Polyploidy

Polyploidy can be induced with chemicals such as colchicine, as O. J. Eigsti first demonstrated in 1935. His work extended that done by F. A. Blakeslee, and the technique he used has been adopted commercially to produce products such as seedless watermelon. The seeds are missing because the embryos abort from aneuploidy before they can form seeds.

In nature there are different kinds of polyploids. An autopolyploid plant has all its chromosomes derived from one haploid set. An allopolyploid plant has its sets derived from two different plant species. In general, allopolyploids are fertile and survive, whereas autopolyploids are sterile and must be propagated as clones (identical twins), by cuttings.

The difference between autopolyploidy and allopolyploidy can be appreciated by an example. No one knows the reasons for mitotic failure leading to spontaneous tetraploids, but artificial ones are induced by mitotic poisons, like colchicine, that prevent spindle fiber formation. If one species has chromosomes ABCD in a (haploid) gamete, and a related species has chromosomes FGHI, the resulting (diploid) zygote will have a chromosome set consisting of ABCDFGHI. If that collection of chromosomes undergoes a spontaneous doubling, the resulting plant is AABBCCDDFFGGHHII. Such a plant will produce ABCDFGHI gametes and by self-pollination, which is common in many flowering plants, the new allopolyploid will be fertile.

In the case of autopolyploids, by contrast, the chromosomes ABCD become triplicated (3N: AAABBBCCCDDD) or quadruplicated (4N: AAAABBBBCCCCDDDD). This may lead to nondisjunctional separations during meiosis, wherein the chromosomes will divide improperly or incompletely. In the 3N plant many of the gametes may be AABCDD or ABCCD or other variations of aneuploidy that will disturb embryonic development.

Among familiar plant polyploids are strains of wheat with chromosome numbers of 14 (2N), 28 (4N), and 42(6N), all of which are based on an ancestral form whose haploid number was 7. Chrysanthemums have a series of varieties with a range of chromosome numbers: 18, 36, 54, 72, and 90. The ancestral haploid is assumed to be 9. About half of all flowering plant species are believed to have polyploid varieties. If an accidental doubling of the zygote chromosome number is the major mechanism involved, most of these forms are tetraploid.