Is there a practical upper limit to ploidy?

Question

In my AP Biology class, we were discussing polyploidy, specifically, its deleterious nature in mammals and its prevalence in plants. We also learned that commercial crops, especially fruit, are often selectively bred for high magnitudes of ploidy. Fascinated, I googled the subject and ended up reviewing the Wikipedia article to get a little more background info, where I read this:> Polytene chromosomes of plants and fruit flies can be 1024-ploid.[44][45] Ploidy of systems such as the salivary gland, elaiosome, endosperm, and trophoblast can exceed this, up to 1048576-ploid in the silk glands of the commercial silkworm Bombyx mori.[20]From this, I have three questions. The first is, is there an upper limit to ploidy that can occur in nature? I understand that polyploids can "lose" ploidy by haploidisation, but is this as common as increases in ploidy? I believe that the answer to this would imply the answer to my first question, however, I could not find much literature in the way of this area.My second question is, is there an upper limit to synthetically induced polyploidy? Is the Bombyx mori's ploidy arbitrary, or is it at the max that technology can produce? Can it's ploidy be expanded further? Lastly, does arbitrarily high ploidy serve any purpose in agriculture or science? Is there a threshold where utility is no longer maximized?Thank you in advance.

Answer 1

Hi Asked,

You can answer this question yourself, based on what you know of basic biology: offspring of parents of different ploidy are almost always either non-viable, or non-fertile. Cells don't know how to segregate an odd number of chromosomes (or equivalent). Hence, polyploidy tends to be utilized in limited specific situations in nature. You might note that silkworms would not be viable in nature -- the females are far too massive to fly. Though they possibly could be bred back to a viable wild-type, a la Russian blue foxes.

Now, with respect to agricultural use, polyploid vegetables are quite common. The reason: polyploid plants have short internodes and thick leaves. So for something like a lettuce, that transforms an erect, spindly, stringy plant into a short, thick, juicy plant. Cabbage, in one wild type, tops 12 feet, with a single stalk and spindly leaves. Quite a difference from a head of commercial cabbage!

Occasionally, additional benefits obtain -- triploid commercial bananas are seedless, for example, though sterile. So triploidization might be a useful tool to produce a crop with limited invasiveness, for example.

-- Cheers, -- Mr. d.