Mulberries cross-pollinate between the various species (forming hybrids) quite readily, except perhaps for a few of the twenty or so generally recognized species. It is quite possible that there are very few "pure" representatives of any one species (at least until genetic testing proves otherwise).
Hybrids offer the possibility of combining good traits of one species with desirable traits of another species. An example of this might be if a cold intolerant Pakistan mulberry (Morus macroura) was crossed with a cold-hardy Morus alba in a way that still preserved Pakistan's good tasting long fruit, and provided the hybrid with cold tolerance. Unfortunately, most crosses do not result in the desired outcome, but with enough trials, a good outcome is quite possible.
While observing various traits of a particular cultivar, it might be assumed that the reason for a particular trait is because of having been crossed with a species known for those traits (i.e.: Large leaves from a Morus macroura or Morus rubra). As this assumption is possible or even probable, another characteristic of mulberries needs to be mentioned: They are prone to polyploidy occurring in a diploid cultivar... that is, during mitosis, chromosomes in cellular nuclei can duplicate (even repeatedly) without the cell splitting. Although this polyploid condition may be restricted to a minority of the cells of a cultivar, it is often prevalent enough to affect various attributes of the plant (Large leaves, large fruit). "Polyploidy" often translates to "larger" in plants. Although polyploidy happens quite naturally in mulberries, it can be induced by a chemical treatment on seeds using colchicine.
Until 2020 I was convinced that a cross between a highly polyploid 308 chromosome Morus nigra and a 28 chromosome mulberry species was impossible (outside of a laboratory). I wish to present the verified work of some of the various hybrids that a Belgium nurseryman (Dithmar Guillaume) has obtained with Morus nigra as one of the parents.
Morus nigra x Pakistan mulberry (Morus macroura)
The pictures presented here are seedling hybrid mulberry plants from crosses between Morus macroura 'Pakistan' female (2N=28 chromosomes) and a Morus nigra male pollinator (22N=308 chromosomes). Analysis performed by a Belgium university showed that the offspring had a polyploidy level of 12N, or 168 chromosomes, with a 95% confidence (1N from mother and 11N from father).
Although the plants display extreme variability, they are all 'Pakistan' x M.nigra hybrids!
Early fruits are small (true for all seedling mulberries), but are reported to have an excellent flavor.
2020 was the first year Dithmar's 15-year-old hybrid fruited, and strangely enough, two of his two years old's fruited. So far, none have resembled the long 'Pakistan' fruit of the mother tree.
An objective of Dithmar is to create a cold hard 'Pakistan' like fruited mulberry plant.
As apparent in the top photo, his 15-year-old hybrid is still quite small. A dwarf mulberry tree of any type is a highly desirable characteristic.