiting similar growth patterns as shown on the following link. Could your leaf be just a laevigata or actually a nigra? Or a hybrid?