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Causes of Polyploidy

Ploidy is a term that defines how many sets of any given chromosome an individual possesses. A diploid organism has two sets of each chromosome, while a triploid cell has three. Polyploidy (literally, many ploidy) is a catch-all term that means a cell simply has more than one set, typically more than it is expected to when compared to the ploidy of the parents. Polyploidy is more common in the plant world (commercially grown strawberries are octoploids) than in the animal world.

  1. Polyploidy Basics

    • There are two processes by which cells divide: mitosis and meiosis. Mitosis usually leaves cells with the same number of chromosomes in the end as there were to begin with. Meiosis is the process by which gametes (eggs and sperm) divide and at the end of this process the chromosome number is halved (i.e., becomes haploid). This is useful because two haploid gametes coming together can form an organism with a full complement of chromosome, like its parent. Polyploidy can occur when the chromosomes fail to divide in meiosis, which will effectively double the number of chromosomes the offspring has and cause them to be polyploidy.

    Extraneous Factors

    • Some drugs can cause chromosome doubling. Colchicine affects the way chromosomes separate during the process of mitosis. Other forms of stress such as cold shock may also disrupt the cell division process and result in polyploidy.

    Natural Polyploidy

    • Some tissues within a plant are polyploidy. In most angiosperms, a sperm cell (pollen) will fertilize an egg and create an embryo. Surrounding the egg is another type of tissue that contains chromosomes. Chromosomes from another grain of pollen can fuse with these chromosomes and create what is known as an endosperm. Endosperm surrounds the developing seed and can serve as storage (e.g., much of a kernel of corn is endosperm).

    Hybrid Polyploids

    • Polyploidy can also result when two individuals that have different numbers of chromosomes mate and create an offspring. These offspring are called hybrids and do not have the same chromosome complement as either parent. In many instances, these hybrids are sterile and cannot reproduce because their uneven chromosome numbers do not allow them to form viable gametes.

    Human Polyploids

    • Polyploidy can also occur when more than one sperm cell fertilizes a single egg cell. This is rare, especially in humans and usually results in spontaneous abortion.

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