I will just leave this here for your perusal as per some ideas about the entire morus nigra population in the whole universe being clones of the same plant. Search for “genotypes” and “variation” in morus nigra. https://www.researchgate.net/publication/301353946_Forgotten_and_Less_Utilised_Plant_Species_as_Functional_Food_Resources (btw. the reason why they count genotypes is because of how older nigras split and a group of trees may be just one genotype.)
Is there any proof that can shake your belief? Just checking… There are enough differences in our local trees hat a guy, who owns the tree originally identified as “Trnavska” in the next village, took one look at my trees and immediately assessed it was not the same - the leaf shape (both our trees have a single type of leaves) nor the flowers were the same. Mine are clones of another local tree by this definition: ( an organism or cell, or group of organisms or cells, produced asexually from one ancestor or stock, to which they are genetically identical. - in that they are suckers from the same plant.)
I understand, he is incapable of DNA sequencing - the scientists who published the study are, though.
Anyway, even without coming around and doing your own DNA analysis… Please explain how a sexually reproducing organism that is grown on large scale could be untouched by natural mutations for millenia (asuming for this exercise there had been some kind of a catastrophy killing all but one specimen in the entire world that the Romans got from (where?) and then spread by cuttings around Europe roughly 2000 years ago). What makes morus nigra impervious to natural variation and adaptation? And why do you think it is any different than malus domestica (also reproduces both sexually and asexually). Or are an Orange Pippin and Red Relicious also clones?
I think I may not understand what your definition of a clone is.
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Have you ever seen a male Morus nigra tree?
I ask this because I’ve never heard of one, and if there are no males then maybe they only produce seeds apomictically, rather than sexually. In that case they are all clones in one sense (that the seeds are copies of the parent DNA rather than recombinations of DNA). But even then you are correct that random mutations will occur, just as in bud sports, so despite being “clones” they will not be truly identical across their entire range. But the amount of variation will be a lot less than in a sexually reproducing species.
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Nothing, you are correct.
One problem with clones is they don’t forget their age. Eventually all clones will die from old age. With trees this could be ten thousand years. But you can’t beat Father Time. Like 5 thousand year old aspen trees that can’t sexually produce anymore. They lost the genes to mutation. Eventually other vital functions will stop working too. Bamboo flowers and dies every 120 years. This happened in the USA and every clone taken once mature enough flowered and died. We lost thousands of bamboo plants. Clonal age matters. Nothing lives forever. If morus nigra doesn’t reproduce sexually it will be lost.
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If two different trees in close distance have both male and female flowers (plenty of those around), can they not pollinate each other?
Just taking the situation of my region where some trees are thought to come here via the Romans around 200AD and others as the silver lining of Otoman campaigns from Turkey in 16th century. I can’t imagine either of those “batches” being identical (can hardly imagine them stemming from cuttings rather than seed from dried fruit) and then somehow being prevented from pollinating each other given the relative continuity of the regions where they were grown mostly for silk production.
As for seeing a purely male tree - I haven’t looked hard enough. But all our nigras are grown and protected by people and since there’s no marketable use for a huge non-fruiting tree (other than timber) I can imagine the selection going that way.
I also wonder from the point of evolution, why would certain species produce only male flowers, loads of them, up to a certain age. Why waste the energy by flowering at all?
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So you’ve seen male flowers on your nigra? I’ve searched far and wide for any photos of a male nigra flower and never seen one. If you could share photos that would be wonderful!
In terms of evolution, nigra likely originated from some very strange hybridization, perhaps assisted by a virus or something. I’ve never seen anyone who came up with an explanation for how you could get from the usual diploid mulberry species to a ploidy of 22. The math just doesn’t add up.
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Except a lobster… unless it gets eaten.
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Will do tomorrow. Hopefully, there will be some left on my tree that’s delayed by shade. They aren’t much different from alba male flowers, just more robust and twice as long as the female ones.
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Well, how did dogs stay roughly wolf-shaped for ages and then went morphologically bonkers in relatively no time? Some mysteries are cool. 
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I echo everything @swincher is saying. A. The extra ploidy of M nigra is strong evidence that it is a unique freak mutation rather than a normal variant of a different mulberry species. B. Cannot be reproduced by seed imhe. It can only be perpetuated by grafting. C. All nigras taste the same. I tried fruit in Lebanon, Europe and the US. For a fruit to taste the same, it has got to be the same. D. I do not understand nigra!!! I am saying this humbly. It lives the longest of all fruit trees. Hundreds of years. That alone sets it apart. It is a SuperTree…
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But wait, Drew has his seedling above in this very thread. So that seems to be against that point?
I do wonder if the extremely large number of copies of the chromosome somehow makes it so that seedlings have significantly less variation than with most species. I must admit that my understanding of the significance of ploidy numbers is basically non-existent. I mean I get that it measures how many chromosomes they have, but not how that number itself changes phenotypes.
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I finally get it. 
Here you go @swincher, the dirty on the nigra:
The nigras have been flowering for about a month (which is also how their fruiting period is spread), so these are the last flowers left. We’ve had strong rain for a few days so most male spent flowers were already washed off.
Another one late to the party, yuo can see an unfurled stamen on the bottom. The bastards look similar enough to female flowers when juvenile to confuse people into thinking they will have loads of fruit to only get disappointed later.
Here’s more:
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Pollinated female flowers for comparisson:
Some more spent and discarded males:
And just to avoid the obvious follow-up question, this is what a black alba just 20m away looks like at the same time:
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These do look like male flowers on nigra!!!
Reports like this tourisim promotion video (starts just before a nice close-up on male flowers ) claim that “The nigra mulberry IS only propagated by layering and grafting.” Then someone picks it up and swaps “IS” for “CAN”.
Attention to language is vital here, because it is not a question of biological impossibility, but rather of human convenience. You have to keep in mind that morus nigra trees that have both male and female flowers have mostly male flowers to begin with. You can have a 2m tall tree with all male flowers except 2-3 female ones. (If your seedling has male flowers, patience @Drew51 ! ) So you may well wait for good 20+ years to get a decent crop from a seedling. (Which is more than sorbus domestica and the saying goes that you plant those for your grandchildren.) My trees are still relatively young ( cca.15 years - but they had been tortured in a bonsai bowl for some years) and the one in full sun has about 1:20 female to male flowers. The 100+ tree it has been layered from:
has about 1:2. Plus the fruit starts small and increases in size with the tree’s age. (subject to change at maybe 300 years?) Thus, it is no surprise that people choose to take layered plants and graft the nigras to get strong plants with a head start towards a good crop and that nurseries propagate mostly female trees. Because who in their right mind (unless you are nuts like me or are breeding for variation/adaptation ) would go to the trouble of propagating them by seed if you have a more convenient method and a tree that seems to layer itself (by splitting) into practical immortality from the perspective of human life-span.
It is not that nigras are stupid - we are lazy and life’s too short.
P.S. Sorry for bursting the nigra immaculate conception bubble. I feel like I may have killed Santa.
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I grow my seedling for fun. I have a few other cultivars for production. Alba hybrids and such. Plus my tree did fruit last year. A few anyway. I got a taste of the fruit. Like my Cornus mas seedlings it takes some time. Cornus mas now has fruit every year after nine years. Patience paid off. But even if it doesn’t I’ll still have plenty of mulberries from other trees more adapted to my area. I waited 30 years for my cactus to mature enough to flower. What a sight once it did!
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I take my projects like seedling experiments or sorbus domestica grafts and the likes as fun, too. And as an exercise in practical optimism.
I suppose all the flowers were female, then. That’s a very different situation.
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