Polyploidy
Related Terms
Aneuploid, aneuploidy, colchicine, chromosome, diandry, digyny, diploid, haploid, polyploid, tetraploidy, triploidy.
Background
Polyploidy is a genetic condition in which a cell has one or more extra sets of chromosomes. Nearly every cell in the body contains a nucleus, which contains two sets of chromosomes. These chromosomes contain DNA (deoxyribonucleic acid), the building blocks of life. Each parent provides one set of 23 chromosomes to his or her offspring. Therefore, each person has 23 pairs of chromosomes. The X and Y chromosome are called sex-determining chromosomes because they distinguish males from females. Human females have a pair of X chromosomes, while males have one X and one Y chromosome.
Aneuploidy refers to an abnormal number of chromosomes. Aneuploidy in certain chromosomes, such as trisomy 16 (three copies of chromosome 16), prevents survival of the developing fetus.
Polyploidy occurs frequently in some organisms, such as goldfish, salmon, salamanders, and some plants, typically resulting in survival of these organisms. For example, different kinds of wheat exist because some have two sets of chromosomes, some have four sets, and others have six sets. Polyploidy may, in fact, result in the creation of a new species, as in the case of the plant salsify, in which two new tetraploid species of this plant developed from combinations of the original three species.
Although polyploidy is fairly common in these and other "lower" organisms, polyploidy is rare in humans. Some instances of polyploidy do exist in humans, particularly in some tissues, such as the liver, and in some cancers. Polyploidy in the liver indicates growth and can be observed when part of the liver is removed; polyploidy is increased when the cells begin to grow. Triploidy is a condition in which there are three complete sets of chromosomes in a single cell; in humans, this would be 69 sets of chromosomes per cell. Tetraploidy is a condition in which there are four complete sets of chromosomes in a single cell; in humans, this would be 92 sets of chromosomes per cell. A great majority of pregnancies in which the fetus has triploidy or tetraploidy end in miscarriage, or if the pregnancy goes to full term, will result in the infant's death shortly after birth.
Polyploidy may occur if chromosomes duplicate spontaneously, if a cell fails to divide after its chromosomes are duplicated, or if two sperm cells fertilize one egg during reproduction, which results in a fertilized egg with three sets of chromosomes (triploidy). Polyploidy can also occur through exposure some chemicals, including some used for chemotherapy in humans, and through exposure to colchicine in some plants.
Many cancer cells exhibit polyploidy, and laboratory studies in breast cancer cells have shown that suppression of these genes results in suppressed polyploidy.
Methods
In humans, polyploidy most often occurs when two sperm cells fertilize one egg. This results in the fertilized egg having three sets of chromosomes rather than two sets. Polyploidy is rare in humans; not only because the fetus typically cannot survive, but also because of the ability of the egg to shut itself off from accepting any more sperm once one has fertilized it. However, this mechanism does not work if two sperm happen to enter at the same exact time.
In plants, polyploidy can be induced through exposing the seeds of the plant to colchicine. Colchicine is a poisonous natural product from a plant called autumn crocus. Colchicine interferes with processes that occur during cellular division, resulting in half of the sex cells having no chromosomes and the other half having double the number. Although this would be fatal in humans, it typically produces plants with more desirable characteristics than the parent plants. Polyploid plants can also be bred with a plant with a normal number of chromosomes to produce desirable plants, such as seedless watermelons.
Polyploidy is detected through a chromosome analysis, or a karyotype. A karyotype is an organized profile of an individual's chromosomes. In this type of analysis, a person's chromosomes are stained so they can be seen under a microscope and a picture of them taken. Chromosomes are then cut out from the picture and arranged and numbered by size. Instead of having two identical chromosomes, an organism with polyploidy will have three identical chromosomes (triploidy) or four identical chromosomes (tetraploidy).
Research
It is thought that polyploid cells are a common feature of many types of cancer cells. Research is being conducted to examine the effects of suppressing the breast cancer gene as a way to kill the cancer cells, but this research has not yet been conducted in humans. This information may ultimately lead to new targets specific to polyploidy cancer cells. Other studies have induced tetraploidy and aneuploidy to determine whether this may ultimately lead to cancerous cell development.
Implications
Triploidy occurs in about 2%-3% of human pregnancies and in about 15% of miscarriages, whereas tetraploidy occurs less frequently in pregnancies and in about 1%-2% of miscarriages. Most triploid conceptions end as miscarriage and those that do survive to term usually die shortly after birth. Polyploids tend to have defects in nearly all organs of the body, particularly the heart and central nervous system.
Triploidy can result when the extra set of chromosomes is received from the mother (digyny) or the father (diandry). Diandry usually occurs when an egg is fertilized by two sperm cells, while digyny is usually caused by failure of the egg to divide during cell division. Diandry is more common among early miscarriages, while digyny is more common in cases that survive into the fetal period. Triploidy should be distinguished from trisomy, which is when only one chromosome has three copies. Down syndrome is an example of trisomy, in which chromosome 21 has three copies instead of two; a person with trisomy 21 can survive well into adulthood and live a full life with relatively minor abnormalities.
Digyny triploidy typically produces a fetus that grows abnormally with marked underdevelopment of the adrenal gland, which is responsible for regulating the stress response, and a very small placenta. Among diandry triploids, the fetus appears to grow normally with normal adrenal development; however, they typically have abnormally large placental cysts.
Future research
Polyploidy rarely occurs among humans, although it can be found in certain tissues such as the liver. Polyploidy can be observed in liver tissue after a section of the liver is removed, and the cells start to grow. Many cancer cells exhibit polyploidy, which suggests that suppressing these particular genes may be an effective anticancer strategy. Studies have been conducted to examine the effects of suppressing the breast cancer gene as a way to kill the cancer cells, but this research has not yet been conducted in humans.
Author information
This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).
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