That was my assumption also since I believed that, though Gerardi was often referred to as a Morus nigra cross, the extremely high polyploidy level (22n=308 chromosomes) would prevent crosses from occurring between 2n mulberries (as it does with many other plant species). But the work of Dithmar (@Mikatani) has proved me wrong because he has successfully crossed nigra with macroura.
A Florida woman, Jan Doolin, that has a passion for mulberries has been in discussions with a fellow named Johnny Bee, who says that his aunt’s sister knew Louis Edward Gerardi, who she said had crossed a ‘Himalayan long’ (Morus macroura) with ‘Chelsea’ (Morus nigra) to create ‘Gerardi’. Unfortunately, Mr. Gerardi passed away in 2012.
I have examined ‘Gerardi’ microscopically and, although it does have a fair amount of polyploidy, its nuclei size appears to be no greater than 2n (28 chromosomes), so it doesn’t have the thick leaves of its “Mother” (large nuclei=large cell size=thick leaves).
What is interesting (In my opinion) is that both Morus nigras and Morus macrouras are not very cold tolerant below zone 7, but ‘Gerardi’ appears to be hardy to zone 5.