Plants differ from animals in many ways, one of which is the incidence of viable polyploidy. Ploidy refers to the number of chromosome pairs an organism has and polyploidy is the phenomenon wherein an organism will have more than one complete set of homologous chromosomes. Over 80% of flowering plants are polyploids, while the occurrence of animal polyploidy is a phenomenon of great rarity, almost absent in mammals and scarce in fish. One of the major reasons for this difference is the fact that plants rarely require a mate to create offspring, whereas animals often need a mate to produce viable offspring. Thus, should a polyploid animal mate with an animal of normal ploidy, it will create an aneuploidic zygote that cannot create viable gametes. Plants can undergo two kinds of polyploidization, allopolyploidization and autopolyploidization. Allopolyploidization occurs when two different species hybridize and the resulting offspring is viable and reproductively isolated. Autopolyploidization occurs when a plant species self fertilizes to create offspring that is reproductively isolated from its parent species. There are two major ways polyploidization can occur. The first is mitotic nondisjunction and can best be explained when observing allopolyploidization. When the haploid gametes of two different species hybridize, the resulting zygote has one chromosome of each kind, thus lacking homologous pairings. An error in mitosis can occur where the doubled chromosomes end up in the same daughter cell, thus yielding a diploid individual with two chromosomes of each type. The chromosomes in the new plant species can now synapse properly during meiosis and create viable haploid gametes that allow for self-fertilization. The second method involves meiotic nondisjunction. This type of nondisjunction can occur during meiosis I or meiosis II, but in both cases the result is diploid gametes. These diploid gametes can be fertilized by haploid gametes to yield nonviable triploid individuals, or they can fuse with another diploid gamete to create a viable tetraploid individual. So, if a plant produces two diploid gametes through nondisjunction and they self-fertilize, the result would be a new, tetraploid species. This species is now reproductively isolated from its parent as if the two were to mate they would create nonviable triploid individuals.
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